[PATCH v12 0/4] kunit: Add support for suppressing warning backtraces

[PATCH v12 0/4] kunit: Add support for suppressing warning backtraces

[Albert Esteve]

Some unit tests intentionally trigger warning backtraces by passing bad
parameters to kernel API functions. Such unit tests typically check the
return value from such calls, not the existence of the warning backtrace.

Such intentionally generated warning backtraces are neither desirable
nor useful for a number of reasons:
- They can result in overlooked real problems.
- A warning that suddenly starts to show up in unit tests needs to be
  investigated and has to be marked to be ignored, for example by
  adjusting filter scripts. Such filters are ad hoc because there is
  no real standard format for warnings. On top of that, such filter
  scripts would require constant maintenance.

One option to address the problem would be to add messages such as
"expected warning backtraces start/end here" to the kernel log.
However, that would again require filter scripts, might result in
missing real problematic warning backtraces triggered while the test
is running, and the irrelevant backtrace(s) would still clog the
kernel log.

Solve the problem by providing a means to suppress warning backtraces
originating from the current kthread while executing test code.
Since each KUnit test runs in its own kthread, this effectively scopes
suppression to the test that enabled it, without requiring any
architecture-specific code.

Overview:
Patch#1 Introduces the suppression infrastructure integrated into
        KUnit's hook mechanism.
Patch#2 Adds selftests to validate the functionality.
Patch#3 Demonstrates real-world usage in the DRM subsystem.
Patch#4 Documents the new API and usage guidelines.

Design Notes:
Suppression is integrated into the existing KUnit hooks infrastructure,
reusing the kunit_running static branch for zero overhead
when no tests are running. The implementation lives entirely in the
kunit module; only a static-inline wrapper and a function pointer
slot are added to built-in code.

Suppression is checked at three points in the warning path:
- In `warn_slowpath_fmt()` (kernel/panic.c), for architectures without
  __WARN_FLAGS. The check runs before any output, fully suppressing
  both message and backtrace.
- In `__warn_printk()` (kernel/panic.c), for architectures that define
  __WARN_FLAGS but not their own __WARN_printf (arm64, loongarch,
  parisc, powerpc, riscv, sh). The check suppresses the warning message
  text that is printed before the trap enters __report_bug().
- In `__report_bug()` (lib/bug.c), for architectures that define
  __WARN_FLAGS. The check runs before `__warn()` is called, suppressing
  the backtrace and stack dump. 

To avoid double-counting on architectures where both `__warn_printk()`
and `__report_bug()` run for the same warning, the hook takes a bool
parameter: true to increment the suppression counter, false to suppress
without counting.

The suppression state is dynamically allocated via kunit_kzalloc() and
tied to the KUnit test lifecycle via `kunit_add_action()`, ensuring
automatic cleanup at test exit. Writer-side access to the global
suppression list is serialized with a spinlock; readers use RCU.

Two API forms are provided:
- kunit_warning_suppress(test) { ... }: scoped blocks with automatic
  cleanup. The suppression handle is not accessible outside the block,
  so warning counts (if needed) must be checked inside. Multiple
  suppression blocks are allowed.
- kunit_start/end_suppress_warning(test): direct functions that return
  an explicit handle. Use when the handle needs to be retained, or passed
  across helpers. Multiple suppression blocks are allowed.

This series is based on the RFC patch and subsequent discussion at
https://patchwork.kernel.org/project/linux-kselftest/patch/02546e59-1afe-4b08-ba81-d94f3b691c9a@moroto.mountain/
and offers a more comprehensive solution of the problem discussed there.

Changes since RFC:
- Introduced CONFIG_KUNIT_SUPPRESS_BACKTRACE
- Minor cleanups and bug fixes
- Added support for all affected architectures
- Added support for counting suppressed warnings
- Added unit tests using those counters
- Added patch to suppress warning backtraces in dev_addr_lists tests

Changes since v1:
- Rebased to v6.9-rc1
- Added Tested-by:, Acked-by:, and Reviewed-by: tags
  [I retained those tags since there have been no functional changes]
- Introduced KUNIT_SUPPRESS_BACKTRACE configuration option, enabled by
  default.

Changes since v2:
- Rebased to v6.9-rc2
- Added comments to drm warning suppression explaining why it is needed.
- Added patch to move conditional code in arch/sh/include/asm/bug.h
  to avoid kerneldoc warning
- Added architecture maintainers to Cc: for architecture specific patches
- No functional changes

Changes since v3:
- Rebased to v6.14-rc6
- Dropped net: "kunit: Suppress lock warning noise at end of dev_addr_lists tests"
  since 3db3b62955cd6d73afde05a17d7e8e106695c3b9
- Added __kunit_ and KUNIT_ prefixes.
- Tested on interessed architectures.

Changes since v4:
- Rebased to v6.15-rc7
- Dropped all code in __report_bug()
- Moved all checks in WARN*() macros.
- Dropped all architecture specific code.
- Made __kunit_is_suppressed_warning nice to noinstr functions.

Changes since v5:
- Rebased to v7.0-rc3
- Added RCU protection for the suppressed warnings list.
- Added static key and branching optimization.
- Removed custom `strcmp` implementation and reworked
  __kunit_is_suppressed_warning() entrypoint function.

Changes since v6:
- Moved suppression checks from WARN*() macros to warn_slowpath_fmt()
  and __report_bug().
- Replaced stack-allocated suppression struct with kunit_kzalloc() heap
  allocation tied to the KUnit test lifecycle.
- Changed suppression strategy from function-name matching to task-scoped:
  all warnings on the current task are suppressed between START and END,
  rather than only warnings originating from a specific named function.
- Simplified macro API: removed KUNIT_DECLARE_SUPPRESSED_WARNING(),
  the START macro now takes (test) and handles allocation internally.
- Removed static key and branching optiomization, as by the time it
  was executed, callers are already in warn slowpaths.
- Link to v6: https://lore.kernel.org/r/20260317-kunit_add_support-v6-0-dd22aeb3fe5d@redhat.com

Changes since v7:
- Integrated suppression into existing KUnit hooks infrastructure
- Removed CONFIG_KUNIT_SUPPRESS_BACKTRACE
- Added suppression check in __warn_printk()
- Added spinlock for writer-side RCU protection
- Replaced explicit rcu_read_lock/unlock with guard(rcu)()
- Added scoped API (kunit_warning_suppress) using __cleanup attribute
- Updated DRM patch to use scoped API
- Expanded self-tests: incremental counting, cross-kthread isolation
- Rewrote documentation covering all three API forms with examples
- Link to v7: https://lore.kernel.org/r/20260420-kunit_add_support-v7-0-e8bc6e0f70de@redhat.com

Changes since v8:
- Rebased to v7.1-rc2
- Remove KUNIT_START/END_SUPPRESSED_WARNING() macros
- Add KUNIT_EXPECT_SUPPRESSED_WARNING_COUNT checks to drm tests
- Link to v8: https://lore.kernel.org/r/20260504-kunit_add_support-v8-0-3e5957cdd235@redhat.com

Changes since v9:
- Fix silent false-pass when kunit_start_suppress_warning() returns NULL
- Fix RCU lockdep splat for kunit_is_suppressed_warning() calls
- Move disable_trace_on_warning() in __report_bug()
- Make suppress counter atomic
- Mark helper warn functions in selftest as noinline
- Add kunit_skip() for CONFIG_BUG=n in selftests
- Fix potentially uninitialized data.was_active in kthread seltest
- Add kthread_stop() in kthread selftest early exit
- Initialize scaling_factor to INT_MIN in DRM scaling tests
- Add include for bool in test-bug.h to fix CONFIG_KUNIT=n case
- Link to v9: https://lore.kernel.org/r/20260508-kunit_add_support-v9-0-99df7aa880f6@redhat.com

Changes since v10:
- Remove synchronize_rcu() to avoid sleeping in atomic context
- Pin task_struct refcount to prevent ABA false-positive matches
- Loop in suppression selftest to prevent use-after-free on kthread exit
- Skip DRM rect tests on CONFIG_BUG=n
- Link to v10: https://lore.kernel.org/r/20260513-kunit_add_support-v10-0-e379d206c8cd@redhat.com

Changes since v11:
- Use call_rcu() to defer free without blocking
- Remove #ifdef CONFIG_KUNIT guard in lib/bug.c
- Remove stale config checks from selftest
- Replace skip on DRM rect tests with conditional expectation
- Link to v11: https://lore.kernel.org/r/20260514-kunit_add_support-v11-0-b36a530a6d8f@redhat.com

--
2.34.1

---
To: Brendan Higgins <brendan.higgins@linux.dev>
To: David Gow <david@davidgow.net>
To: Rae Moar <raemoar63@gmail.com>
To: Andrew Morton <akpm@linux-foundation.org>
To: Paul Walmsley <pjw@kernel.org>
To: Palmer Dabbelt <palmer@dabbelt.com>
To: Albert Ou <aou@eecs.berkeley.edu>
To: Alexandre Ghiti <alex@ghiti.fr>
To: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
To: Maxime Ripard <mripard@kernel.org>
To: Thomas Zimmermann <tzimmermann@suse.de>
To: David Airlie <airlied@gmail.com>
To: Simona Vetter <simona@ffwll.ch>
To: Jonathan Corbet <corbet@lwn.net>
To: Shuah Khan <skhan@linuxfoundation.org>
Cc: linux-kernel@vger.kernel.org
Cc: linux-kselftest@vger.kernel.org
Cc: kunit-dev@googlegroups.com
Cc: linux-riscv@lists.infradead.org
Cc: dri-devel@lists.freedesktop.org
Cc: workflows@vger.kernel.org
Cc: linux-doc@vger.kernel.org

---
Alessandro Carminati (1):
      bug/kunit: Core support for suppressing warning backtraces

Guenter Roeck (3):
      kunit: Add backtrace suppression self-tests
      drm: Suppress intentional warning backtraces in scaling unit tests
      kunit: Add documentation for warning backtrace suppression API

 Documentation/dev-tools/kunit/usage.rst |  46 +++++++-
 drivers/gpu/drm/tests/drm_rect_test.c   |  36 +++++-
 include/kunit/test-bug.h                |  26 +++++
 include/kunit/test.h                    |  98 ++++++++++++++++
 kernel/panic.c                          |  11 ++
 lib/bug.c                               |  12 +-
 lib/kunit/Makefile                      |   4 +-
 lib/kunit/backtrace-suppression-test.c  | 192 ++++++++++++++++++++++++++++++++
 lib/kunit/bug.c                         | 131 ++++++++++++++++++++++
 lib/kunit/hooks-impl.h                  |   2 +
 10 files changed, 548 insertions(+), 10 deletions(-)
---
base-commit: 74fe02ce122a6103f207d29fafc8b3a53de6abaf
change-id: 20260312-kunit_add_support-2f35806b19dd

Best regards,
-- 
Albert Esteve <aesteve@redhat.com>

[PATCH v12 1/4] bug/kunit: Core support for suppressing warning backtraces

[Albert Esteve]

From: Alessandro Carminati <acarmina@redhat.com>

Some unit tests intentionally trigger warning backtraces by passing bad
parameters to kernel API functions. Such unit tests typically check the
return value from such calls, not the existence of the warning backtrace.

Such intentionally generated warning backtraces are neither desirable
nor useful for a number of reasons:
- They can result in overlooked real problems.
- A warning that suddenly starts to show up in unit tests needs to be
  investigated and has to be marked to be ignored, for example by
  adjusting filter scripts. Such filters are ad hoc because there is
  no real standard format for warnings. On top of that, such filter
  scripts would require constant maintenance.

Solve the problem by providing a means to suppress warning backtraces
originating from the current kthread while executing test code. Since
each KUnit test runs in its own kthread, this effectively scopes
suppression to the test that enabled it. Limit changes to generic code
to the absolute minimum.

Implementation details:
Suppression is integrated into the existing KUnit hooks infrastructure
in test-bug.h, reusing the kunit_running static branch for zero
overhead when no tests are running.

Suppression is checked at three points in the warning path:
- In warn_slowpath_fmt(), the check runs before any output, fully
  suppressing both message and backtrace. This covers architectures
  without __WARN_FLAGS.
- In __warn_printk(), the check suppresses the warning message text.
  This covers architectures that define __WARN_FLAGS but not their own
  __WARN_printf (arm64, loongarch, parisc, powerpc, riscv, sh), where
  the message is printed before the trap enters __report_bug().
- In __report_bug(), the check runs before __warn() is called,
  suppressing the backtrace and stack dump.

To avoid double-counting on architectures where both __warn_printk()
and __report_bug() run for the same warning, kunit_is_suppressed_warning()
takes a bool parameter: true to increment the suppression counter
(used in warn_slowpath_fmt and __report_bug), false to check only
(used in __warn_printk).

The suppression state is dynamically allocated via kzalloc() and its
lifetime is managed via kunit_add_action(). On cleanup, the node is
removed with list_del_rcu() and the object is freed asynchronously via
call_rcu(), ensuring that any concurrent RCU reader that obtained a
pointer to the handle before removal can finish safely. Writer-side
access to the global suppression list is serialized with a spinlock;
readers use RCU.

Two API forms are provided:
- kunit_warning_suppress(test) { ... }: scoped, uses __cleanup for
  automatic teardown on scope exit, kunit_add_action() as safety net
  for abnormal exits (e.g. kthread_exit from failed assertions).
  Suppression handle is only accessible inside the block.
- kunit_start/end_suppress_warning(test): direct functions returning
  an explicit handle, for retaining the handle within the test,
  or for cross-function usage.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Alessandro Carminati <acarmina@redhat.com>
Reviewed-by: Kees Cook <kees@kernel.org>
Reviewed-by: David Gow <david@davidgow.net>
Signed-off-by: Albert Esteve <aesteve@redhat.com>
---
 include/kunit/test-bug.h |  26 ++++++++++
 include/kunit/test.h     |  98 +++++++++++++++++++++++++++++++++++
 kernel/panic.c           |  11 ++++
 lib/bug.c                |  12 ++++-
 lib/kunit/Makefile       |   3 +-
 lib/kunit/bug.c          | 131 +++++++++++++++++++++++++++++++++++++++++++++++
 lib/kunit/hooks-impl.h   |   2 +
 7 files changed, 280 insertions(+), 3 deletions(-)

diff --git a/include/kunit/test-bug.h b/include/kunit/test-bug.h
index 47aa8f21ccce8..99869029fc686 100644
--- a/include/kunit/test-bug.h
+++ b/include/kunit/test-bug.h
@@ -10,6 +10,7 @@
 #define _KUNIT_TEST_BUG_H
 
 #include <linux/stddef.h> /* for NULL */
+#include <linux/types.h>  /* for bool */
 
 #if IS_ENABLED(CONFIG_KUNIT)
 
@@ -23,6 +24,7 @@ DECLARE_STATIC_KEY_FALSE(kunit_running);
 extern struct kunit_hooks_table {
 	__printf(3, 4) void (*fail_current_test)(const char*, int, const char*, ...);
 	void *(*get_static_stub_address)(struct kunit *test, void *real_fn_addr);
+	bool (*is_suppressed_warning)(bool count);
 } kunit_hooks;
 
 /**
@@ -60,9 +62,33 @@ static inline struct kunit *kunit_get_current_test(void)
 		}								\
 	} while (0)
 
+/**
+ * kunit_is_suppressed_warning() - Check if warnings are being suppressed
+ *                                 by the current KUnit test.
+ * @count: if true, increment the suppression counter on match.
+ *
+ * Returns true if the current task has active warning suppression.
+ * Uses the kunit_running static branch for zero overhead when no tests run.
+ *
+ * A single WARN*() may traverse multiple call sites in the warning path
+ * (e.g., __warn_printk() and __report_bug()). Pass @count = true at the
+ * primary suppression point to count each warning exactly once, and
+ * @count = false at secondary points to suppress output without
+ * inflating the count.
+ */
+static inline bool kunit_is_suppressed_warning(bool count)
+{
+	if (!static_branch_unlikely(&kunit_running))
+		return false;
+
+	return kunit_hooks.is_suppressed_warning &&
+	       kunit_hooks.is_suppressed_warning(count);
+}
+
 #else
 
 static inline struct kunit *kunit_get_current_test(void) { return NULL; }
+static inline bool kunit_is_suppressed_warning(bool count) { return false; }
 
 #define kunit_fail_current_test(fmt, ...) do {} while (0)
 
diff --git a/include/kunit/test.h b/include/kunit/test.h
index 9cd1594ab697d..be71612f61655 100644
--- a/include/kunit/test.h
+++ b/include/kunit/test.h
@@ -1795,4 +1795,102 @@ do {									       \
 // include resource.h themselves if they need it.
 #include <kunit/resource.h>
 
+/*
+ * Warning backtrace suppression API.
+ *
+ * Suppresses WARN*() backtraces on the current task while active. Two forms
+ * are provided:
+ *
+ * - Scoped: kunit_warning_suppress(test) { ... }
+ *   Suppression is active for the duration of the block. On normal exit,
+ *   the for-loop increment deactivates suppression. On early exit (break,
+ *   return, goto), the __cleanup attribute fires. On kthread_exit() (e.g.,
+ *   a failed KUnit assertion), kunit_add_action() cleans up at test
+ *   teardown. The suppression handle is only accessible inside the block,
+ *   so warning counts must be checked before the block exits.
+ *
+ * - Direct: kunit_start_suppress_warning() / kunit_end_suppress_warning()
+ *   The underlying functions, returning an explicit handle pointer. Use
+ *   when the handle needs to be retained (e.g., for post-suppression
+ *   count checks) or passed across helper functions.
+ */
+struct kunit_suppressed_warning;
+
+struct kunit_suppressed_warning *
+kunit_start_suppress_warning(struct kunit *test);
+void kunit_end_suppress_warning(struct kunit *test,
+				struct kunit_suppressed_warning *w);
+int kunit_suppressed_warning_count(struct kunit_suppressed_warning *w);
+void __kunit_suppress_auto_cleanup(struct kunit_suppressed_warning **wp);
+bool kunit_has_active_suppress_warning(void);
+
+/**
+ * kunit_warning_suppress() - Suppress WARN*() backtraces for the duration
+ *                            of a block.
+ * @test: The test context object.
+ *
+ * Scoped form of the suppression API. Suppression starts when the block is
+ * entered and ends automatically when the block exits through any path. See
+ * the section comment above for the cleanup guarantees on each exit path.
+ * Fails the test if suppression is already active; nesting is not supported.
+ *
+ * The warning count can be checked inside the block via
+ * KUNIT_EXPECT_SUPPRESSED_WARNING_COUNT(). The handle is not accessible
+ * after the block exits.
+ *
+ * Example::
+ *
+ *   kunit_warning_suppress(test) {
+ *       trigger_warning();
+ *       KUNIT_EXPECT_SUPPRESSED_WARNING_COUNT(test, 1);
+ *   }
+ */
+#define kunit_warning_suppress(test)					\
+	for (struct kunit_suppressed_warning *__kunit_suppress		\
+	     __cleanup(__kunit_suppress_auto_cleanup) =			\
+	     kunit_start_suppress_warning(test);			\
+	     __kunit_suppress;						\
+	     kunit_end_suppress_warning(test, __kunit_suppress),	\
+	     __kunit_suppress = NULL)
+
+/**
+ * KUNIT_SUPPRESSED_WARNING_COUNT() - Returns the suppressed warning count.
+ *
+ * Returns the number of WARN*() calls suppressed since the current
+ * suppression block started, or 0 if the handle is NULL. Usable inside a
+ * kunit_warning_suppress() block.
+ */
+#define KUNIT_SUPPRESSED_WARNING_COUNT() \
+	kunit_suppressed_warning_count(__kunit_suppress)
+
+/**
+ * KUNIT_EXPECT_SUPPRESSED_WARNING_COUNT() - Sets an expectation that the
+ *                                           suppressed warning count equals
+ *                                           @expected.
+ * @test: The test context object.
+ * @expected: an expression that evaluates to the expected warning count.
+ *
+ * Sets an expectation that the number of suppressed WARN*() calls equals
+ * @expected. This is semantically equivalent to
+ * KUNIT_EXPECT_EQ(@test, KUNIT_SUPPRESSED_WARNING_COUNT(), @expected).
+ * See KUNIT_EXPECT_EQ() for more information.
+ */
+#define KUNIT_EXPECT_SUPPRESSED_WARNING_COUNT(test, expected) \
+	KUNIT_EXPECT_EQ(test, KUNIT_SUPPRESSED_WARNING_COUNT(), expected)
+
+/**
+ * KUNIT_ASSERT_SUPPRESSED_WARNING_COUNT() - Sets an assertion that the
+ *                                           suppressed warning count equals
+ *                                           @expected.
+ * @test: The test context object.
+ * @expected: an expression that evaluates to the expected warning count.
+ *
+ * Sets an assertion that the number of suppressed WARN*() calls equals
+ * @expected. This is the same as KUNIT_EXPECT_SUPPRESSED_WARNING_COUNT(),
+ * except it causes an assertion failure (see KUNIT_ASSERT_TRUE()) when the
+ * assertion is not met.
+ */
+#define KUNIT_ASSERT_SUPPRESSED_WARNING_COUNT(test, expected) \
+	KUNIT_ASSERT_EQ(test, KUNIT_SUPPRESSED_WARNING_COUNT(), expected)
+
 #endif /* _KUNIT_TEST_H */
diff --git a/kernel/panic.c b/kernel/panic.c
index 20feada5319d4..213725b612aa1 100644
--- a/kernel/panic.c
+++ b/kernel/panic.c
@@ -39,6 +39,7 @@
 #include <linux/sys_info.h>
 #include <trace/events/error_report.h>
 #include <asm/sections.h>
+#include <kunit/test-bug.h>
 
 #define PANIC_TIMER_STEP 100
 #define PANIC_BLINK_SPD 18
@@ -1124,6 +1125,11 @@ void warn_slowpath_fmt(const char *file, int line, unsigned taint,
 	bool rcu = warn_rcu_enter();
 	struct warn_args args;
 
+	if (kunit_is_suppressed_warning(true)) {
+		warn_rcu_exit(rcu);
+		return;
+	}
+
 	pr_warn(CUT_HERE);
 
 	if (!fmt) {
@@ -1146,6 +1152,11 @@ void __warn_printk(const char *fmt, ...)
 	bool rcu = warn_rcu_enter();
 	va_list args;
 
+	if (kunit_is_suppressed_warning(false)) {
+		warn_rcu_exit(rcu);
+		return;
+	}
+
 	pr_warn(CUT_HERE);
 
 	va_start(args, fmt);
diff --git a/lib/bug.c b/lib/bug.c
index 224f4cfa4aa31..874cb4ae4d047 100644
--- a/lib/bug.c
+++ b/lib/bug.c
@@ -48,6 +48,7 @@
 #include <linux/rculist.h>
 #include <linux/ftrace.h>
 #include <linux/context_tracking.h>
+#include <kunit/test-bug.h>
 
 extern struct bug_entry __start___bug_table[], __stop___bug_table[];
 
@@ -209,8 +210,6 @@ static enum bug_trap_type __report_bug(struct bug_entry *bug, unsigned long buga
 			return BUG_TRAP_TYPE_NONE;
 	}
 
-	disable_trace_on_warning();
-
 	bug_get_file_line(bug, &file, &line);
 	fmt = bug_get_format(bug);
 
@@ -220,6 +219,15 @@ static enum bug_trap_type __report_bug(struct bug_entry *bug, unsigned long buga
 	no_cut   = bug->flags & BUGFLAG_NO_CUT_HERE;
 	has_args = bug->flags & BUGFLAG_ARGS;
 
+	/*
+	 * Before the once logic so suppressed warnings do not consume
+	 * the single-fire budget of WARN_ON_ONCE().
+	 */
+	if (warning && kunit_is_suppressed_warning(true))
+		return BUG_TRAP_TYPE_WARN;
+
+	disable_trace_on_warning();
+
 	if (warning && once) {
 		if (done)
 			return BUG_TRAP_TYPE_WARN;
diff --git a/lib/kunit/Makefile b/lib/kunit/Makefile
index 656f1fa35abcc..4592f9d0aa8dd 100644
--- a/lib/kunit/Makefile
+++ b/lib/kunit/Makefile
@@ -10,7 +10,8 @@ kunit-objs +=				test.o \
 					executor.o \
 					attributes.o \
 					device.o \
-					platform.o
+					platform.o \
+					bug.o
 
 ifeq ($(CONFIG_KUNIT_DEBUGFS),y)
 kunit-objs +=				debugfs.o
diff --git a/lib/kunit/bug.c b/lib/kunit/bug.c
new file mode 100644
index 0000000000000..8a5725aebcaa4
--- /dev/null
+++ b/lib/kunit/bug.c
@@ -0,0 +1,131 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * KUnit helpers for backtrace suppression
+ *
+ * Copyright (C) 2025 Alessandro Carminati <acarmina@redhat.com>
+ * Copyright (C) 2024 Guenter Roeck <linux@roeck-us.net>
+ */
+
+#include <kunit/resource.h>
+#include <linux/export.h>
+#include <linux/rculist.h>
+#include <linux/sched.h>
+#include <linux/sched/task.h>
+#include <linux/slab.h>
+#include <linux/spinlock.h>
+
+#include "hooks-impl.h"
+
+struct kunit_suppressed_warning {
+	struct list_head node;
+	struct rcu_head rcu;
+	struct task_struct *task;
+	struct kunit *test;
+	atomic_t counter;
+};
+
+static LIST_HEAD(suppressed_warnings);
+static DEFINE_SPINLOCK(suppressed_warnings_lock);
+
+static void kunit_suppress_warning_free(struct rcu_head *rcu)
+{
+	struct kunit_suppressed_warning *w =
+		container_of(rcu, struct kunit_suppressed_warning, rcu);
+
+	put_task_struct(w->task);
+	kfree(w);
+}
+
+static void kunit_suppress_warning_remove(struct kunit_suppressed_warning *w)
+{
+	unsigned long flags;
+
+	spin_lock_irqsave(&suppressed_warnings_lock, flags);
+	list_del_rcu(&w->node);
+	spin_unlock_irqrestore(&suppressed_warnings_lock, flags);
+	call_rcu(&w->rcu, kunit_suppress_warning_free);
+}
+
+KUNIT_DEFINE_ACTION_WRAPPER(kunit_suppress_warning_cleanup,
+			    kunit_suppress_warning_remove,
+			    struct kunit_suppressed_warning *);
+
+bool kunit_has_active_suppress_warning(void)
+{
+	return __kunit_is_suppressed_warning_impl(false);
+}
+EXPORT_SYMBOL_GPL(kunit_has_active_suppress_warning);
+
+struct kunit_suppressed_warning *
+kunit_start_suppress_warning(struct kunit *test)
+{
+	struct kunit_suppressed_warning *w;
+	unsigned long flags;
+	int ret;
+
+	if (kunit_has_active_suppress_warning()) {
+		KUNIT_FAIL(test, "Another suppression block is already active");
+		return NULL;
+	}
+
+	w = kzalloc(sizeof(*w), GFP_KERNEL);
+	if (!w) {
+		KUNIT_FAIL(test, "Failed to allocate suppression handle.");
+		return NULL;
+	}
+
+	w->task = get_task_struct(current);
+	w->test = test;
+
+	spin_lock_irqsave(&suppressed_warnings_lock, flags);
+	list_add_rcu(&w->node, &suppressed_warnings);
+	spin_unlock_irqrestore(&suppressed_warnings_lock, flags);
+
+	ret = kunit_add_action_or_reset(test,
+					kunit_suppress_warning_cleanup, w);
+	if (ret) {
+		KUNIT_FAIL(test, "Failed to add suppression cleanup action.");
+		return NULL;
+	}
+
+	return w;
+}
+EXPORT_SYMBOL_GPL(kunit_start_suppress_warning);
+
+void kunit_end_suppress_warning(struct kunit *test,
+				struct kunit_suppressed_warning *w)
+{
+	if (!w)
+		return;
+	kunit_release_action(test, kunit_suppress_warning_cleanup, w);
+}
+EXPORT_SYMBOL_GPL(kunit_end_suppress_warning);
+
+void __kunit_suppress_auto_cleanup(struct kunit_suppressed_warning **wp)
+{
+	if (*wp)
+		kunit_end_suppress_warning((*wp)->test, *wp);
+}
+EXPORT_SYMBOL_GPL(__kunit_suppress_auto_cleanup);
+
+int kunit_suppressed_warning_count(struct kunit_suppressed_warning *w)
+{
+	return w ? atomic_read(&w->counter) : 0;
+}
+EXPORT_SYMBOL_GPL(kunit_suppressed_warning_count);
+
+bool __kunit_is_suppressed_warning_impl(bool count)
+{
+	struct kunit_suppressed_warning *w;
+
+	guard(rcu)();
+	list_for_each_entry_rcu(w, &suppressed_warnings, node) {
+		if (w->task == current) {
+			if (count)
+				atomic_inc(&w->counter);
+			return true;
+		}
+	}
+
+	return false;
+}
diff --git a/lib/kunit/hooks-impl.h b/lib/kunit/hooks-impl.h
index 4e71b2d0143ba..d8720f2616925 100644
--- a/lib/kunit/hooks-impl.h
+++ b/lib/kunit/hooks-impl.h
@@ -19,6 +19,7 @@ void __printf(3, 4) __kunit_fail_current_test_impl(const char *file,
 						   int line,
 						   const char *fmt, ...);
 void *__kunit_get_static_stub_address_impl(struct kunit *test, void *real_fn_addr);
+bool __kunit_is_suppressed_warning_impl(bool count);
 
 /* Code to set all of the function pointers. */
 static inline void kunit_install_hooks(void)
@@ -26,6 +27,7 @@ static inline void kunit_install_hooks(void)
 	/* Install the KUnit hook functions. */
 	kunit_hooks.fail_current_test = __kunit_fail_current_test_impl;
 	kunit_hooks.get_static_stub_address = __kunit_get_static_stub_address_impl;
+	kunit_hooks.is_suppressed_warning = __kunit_is_suppressed_warning_impl;
 }
 
 #endif /* _KUNIT_HOOKS_IMPL_H */

-- 
2.53.0

Claude review: bug/kunit: Core support for suppressing warning backtraces

[Claude Code Review Bot]

Patch Review

**Design and correctness look good.** The hook integration is clean and follows the existing pattern exactly.

**1. Suppression check placement in `__report_bug()` is well-placed:**

```c
+	/*
+	 * Before the once logic so suppressed warnings do not consume
+	 * the single-fire budget of WARN_ON_ONCE().
+	 */
+	if (warning && kunit_is_suppressed_warning(true))
+		return BUG_TRAP_TYPE_WARN;
+
+	disable_trace_on_warning();
```

Good decision to return before both the `once` flag consumption and `disable_trace_on_warning()`. A suppressed warning shouldn't eat the `WARN_ON_ONCE()` budget or disable tracing.

**2. `kunit_start_suppress_warning` uses plain `kzalloc` (not `kunit_kzalloc`):**

```c
+	w = kzalloc(sizeof(*w), GFP_KERNEL);
```

This is correct since the lifetime is manually managed via `call_rcu` -> `kfree`, but it's worth noting the cover letter says "dynamically allocated via kunit_kzalloc()" which is inaccurate — only the overall cover letter, not the patch 1 commit message which correctly says `kzalloc()`.

**3. The `kunit_is_suppressed_warning` inline checks the function pointer before calling:**

```c
+	return kunit_hooks.is_suppressed_warning &&
+	       kunit_hooks.is_suppressed_warning(count);
```

This NULL check is correct — the hook may not be installed if the kunit module hasn't loaded yet but `kunit_running` was somehow set. Defensive and appropriate.

**4. `kunit_end_suppress_warning` silently returns on NULL:**

```c
+void kunit_end_suppress_warning(struct kunit *test,
+				struct kunit_suppressed_warning *w)
+{
+	if (!w)
+		return;
+	kunit_release_action(test, kunit_suppress_warning_cleanup, w);
+}
```

This makes the direct API forgiving when `kunit_start_suppress_warning` returned NULL on failure. That's the right choice since `kunit_start_suppress_warning` already calls `KUNIT_FAIL`.

**5. The `__kunit_suppress_auto_cleanup` accesses `(*wp)->test`:**

```c
+void __kunit_suppress_auto_cleanup(struct kunit_suppressed_warning **wp)
+{
+	if (*wp)
+		kunit_end_suppress_warning((*wp)->test, *wp);
+}
```

This is fine — the `__cleanup` attribute fires on scope exit, before the action teardown, so the handle is still valid at this point.

**6. Minor: `__kunit_is_suppressed_warning_impl` is not exported:**

```c
+bool __kunit_is_suppressed_warning_impl(bool count)
+{
```

No `EXPORT_SYMBOL_GPL`. This is correct since it's only called indirectly via the hook function pointer, and `kunit_has_active_suppress_warning()` (which calls it) IS exported. But `kunit_has_active_suppress_warning` is defined in `bug.c` and calls `__kunit_is_suppressed_warning_impl` directly, so they're in the same compilation unit — no export needed.

---
Generated by Claude Code Patch Reviewer

[PATCH v12 2/4] kunit: Add backtrace suppression self-tests

[Albert Esteve]

From: Guenter Roeck <linux@roeck-us.net>

Add unit tests to verify that warning backtrace suppression works.

Tests cover both API forms:
- Scoped: kunit_warning_suppress() with in-block count verification
  and post-block inactivity check.
- Direct functions: kunit_start/end_suppress_warning() with
  sequential independent suppression blocks and per-block counts.

Furthermore, tests verify incremental warning counting, that
kunit_has_active_suppress_warning() transitions correctly around
suppression boundaries, and that suppression active in the test
kthread does not leak to a separate kthread.

If backtrace suppression does _not_ work, the unit tests will likely
trigger unsuppressed backtraces, which should actually help to get
the affected architectures / platforms fixed.

Tested-by: Linux Kernel Functional Testing <lkft@linaro.org>
Acked-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Alessandro Carminati <acarmina@redhat.com>
Reviewed-by: David Gow <david@davidgow.net>
Signed-off-by: Albert Esteve <aesteve@redhat.com>
---
 lib/kunit/Makefile                     |   1 +
 lib/kunit/backtrace-suppression-test.c | 192 +++++++++++++++++++++++++++++++++
 2 files changed, 193 insertions(+)

diff --git a/lib/kunit/Makefile b/lib/kunit/Makefile
index 4592f9d0aa8dd..2e8a6b71a2ab0 100644
--- a/lib/kunit/Makefile
+++ b/lib/kunit/Makefile
@@ -22,6 +22,7 @@ obj-$(if $(CONFIG_KUNIT),y) +=		hooks.o
 
 obj-$(CONFIG_KUNIT_TEST) +=		kunit-test.o
 obj-$(CONFIG_KUNIT_TEST) +=		platform-test.o
+obj-$(CONFIG_KUNIT_TEST) +=		backtrace-suppression-test.o
 
 # string-stream-test compiles built-in only.
 ifeq ($(CONFIG_KUNIT_TEST),y)
diff --git a/lib/kunit/backtrace-suppression-test.c b/lib/kunit/backtrace-suppression-test.c
new file mode 100644
index 0000000000000..59a038b2739f5
--- /dev/null
+++ b/lib/kunit/backtrace-suppression-test.c
@@ -0,0 +1,192 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * KUnit test for suppressing warning tracebacks.
+ *
+ * Copyright (C) 2024, Guenter Roeck
+ * Author: Guenter Roeck <linux@roeck-us.net>
+ */
+
+#include <kunit/test.h>
+#include <linux/bug.h>
+#include <linux/completion.h>
+#include <linux/kthread.h>
+
+static void backtrace_suppression_test_warn_direct(struct kunit *test)
+{
+	if (!IS_ENABLED(CONFIG_BUG))
+		kunit_skip(test, "requires CONFIG_BUG");
+
+	kunit_warning_suppress(test) {
+		WARN(1, "This backtrace should be suppressed");
+		/*
+		 * Count must be checked inside the scope; the handle
+		 * is not accessible after the block exits.
+		 */
+		KUNIT_EXPECT_SUPPRESSED_WARNING_COUNT(test, 1);
+	}
+	KUNIT_EXPECT_FALSE(test, kunit_has_active_suppress_warning());
+}
+
+static noinline void trigger_backtrace_warn(void)
+{
+	WARN(1, "This backtrace should be suppressed");
+}
+
+static void backtrace_suppression_test_warn_indirect(struct kunit *test)
+{
+	if (!IS_ENABLED(CONFIG_BUG))
+		kunit_skip(test, "requires CONFIG_BUG");
+
+	kunit_warning_suppress(test) {
+		trigger_backtrace_warn();
+		KUNIT_EXPECT_SUPPRESSED_WARNING_COUNT(test, 1);
+	}
+}
+
+static void backtrace_suppression_test_warn_multi(struct kunit *test)
+{
+	if (!IS_ENABLED(CONFIG_BUG))
+		kunit_skip(test, "requires CONFIG_BUG");
+
+	kunit_warning_suppress(test) {
+		WARN(1, "This backtrace should be suppressed");
+		trigger_backtrace_warn();
+		KUNIT_EXPECT_SUPPRESSED_WARNING_COUNT(test, 2);
+	}
+}
+
+static void backtrace_suppression_test_warn_on_direct(struct kunit *test)
+{
+	if (!IS_ENABLED(CONFIG_BUG))
+		kunit_skip(test, "requires CONFIG_BUG");
+
+	kunit_warning_suppress(test) {
+		WARN_ON(1);
+		KUNIT_EXPECT_SUPPRESSED_WARNING_COUNT(test, 1);
+	}
+}
+
+static noinline void trigger_backtrace_warn_on(void)
+{
+	WARN_ON(1);
+}
+
+static void backtrace_suppression_test_warn_on_indirect(struct kunit *test)
+{
+	if (!IS_ENABLED(CONFIG_BUG))
+		kunit_skip(test, "requires CONFIG_BUG");
+
+	kunit_warning_suppress(test) {
+		trigger_backtrace_warn_on();
+		KUNIT_EXPECT_SUPPRESSED_WARNING_COUNT(test, 1);
+	}
+}
+
+static void backtrace_suppression_test_count(struct kunit *test)
+{
+	if (!IS_ENABLED(CONFIG_BUG))
+		kunit_skip(test, "requires CONFIG_BUG");
+
+	kunit_warning_suppress(test) {
+		KUNIT_EXPECT_SUPPRESSED_WARNING_COUNT(test, 0);
+
+		WARN(1, "suppressed");
+		KUNIT_EXPECT_SUPPRESSED_WARNING_COUNT(test, 1);
+
+		WARN(1, "suppressed again");
+		KUNIT_EXPECT_SUPPRESSED_WARNING_COUNT(test, 2);
+	}
+}
+
+static void backtrace_suppression_test_active_state(struct kunit *test)
+{
+	KUNIT_EXPECT_FALSE(test, kunit_has_active_suppress_warning());
+
+	kunit_warning_suppress(test) {
+		KUNIT_EXPECT_TRUE(test, kunit_has_active_suppress_warning());
+	}
+
+	KUNIT_EXPECT_FALSE(test, kunit_has_active_suppress_warning());
+
+	kunit_warning_suppress(test) {
+		KUNIT_EXPECT_TRUE(test, kunit_has_active_suppress_warning());
+	}
+
+	KUNIT_EXPECT_FALSE(test, kunit_has_active_suppress_warning());
+}
+
+static void backtrace_suppression_test_multi_scope(struct kunit *test)
+{
+	struct kunit_suppressed_warning *sw1, *sw2;
+
+	if (!IS_ENABLED(CONFIG_BUG))
+		kunit_skip(test, "requires CONFIG_BUG");
+
+	sw1 = kunit_start_suppress_warning(test);
+	trigger_backtrace_warn_on();
+	WARN(1, "suppressed by sw1");
+	kunit_end_suppress_warning(test, sw1);
+
+	sw2 = kunit_start_suppress_warning(test);
+	WARN(1, "suppressed by sw2");
+	kunit_end_suppress_warning(test, sw2);
+
+	KUNIT_EXPECT_EQ(test, kunit_suppressed_warning_count(sw1), 2);
+	KUNIT_EXPECT_EQ(test, kunit_suppressed_warning_count(sw2), 1);
+}
+
+struct cross_kthread_data {
+	bool was_active;
+	struct completion done;
+};
+
+static int cross_kthread_fn(void *data)
+{
+	struct cross_kthread_data *d = data;
+
+	d->was_active = kunit_has_active_suppress_warning();
+	complete(&d->done);
+	while (!kthread_should_stop())
+		schedule();
+	return 0;
+}
+
+static void backtrace_suppression_test_cross_kthread(struct kunit *test)
+{
+	struct cross_kthread_data data;
+	struct task_struct *task;
+
+	data.was_active = false;
+	init_completion(&data.done);
+
+	kunit_warning_suppress(test) {
+		task = kthread_run(cross_kthread_fn, &data, "kunit-cross-test");
+		KUNIT_ASSERT_FALSE(test, IS_ERR(task));
+		wait_for_completion(&data.done);
+		kthread_stop(task);
+	}
+
+	KUNIT_EXPECT_FALSE(test, data.was_active);
+}
+
+static struct kunit_case backtrace_suppression_test_cases[] = {
+	KUNIT_CASE(backtrace_suppression_test_warn_direct),
+	KUNIT_CASE(backtrace_suppression_test_warn_indirect),
+	KUNIT_CASE(backtrace_suppression_test_warn_multi),
+	KUNIT_CASE(backtrace_suppression_test_warn_on_direct),
+	KUNIT_CASE(backtrace_suppression_test_warn_on_indirect),
+	KUNIT_CASE(backtrace_suppression_test_count),
+	KUNIT_CASE(backtrace_suppression_test_active_state),
+	KUNIT_CASE(backtrace_suppression_test_multi_scope),
+	KUNIT_CASE(backtrace_suppression_test_cross_kthread),
+	{}
+};
+
+static struct kunit_suite backtrace_suppression_test_suite = {
+	.name = "backtrace-suppression-test",
+	.test_cases = backtrace_suppression_test_cases,
+};
+kunit_test_suites(&backtrace_suppression_test_suite);
+
+MODULE_LICENSE("GPL");
+MODULE_DESCRIPTION("KUnit test to verify warning backtrace suppression");

-- 
2.53.0

Claude review: kunit: Add backtrace suppression self-tests

[Claude Code Review Bot]

Patch Review

Good test coverage. Tests both API forms, incremental counting, active-state transitions, and cross-kthread isolation.

**1. `backtrace_suppression_test_multi_scope` accesses handles after cleanup:**

```c
+	sw1 = kunit_start_suppress_warning(test);
+	trigger_backtrace_warn_on();
+	WARN(1, "suppressed by sw1");
+	kunit_end_suppress_warning(test, sw1);
+
+	sw2 = kunit_start_suppress_warning(test);
+	WARN(1, "suppressed by sw2");
+	kunit_end_suppress_warning(test, sw2);
+
+	KUNIT_EXPECT_EQ(test, kunit_suppressed_warning_count(sw1), 2);
+	KUNIT_EXPECT_EQ(test, kunit_suppressed_warning_count(sw2), 1);
```

After `kunit_end_suppress_warning(test, sw1)`, the cleanup path runs `kunit_suppress_warning_remove(sw1)` which calls `call_rcu(&w->rcu, kunit_suppress_warning_free)`. The `sw1` pointer remains valid because the RCU grace period cannot complete while the current task is still executing without scheduling — so this is safe *in practice*. However, it's a fragile pattern: if someone inserts a `schedule()`, `msleep()`, or even `cond_resched()` between the end calls and the count checks, it becomes a use-after-free. A brief comment here explaining the RCU grace period guarantee would help future maintainers. Alternatively, reading the count *before* calling `kunit_end_suppress_warning` would be more robust.

**2. `cross_kthread_fn` uses busy-loop pattern:**

```c
+	while (!kthread_should_stop())
+		schedule();
```

This was noted in v10's changelog as fixing a use-after-free on kthread exit. The pattern is correct — the thread waits until `kthread_stop()` is called, preventing the stack-allocated `data` from going out of scope before the thread accesses it.

**3. Good use of `noinline` on helper trigger functions:**

```c
+static noinline void trigger_backtrace_warn(void)
+static noinline void trigger_backtrace_warn_on(void)
```

Prevents the compiler from inlining these into the suppression scope, ensuring the warning actually goes through the full WARN path. Good.

---
Generated by Claude Code Patch Reviewer

[PATCH v12 3/4] drm: Suppress intentional warning backtraces in scaling unit tests

[Albert Esteve]

From: Guenter Roeck <linux@roeck-us.net>

The drm_test_rect_calc_hscale and drm_test_rect_calc_vscale unit tests
intentionally trigger warning backtraces by providing invalid parameters
the tested functions. Suppress the backtraces to avoid clogging the
kernel log and distracting from real problems.

The suppression API also exposes a warning counter, which is used
to assert that the expected warning was actually triggered. On
CONFIG_BUG=n, WARN_ON() is a no-op and the counter stays zero;
the expected count is adjusted accordingly, preserving the return
value check on all configurations.

Tested-by: Linux Kernel Functional Testing <lkft@linaro.org>
Acked-by: Dan Carpenter <dan.carpenter@linaro.org>
Acked-by: Maíra Canal <mcanal@igalia.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: David Airlie <airlied@gmail.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Alessandro Carminati <acarmina@redhat.com>
Acked-by: David Gow <david@davidgow.net>
Signed-off-by: Albert Esteve <aesteve@redhat.com>
---
 drivers/gpu/drm/tests/drm_rect_test.c | 36 +++++++++++++++++++++++++++++------
 1 file changed, 30 insertions(+), 6 deletions(-)

diff --git a/drivers/gpu/drm/tests/drm_rect_test.c b/drivers/gpu/drm/tests/drm_rect_test.c
index 17e1f34b76101..5aa8ec5fc4d64 100644
--- a/drivers/gpu/drm/tests/drm_rect_test.c
+++ b/drivers/gpu/drm/tests/drm_rect_test.c
@@ -10,6 +10,7 @@
 #include <drm/drm_rect.h>
 #include <drm/drm_mode.h>
 
+#include <linux/limits.h>
 #include <linux/string_helpers.h>
 #include <linux/errno.h>
 
@@ -407,10 +408,22 @@ KUNIT_ARRAY_PARAM(drm_rect_scale, drm_rect_scale_cases, drm_rect_scale_case_desc
 static void drm_test_rect_calc_hscale(struct kunit *test)
 {
 	const struct drm_rect_scale_case *params = test->param_value;
-	int scaling_factor;
+	/* With CONFIG_BUG=n, WARN_ON() is a no-op so no warning fires. */
+	int expected_warnings = IS_ENABLED(CONFIG_BUG) ?
+			(params->expected_scaling_factor == -EINVAL) : 0;
+	int scaling_factor = INT_MIN;
 
-	scaling_factor = drm_rect_calc_hscale(&params->src, &params->dst,
-					      params->min_range, params->max_range);
+	/*
+	 * drm_rect_calc_hscale() generates a warning backtrace whenever bad
+	 * parameters are passed to it. This affects unit tests with -EINVAL
+	 * error code in expected_scaling_factor.
+	 */
+	kunit_warning_suppress(test) {
+		scaling_factor = drm_rect_calc_hscale(&params->src, &params->dst,
+						      params->min_range,
+						      params->max_range);
+		KUNIT_EXPECT_SUPPRESSED_WARNING_COUNT(test, expected_warnings);
+	}
 
 	KUNIT_EXPECT_EQ(test, scaling_factor, params->expected_scaling_factor);
 }
@@ -418,10 +431,21 @@ static void drm_test_rect_calc_hscale(struct kunit *test)
 static void drm_test_rect_calc_vscale(struct kunit *test)
 {
 	const struct drm_rect_scale_case *params = test->param_value;
-	int scaling_factor;
+	/* With CONFIG_BUG=n, WARN_ON() is a no-op so no warning fires. */
+	int expected_warnings = IS_ENABLED(CONFIG_BUG) ?
+			(params->expected_scaling_factor == -EINVAL) : 0;
+	int scaling_factor = INT_MIN;
 
-	scaling_factor = drm_rect_calc_vscale(&params->src, &params->dst,
-					      params->min_range, params->max_range);
+	/*
+	 * drm_rect_calc_vscale() generates a warning backtrace whenever bad
+	 * parameters are passed to it. This affects unit tests with -EINVAL
+	 * error code in expected_scaling_factor.
+	 */
+	kunit_warning_suppress(test) {
+		scaling_factor = drm_rect_calc_vscale(&params->src, &params->dst,
+						      params->min_range, params->max_range);
+		KUNIT_EXPECT_SUPPRESSED_WARNING_COUNT(test, expected_warnings);
+	}
 
 	KUNIT_EXPECT_EQ(test, scaling_factor, params->expected_scaling_factor);
 }

-- 
2.53.0

Claude review: drm: Suppress intentional warning backtraces in scaling unit tests

[Claude Code Review Bot]

Patch Review

Clean application of the scoped API.

**1. The `expected_warnings` calculation is clever:**

```c
+	int expected_warnings = IS_ENABLED(CONFIG_BUG) ?
+			(params->expected_scaling_factor == -EINVAL) : 0;
```

When `CONFIG_BUG=n`, `WARN_ON()` is a no-op, so no warnings fire and the count stays at 0. This handles the conditional cleanly without `#ifdef`. The expression `(params->expected_scaling_factor == -EINVAL)` evaluates to 0 or 1, matching whether the specific test case triggers a warning or not.

**2. `scaling_factor = INT_MIN` initialization:**

```c
+	int scaling_factor = INT_MIN;
```

Necessary because if `kunit_start_suppress_warning()` fails (returns NULL), the for-loop body doesn't execute, and `scaling_factor` would be uninitialized at the `KUNIT_EXPECT_EQ` check below. `INT_MIN` is a safe sentinel (no test case expects it). Good fix from v10.

**3. The `KUNIT_EXPECT_EQ` for `scaling_factor` is outside the suppression block:**

```c
+	kunit_warning_suppress(test) {
+		scaling_factor = drm_rect_calc_hscale(...);
+		KUNIT_EXPECT_SUPPRESSED_WARNING_COUNT(test, expected_warnings);
+	}
 	KUNIT_EXPECT_EQ(test, scaling_factor, params->expected_scaling_factor);
```

This is the right structure — the warning count must be inside the block, but the functional assertion can (and should) be outside it. Clean.

**4. Nit: the `#include <linux/limits.h>` addition for `INT_MIN`:**

```c
+#include <linux/limits.h>
```

Correct include for `INT_MIN`.

---
Generated by Claude Code Patch Reviewer

[PATCH v12 4/4] kunit: Add documentation for warning backtrace suppression API

[Albert Esteve]

From: Guenter Roeck <linux@roeck-us.net>

Document API functions for suppressing warning backtraces.

Tested-by: Linux Kernel Functional Testing <lkft@linaro.org>
Acked-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Alessandro Carminati <acarmina@redhat.com>
Reviewed-by: David Gow <david@davidgow.net>
Signed-off-by: Albert Esteve <aesteve@redhat.com>
---
 Documentation/dev-tools/kunit/usage.rst | 46 ++++++++++++++++++++++++++++++++-
 1 file changed, 45 insertions(+), 1 deletion(-)

diff --git a/Documentation/dev-tools/kunit/usage.rst b/Documentation/dev-tools/kunit/usage.rst
index ebd06f5ea4550..1c78dfff94e8a 100644
--- a/Documentation/dev-tools/kunit/usage.rst
+++ b/Documentation/dev-tools/kunit/usage.rst
@@ -157,6 +157,50 @@ Alternatively, one can take full control over the error message by using
 	if (some_setup_function())
 		KUNIT_FAIL(test, "Failed to setup thing for testing");
 
+Suppressing warning backtraces
+------------------------------
+
+Some unit tests trigger warning backtraces either intentionally or as a side
+effect. Such backtraces are normally undesirable since they distract from
+the actual test and may result in the impression that there is a problem.
+
+Backtraces can be suppressed with **task-scoped suppression**: while
+suppression is active on the current task, the backtrace and stack dump from
+``WARN*()``, ``WARN_ON*()``, and related macros on that task are suppressed.
+Two API forms are available.
+
+- Scoped suppression is the simplest form. Wrap the code that triggers
+  warnings in a ``kunit_warning_suppress()`` block:
+
+.. code-block:: c
+
+	static void some_test(struct kunit *test)
+	{
+		kunit_warning_suppress(test) {
+			trigger_backtrace();
+			KUNIT_EXPECT_SUPPRESSED_WARNING_COUNT(test, 1);
+		}
+	}
+
+.. note::
+   The warning count must be checked inside the block; the suppression handle
+   is not accessible after the block exits.
+
+- Direct functions return an explicit handle pointer. Use them when the handle
+  needs to be retained or passed across helper functions:
+
+.. code-block:: c
+
+	static void some_test(struct kunit *test)
+	{
+		struct kunit_suppressed_warning *w;
+
+		w = kunit_start_suppress_warning(test);
+		trigger_backtrace();
+		kunit_end_suppress_warning(test, w);
+
+		KUNIT_EXPECT_EQ(test, kunit_suppressed_warning_count(w), 1);
+	}
 
 Test Suites
 ~~~~~~~~~~~
@@ -1211,4 +1255,4 @@ For example:
 		dev_managed_string = devm_kstrdup(fake_device, "Hello, World!");
 
 		// Everything is cleaned up automatically when the test ends.
-	}
\ No newline at end of file
+	}

-- 
2.53.0

Claude review: kunit: Add documentation for warning backtrace suppression API

[Claude Code Review Bot]

Patch Review

Documentation is clear and covers both API forms with examples.

**1. Fixes a pre-existing missing newline at EOF:**

```c
-	}
\ No newline at end of file
+	}
```

Minor bonus fix.

**2. The documentation correctly notes the handle scope constraint:**

```
+.. note::
+   The warning count must be checked inside the block; the suppression handle
+   is not accessible after the block exits.
```

This is important for users of the scoped API and is well-called-out.

**3. The direct-function example shows count access after `kunit_end_suppress_warning`:**

```c
+		w = kunit_start_suppress_warning(test);
+		trigger_backtrace();
+		kunit_end_suppress_warning(test, w);
+
+		KUNIT_EXPECT_EQ(test, kunit_suppressed_warning_count(w), 1);
```

This mirrors the pattern in `backtrace_suppression_test_multi_scope`. As noted in my patch 2 review, this works because the RCU grace period hasn't completed, but it's a fragile pattern to document as canonical usage. Consider adding a note that the handle memory remains valid briefly after `kunit_end_suppress_warning()` due to RCU-deferred freeing, or restructure the example to read the count before ending suppression.

---
Generated by Claude Code Patch Reviewer

Re: [PATCH v12 0/4] kunit: Add support for suppressing warning backtraces

[Albert Esteve]

On Fri, May 15, 2026 at 10:53 AM Albert Esteve <aesteve@redhat.com> wrote:
>
> Some unit tests intentionally trigger warning backtraces by passing bad
> parameters to kernel API functions. Such unit tests typically check the
> return value from such calls, not the existence of the warning backtrace.
>
> Such intentionally generated warning backtraces are neither desirable
> nor useful for a number of reasons:
> - They can result in overlooked real problems.
> - A warning that suddenly starts to show up in unit tests needs to be
>   investigated and has to be marked to be ignored, for example by
>   adjusting filter scripts. Such filters are ad hoc because there is
>   no real standard format for warnings. On top of that, such filter
>   scripts would require constant maintenance.
>
> One option to address the problem would be to add messages such as
> "expected warning backtraces start/end here" to the kernel log.
> However, that would again require filter scripts, might result in
> missing real problematic warning backtraces triggered while the test
> is running, and the irrelevant backtrace(s) would still clog the
> kernel log.
>
> Solve the problem by providing a means to suppress warning backtraces
> originating from the current kthread while executing test code.
> Since each KUnit test runs in its own kthread, this effectively scopes
> suppression to the test that enabled it, without requiring any
> architecture-specific code.
>
> Overview:
> Patch#1 Introduces the suppression infrastructure integrated into
>         KUnit's hook mechanism.
> Patch#2 Adds selftests to validate the functionality.
> Patch#3 Demonstrates real-world usage in the DRM subsystem.
> Patch#4 Documents the new API and usage guidelines.
>
> Design Notes:
> Suppression is integrated into the existing KUnit hooks infrastructure,
> reusing the kunit_running static branch for zero overhead
> when no tests are running. The implementation lives entirely in the
> kunit module; only a static-inline wrapper and a function pointer
> slot are added to built-in code.
>
> Suppression is checked at three points in the warning path:
> - In `warn_slowpath_fmt()` (kernel/panic.c), for architectures without
>   __WARN_FLAGS. The check runs before any output, fully suppressing
>   both message and backtrace.
> - In `__warn_printk()` (kernel/panic.c), for architectures that define
>   __WARN_FLAGS but not their own __WARN_printf (arm64, loongarch,
>   parisc, powerpc, riscv, sh). The check suppresses the warning message
>   text that is printed before the trap enters __report_bug().
> - In `__report_bug()` (lib/bug.c), for architectures that define
>   __WARN_FLAGS. The check runs before `__warn()` is called, suppressing
>   the backtrace and stack dump.
>
> To avoid double-counting on architectures where both `__warn_printk()`
> and `__report_bug()` run for the same warning, the hook takes a bool
> parameter: true to increment the suppression counter, false to suppress
> without counting.
>
> The suppression state is dynamically allocated via kunit_kzalloc() and
> tied to the KUnit test lifecycle via `kunit_add_action()`, ensuring
> automatic cleanup at test exit. Writer-side access to the global
> suppression list is serialized with a spinlock; readers use RCU.
>
> Two API forms are provided:
> - kunit_warning_suppress(test) { ... }: scoped blocks with automatic
>   cleanup. The suppression handle is not accessible outside the block,
>   so warning counts (if needed) must be checked inside. Multiple
>   suppression blocks are allowed.
> - kunit_start/end_suppress_warning(test): direct functions that return
>   an explicit handle. Use when the handle needs to be retained, or passed
>   across helpers. Multiple suppression blocks are allowed.
>
> This series is based on the RFC patch and subsequent discussion at
> https://patchwork.kernel.org/project/linux-kselftest/patch/02546e59-1afe-4b08-ba81-d94f3b691c9a@moroto.mountain/
> and offers a more comprehensive solution of the problem discussed there.
>
> Changes since RFC:
> - Introduced CONFIG_KUNIT_SUPPRESS_BACKTRACE
> - Minor cleanups and bug fixes
> - Added support for all affected architectures
> - Added support for counting suppressed warnings
> - Added unit tests using those counters
> - Added patch to suppress warning backtraces in dev_addr_lists tests
>
> Changes since v1:
> - Rebased to v6.9-rc1
> - Added Tested-by:, Acked-by:, and Reviewed-by: tags
>   [I retained those tags since there have been no functional changes]
> - Introduced KUNIT_SUPPRESS_BACKTRACE configuration option, enabled by
>   default.
>
> Changes since v2:
> - Rebased to v6.9-rc2
> - Added comments to drm warning suppression explaining why it is needed.
> - Added patch to move conditional code in arch/sh/include/asm/bug.h
>   to avoid kerneldoc warning
> - Added architecture maintainers to Cc: for architecture specific patches
> - No functional changes
>
> Changes since v3:
> - Rebased to v6.14-rc6
> - Dropped net: "kunit: Suppress lock warning noise at end of dev_addr_lists tests"
>   since 3db3b62955cd6d73afde05a17d7e8e106695c3b9
> - Added __kunit_ and KUNIT_ prefixes.
> - Tested on interessed architectures.
>
> Changes since v4:
> - Rebased to v6.15-rc7
> - Dropped all code in __report_bug()
> - Moved all checks in WARN*() macros.
> - Dropped all architecture specific code.
> - Made __kunit_is_suppressed_warning nice to noinstr functions.
>
> Changes since v5:
> - Rebased to v7.0-rc3
> - Added RCU protection for the suppressed warnings list.
> - Added static key and branching optimization.
> - Removed custom `strcmp` implementation and reworked
>   __kunit_is_suppressed_warning() entrypoint function.
>
> Changes since v6:
> - Moved suppression checks from WARN*() macros to warn_slowpath_fmt()
>   and __report_bug().
> - Replaced stack-allocated suppression struct with kunit_kzalloc() heap
>   allocation tied to the KUnit test lifecycle.
> - Changed suppression strategy from function-name matching to task-scoped:
>   all warnings on the current task are suppressed between START and END,
>   rather than only warnings originating from a specific named function.
> - Simplified macro API: removed KUNIT_DECLARE_SUPPRESSED_WARNING(),
>   the START macro now takes (test) and handles allocation internally.
> - Removed static key and branching optiomization, as by the time it
>   was executed, callers are already in warn slowpaths.
> - Link to v6: https://lore.kernel.org/r/20260317-kunit_add_support-v6-0-dd22aeb3fe5d@redhat.com
>
> Changes since v7:
> - Integrated suppression into existing KUnit hooks infrastructure
> - Removed CONFIG_KUNIT_SUPPRESS_BACKTRACE
> - Added suppression check in __warn_printk()
> - Added spinlock for writer-side RCU protection
> - Replaced explicit rcu_read_lock/unlock with guard(rcu)()
> - Added scoped API (kunit_warning_suppress) using __cleanup attribute
> - Updated DRM patch to use scoped API
> - Expanded self-tests: incremental counting, cross-kthread isolation
> - Rewrote documentation covering all three API forms with examples
> - Link to v7: https://lore.kernel.org/r/20260420-kunit_add_support-v7-0-e8bc6e0f70de@redhat.com
>
> Changes since v8:
> - Rebased to v7.1-rc2
> - Remove KUNIT_START/END_SUPPRESSED_WARNING() macros
> - Add KUNIT_EXPECT_SUPPRESSED_WARNING_COUNT checks to drm tests
> - Link to v8: https://lore.kernel.org/r/20260504-kunit_add_support-v8-0-3e5957cdd235@redhat.com
>
> Changes since v9:
> - Fix silent false-pass when kunit_start_suppress_warning() returns NULL
> - Fix RCU lockdep splat for kunit_is_suppressed_warning() calls
> - Move disable_trace_on_warning() in __report_bug()
> - Make suppress counter atomic
> - Mark helper warn functions in selftest as noinline
> - Add kunit_skip() for CONFIG_BUG=n in selftests
> - Fix potentially uninitialized data.was_active in kthread seltest
> - Add kthread_stop() in kthread selftest early exit
> - Initialize scaling_factor to INT_MIN in DRM scaling tests
> - Add include for bool in test-bug.h to fix CONFIG_KUNIT=n case
> - Link to v9: https://lore.kernel.org/r/20260508-kunit_add_support-v9-0-99df7aa880f6@redhat.com
>
> Changes since v10:
> - Remove synchronize_rcu() to avoid sleeping in atomic context
> - Pin task_struct refcount to prevent ABA false-positive matches
> - Loop in suppression selftest to prevent use-after-free on kthread exit
> - Skip DRM rect tests on CONFIG_BUG=n
> - Link to v10: https://lore.kernel.org/r/20260513-kunit_add_support-v10-0-e379d206c8cd@redhat.com
>
> Changes since v11:
> - Use call_rcu() to defer free without blocking
> - Remove #ifdef CONFIG_KUNIT guard in lib/bug.c
> - Remove stale config checks from selftest
> - Replace skip on DRM rect tests with conditional expectation
> - Link to v11: https://lore.kernel.org/r/20260514-kunit_add_support-v11-0-b36a530a6d8f@redhat.com

A regression occurred in this version because `kunit_kzalloc` was
removed, following sashiko's suggestions in the last version. The
handle cannot be used after the kunit_end_suppress_warning() call
because we explicitly kfree(), resulting in a use-after-free. I
should've seen it, but the tests passed so it did not click (not sure
why exactly, probably the memory was still mapped during the grace
period so the access succeeded spuriously). This back and forth is not
going well, so I will revert lib/kunit/bug.c to v9, before removing
`synchronize_rcu()`, which was working fine. It used `kunit_kzalloc`
so KUnit correctly handled the memory lifetime. The problem is that
`synchronize_rcu()` requires process context, which is always the case
here, it should never be called under a spinlock or RCU read lock.
Re-adding may trigger a comment from the automatic review, but I think
it is the safest option.

This time sashiko found another issue and suggested `in_task()`, which
sounds like a nice addition to avoid suppressing real warnings within
interrupt handlers. So one more version it is...

>
> --
> 2.34.1
>
> ---
> To: Brendan Higgins <brendan.higgins@linux.dev>
> To: David Gow <david@davidgow.net>
> To: Rae Moar <raemoar63@gmail.com>
> To: Andrew Morton <akpm@linux-foundation.org>
> To: Paul Walmsley <pjw@kernel.org>
> To: Palmer Dabbelt <palmer@dabbelt.com>
> To: Albert Ou <aou@eecs.berkeley.edu>
> To: Alexandre Ghiti <alex@ghiti.fr>
> To: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> To: Maxime Ripard <mripard@kernel.org>
> To: Thomas Zimmermann <tzimmermann@suse.de>
> To: David Airlie <airlied@gmail.com>
> To: Simona Vetter <simona@ffwll.ch>
> To: Jonathan Corbet <corbet@lwn.net>
> To: Shuah Khan <skhan@linuxfoundation.org>
> Cc: linux-kernel@vger.kernel.org
> Cc: linux-kselftest@vger.kernel.org
> Cc: kunit-dev@googlegroups.com
> Cc: linux-riscv@lists.infradead.org
> Cc: dri-devel@lists.freedesktop.org
> Cc: workflows@vger.kernel.org
> Cc: linux-doc@vger.kernel.org
>
> ---
> Alessandro Carminati (1):
>       bug/kunit: Core support for suppressing warning backtraces
>
> Guenter Roeck (3):
>       kunit: Add backtrace suppression self-tests
>       drm: Suppress intentional warning backtraces in scaling unit tests
>       kunit: Add documentation for warning backtrace suppression API
>
>  Documentation/dev-tools/kunit/usage.rst |  46 +++++++-
>  drivers/gpu/drm/tests/drm_rect_test.c   |  36 +++++-
>  include/kunit/test-bug.h                |  26 +++++
>  include/kunit/test.h                    |  98 ++++++++++++++++
>  kernel/panic.c                          |  11 ++
>  lib/bug.c                               |  12 +-
>  lib/kunit/Makefile                      |   4 +-
>  lib/kunit/backtrace-suppression-test.c  | 192 ++++++++++++++++++++++++++++++++
>  lib/kunit/bug.c                         | 131 ++++++++++++++++++++++
>  lib/kunit/hooks-impl.h                  |   2 +
>  10 files changed, 548 insertions(+), 10 deletions(-)
> ---
> base-commit: 74fe02ce122a6103f207d29fafc8b3a53de6abaf
> change-id: 20260312-kunit_add_support-2f35806b19dd
>
> Best regards,
> --
> Albert Esteve <aesteve@redhat.com>
>

Claude review: kunit: Add support for suppressing warning backtraces

[Claude Code Review Bot]

Overall Series Review

Subject: kunit: Add support for suppressing warning backtraces
Author: Albert Esteve <aesteve@redhat.com>
Patches: 6
Reviewed: 2026-05-16T09:36:17.052981

---

This is a well-engineered v12 series adding KUnit warning backtrace suppression, with reviews from Kees Cook and David Gow. The design is solid: task-scoped suppression via the existing KUnit hooks mechanism, RCU for readers/spinlock for writers, `call_rcu()` for safe deferred freeing, and `get_task_struct()` to prevent ABA false positives on task matching. The three suppression injection points (`warn_slowpath_fmt`, `__warn_printk`, `__report_bug`) correctly cover all architecture variants with the `count` bool preventing double-counting.

The series is in good shape overall. I have one substantive concern about the multi-scope self-test accessing handles after their deferred free has been scheduled, plus a few minor observations.

---
Generated by Claude Code Patch Reviewer

[PATCH v13 0/4] kunit: Add support for suppressing warning backtraces

[Albert Esteve]

Some unit tests intentionally trigger warning backtraces by passing bad
parameters to kernel API functions. Such unit tests typically check the
return value from such calls, not the existence of the warning backtrace.

Such intentionally generated warning backtraces are neither desirable
nor useful for a number of reasons:
- They can result in overlooked real problems.
- A warning that suddenly starts to show up in unit tests needs to be
  investigated and has to be marked to be ignored, for example by
  adjusting filter scripts. Such filters are ad hoc because there is
  no real standard format for warnings. On top of that, such filter
  scripts would require constant maintenance.

One option to address the problem would be to add messages such as
"expected warning backtraces start/end here" to the kernel log.
However, that would again require filter scripts, might result in
missing real problematic warning backtraces triggered while the test
is running, and the irrelevant backtrace(s) would still clog the
kernel log.

Solve the problem by providing a means to suppress warning backtraces
originating from the current kthread while executing test code.
Since each KUnit test runs in its own kthread, this effectively scopes
suppression to the test that enabled it, without requiring any
architecture-specific code.

Overview:
Patch#1 Introduces the suppression infrastructure integrated into
        KUnit's hook mechanism.
Patch#2 Adds selftests to validate the functionality.
Patch#3 Demonstrates real-world usage in the DRM subsystem.
Patch#4 Documents the new API and usage guidelines.

Design Notes:
Suppression is integrated into the existing KUnit hooks infrastructure,
reusing the kunit_running static branch for zero overhead
when no tests are running. The implementation lives entirely in the
kunit module; only a static-inline wrapper and a function pointer
slot are added to built-in code.

Suppression is checked at three points in the warning path:
- In `warn_slowpath_fmt()` (kernel/panic.c), for architectures without
  __WARN_FLAGS. The check runs before any output, fully suppressing
  both message and backtrace.
- In `__warn_printk()` (kernel/panic.c), for architectures that define
  __WARN_FLAGS but not their own __WARN_printf (arm64, loongarch,
  parisc, powerpc, riscv, sh). The check suppresses the warning message
  text that is printed before the trap enters __report_bug().
- In `__report_bug()` (lib/bug.c), for architectures that define
  __WARN_FLAGS. The check runs before `__warn()` is called, suppressing
  the backtrace and stack dump. 

To avoid double-counting on architectures where both `__warn_printk()`
and `__report_bug()` run for the same warning, the hook takes a bool
parameter: true to increment the suppression counter, false to suppress
without counting.

The suppression state is dynamically allocated via kunit_kzalloc() and
tied to the KUnit test lifecycle via `kunit_add_action()`, ensuring
automatic cleanup at test exit. Writer-side access to the global
suppression list is serialized with a spinlock; readers use RCU.

Two API forms are provided:
- kunit_warning_suppress(test) { ... }: scoped blocks with automatic
  cleanup. The suppression handle is not accessible outside the block,
  so warning counts (if needed) must be checked inside. Multiple
  sequential suppression blocks are allowed.
- kunit_start/end_suppress_warning(test): direct functions that return
  an explicit handle. Use when the handle needs to be retained, or passed
  across helpers. Multiple sequential suppression blocks are allowed.

This series is based on the RFC patch and subsequent discussion at
https://patchwork.kernel.org/project/linux-kselftest/patch/02546e59-1afe-4b08-ba81-d94f3b691c9a@moroto.mountain/
and offers a more comprehensive solution of the problem discussed there.

Changes since RFC:
- Introduced CONFIG_KUNIT_SUPPRESS_BACKTRACE
- Minor cleanups and bug fixes
- Added support for all affected architectures
- Added support for counting suppressed warnings
- Added unit tests using those counters
- Added patch to suppress warning backtraces in dev_addr_lists tests

Changes since v1:
- Rebased to v6.9-rc1
- Added Tested-by:, Acked-by:, and Reviewed-by: tags
  [I retained those tags since there have been no functional changes]
- Introduced KUNIT_SUPPRESS_BACKTRACE configuration option, enabled by
  default.

Changes since v2:
- Rebased to v6.9-rc2
- Added comments to drm warning suppression explaining why it is needed.
- Added patch to move conditional code in arch/sh/include/asm/bug.h
  to avoid kerneldoc warning
- Added architecture maintainers to Cc: for architecture specific patches
- No functional changes

Changes since v3:
- Rebased to v6.14-rc6
- Dropped net: "kunit: Suppress lock warning noise at end of dev_addr_lists tests"
  since 3db3b62955cd6d73afde05a17d7e8e106695c3b9
- Added __kunit_ and KUNIT_ prefixes.
- Tested on interessed architectures.

Changes since v4:
- Rebased to v6.15-rc7
- Dropped all code in __report_bug()
- Moved all checks in WARN*() macros.
- Dropped all architecture specific code.
- Made __kunit_is_suppressed_warning nice to noinstr functions.

Changes since v5:
- Rebased to v7.0-rc3
- Added RCU protection for the suppressed warnings list.
- Added static key and branching optimization.
- Removed custom `strcmp` implementation and reworked
  __kunit_is_suppressed_warning() entrypoint function.

Changes since v6:
- Moved suppression checks from WARN*() macros to warn_slowpath_fmt()
  and __report_bug().
- Replaced stack-allocated suppression struct with kunit_kzalloc() heap
  allocation tied to the KUnit test lifecycle.
- Changed suppression strategy from function-name matching to task-scoped:
  all warnings on the current task are suppressed between START and END,
  rather than only warnings originating from a specific named function.
- Simplified macro API: removed KUNIT_DECLARE_SUPPRESSED_WARNING(),
  the START macro now takes (test) and handles allocation internally.
- Removed static key and branching optiomization, as by the time it
  was executed, callers are already in warn slowpaths.
- Link to v6: https://lore.kernel.org/r/20260317-kunit_add_support-v6-0-dd22aeb3fe5d@redhat.com

Changes since v7:
- Integrated suppression into existing KUnit hooks infrastructure
- Removed CONFIG_KUNIT_SUPPRESS_BACKTRACE
- Added suppression check in __warn_printk()
- Added spinlock for writer-side RCU protection
- Replaced explicit rcu_read_lock/unlock with guard(rcu)()
- Added scoped API (kunit_warning_suppress) using __cleanup attribute
- Updated DRM patch to use scoped API
- Expanded self-tests: incremental counting, cross-kthread isolation
- Rewrote documentation covering all three API forms with examples
- Link to v7: https://lore.kernel.org/r/20260420-kunit_add_support-v7-0-e8bc6e0f70de@redhat.com

Changes since v8:
- Rebased to v7.1-rc2
- Remove KUNIT_START/END_SUPPRESSED_WARNING() macros
- Add KUNIT_EXPECT_SUPPRESSED_WARNING_COUNT checks to drm tests
- Link to v8: https://lore.kernel.org/r/20260504-kunit_add_support-v8-0-3e5957cdd235@redhat.com

Changes since v9:
- Fix silent false-pass when kunit_start_suppress_warning() returns NULL
- Fix RCU lockdep splat for kunit_is_suppressed_warning() calls
- Move disable_trace_on_warning() in __report_bug()
- Make suppress counter atomic
- Mark helper warn functions in selftest as noinline
- Add kunit_skip() for CONFIG_BUG=n in selftests
- Fix potentially uninitialized data.was_active in kthread seltest
- Add kthread_stop() in kthread selftest early exit
- Initialize scaling_factor to INT_MIN in DRM scaling tests
- Add include for bool in test-bug.h to fix CONFIG_KUNIT=n case
- Link to v9: https://lore.kernel.org/r/20260508-kunit_add_support-v9-0-99df7aa880f6@redhat.com

Changes since v10:
- Remove synchronize_rcu() to avoid sleeping in atomic context
- Pin task_struct refcount to prevent ABA false-positive matches
- Loop in suppression selftest to prevent use-after-free on kthread exit
- Skip DRM rect tests on CONFIG_BUG=n
- Link to v10: https://lore.kernel.org/r/20260513-kunit_add_support-v10-0-e379d206c8cd@redhat.com

Changes since v11:
- Use call_rcu() to defer free without blocking
- Remove #ifdef CONFIG_KUNIT guard in lib/bug.c
- Remove stale config checks from selftest
- Replace skip on DRM rect tests with conditional expectation
- Link to v11: https://lore.kernel.org/r/20260514-kunit_add_support-v11-0-b36a530a6d8f@redhat.com

Changes since v12:
- Reverted to the v9 synchronize_rcu() approach
- Add in_task() check at the top of __kunit_is_suppressed_warning_impl()
- Link to v12: https://lore.kernel.org/r/20260515-kunit_add_support-v12-0-a216dc228be8@redhat.com
--
2.34.1

---
To: Brendan Higgins <brendan.higgins@linux.dev>
To: David Gow <david@davidgow.net>
To: Rae Moar <raemoar63@gmail.com>
To: Andrew Morton <akpm@linux-foundation.org>
To: Paul Walmsley <pjw@kernel.org>
To: Palmer Dabbelt <palmer@dabbelt.com>
To: Albert Ou <aou@eecs.berkeley.edu>
To: Alexandre Ghiti <alex@ghiti.fr>
To: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
To: Maxime Ripard <mripard@kernel.org>
To: Thomas Zimmermann <tzimmermann@suse.de>
To: David Airlie <airlied@gmail.com>
To: Simona Vetter <simona@ffwll.ch>
To: Jonathan Corbet <corbet@lwn.net>
To: Shuah Khan <skhan@linuxfoundation.org>
Cc: linux-kernel@vger.kernel.org
Cc: linux-kselftest@vger.kernel.org
Cc: kunit-dev@googlegroups.com
Cc: linux-riscv@lists.infradead.org
Cc: dri-devel@lists.freedesktop.org
Cc: workflows@vger.kernel.org
Cc: linux-doc@vger.kernel.org

---

---
Alessandro Carminati (1):
      bug/kunit: Core support for suppressing warning backtraces

Guenter Roeck (3):
      kunit: Add backtrace suppression self-tests
      drm: Suppress intentional warning backtraces in scaling unit tests
      kunit: Add documentation for warning backtrace suppression API

 Documentation/dev-tools/kunit/usage.rst |  46 +++++++-
 drivers/gpu/drm/tests/drm_rect_test.c   |  36 +++++-
 include/kunit/test-bug.h                |  26 +++++
 include/kunit/test.h                    |  98 ++++++++++++++++
 kernel/panic.c                          |  11 ++
 lib/bug.c                               |  12 +-
 lib/kunit/Makefile                      |   4 +-
 lib/kunit/backtrace-suppression-test.c  | 192 ++++++++++++++++++++++++++++++++
 lib/kunit/bug.c                         | 127 +++++++++++++++++++++
 lib/kunit/hooks-impl.h                  |   2 +
 10 files changed, 544 insertions(+), 10 deletions(-)
---
base-commit: 74fe02ce122a6103f207d29fafc8b3a53de6abaf
change-id: 20260312-kunit_add_support-2f35806b19dd

Best regards,
-- 
Albert Esteve <aesteve@redhat.com>

[PATCH v13 4/4] kunit: Add documentation for warning backtrace suppression API

[Albert Esteve]

From: Guenter Roeck <linux@roeck-us.net>

Document API functions for suppressing warning backtraces.

Tested-by: Linux Kernel Functional Testing <lkft@linaro.org>
Acked-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Alessandro Carminati <acarmina@redhat.com>
Reviewed-by: David Gow <david@davidgow.net>
Signed-off-by: Albert Esteve <aesteve@redhat.com>
---
 Documentation/dev-tools/kunit/usage.rst | 46 ++++++++++++++++++++++++++++++++-
 1 file changed, 45 insertions(+), 1 deletion(-)

diff --git a/Documentation/dev-tools/kunit/usage.rst b/Documentation/dev-tools/kunit/usage.rst
index ebd06f5ea4550..1c78dfff94e8a 100644
--- a/Documentation/dev-tools/kunit/usage.rst
+++ b/Documentation/dev-tools/kunit/usage.rst
@@ -157,6 +157,50 @@ Alternatively, one can take full control over the error message by using
 	if (some_setup_function())
 		KUNIT_FAIL(test, "Failed to setup thing for testing");
 
+Suppressing warning backtraces
+------------------------------
+
+Some unit tests trigger warning backtraces either intentionally or as a side
+effect. Such backtraces are normally undesirable since they distract from
+the actual test and may result in the impression that there is a problem.
+
+Backtraces can be suppressed with **task-scoped suppression**: while
+suppression is active on the current task, the backtrace and stack dump from
+``WARN*()``, ``WARN_ON*()``, and related macros on that task are suppressed.
+Two API forms are available.
+
+- Scoped suppression is the simplest form. Wrap the code that triggers
+  warnings in a ``kunit_warning_suppress()`` block:
+
+.. code-block:: c
+
+	static void some_test(struct kunit *test)
+	{
+		kunit_warning_suppress(test) {
+			trigger_backtrace();
+			KUNIT_EXPECT_SUPPRESSED_WARNING_COUNT(test, 1);
+		}
+	}
+
+.. note::
+   The warning count must be checked inside the block; the suppression handle
+   is not accessible after the block exits.
+
+- Direct functions return an explicit handle pointer. Use them when the handle
+  needs to be retained or passed across helper functions:
+
+.. code-block:: c
+
+	static void some_test(struct kunit *test)
+	{
+		struct kunit_suppressed_warning *w;
+
+		w = kunit_start_suppress_warning(test);
+		trigger_backtrace();
+		kunit_end_suppress_warning(test, w);
+
+		KUNIT_EXPECT_EQ(test, kunit_suppressed_warning_count(w), 1);
+	}
 
 Test Suites
 ~~~~~~~~~~~
@@ -1211,4 +1255,4 @@ For example:
 		dev_managed_string = devm_kstrdup(fake_device, "Hello, World!");
 
 		// Everything is cleaned up automatically when the test ends.
-	}
\ No newline at end of file
+	}

-- 
2.53.0

Claude review: kunit: Add documentation for warning backtrace suppression API

[Claude Code Review Bot]

Patch Review

The documentation is clear and shows both API forms with examples. 

**Minor nit:** The documentation shows the direct API example checking the warning count *after* `kunit_end_suppress_warning()`:

```rst
+		w = kunit_start_suppress_warning(test);
+		trigger_backtrace();
+		kunit_end_suppress_warning(test, w);
+
+		KUNIT_EXPECT_EQ(test, kunit_suppressed_warning_count(w), 1);
```

This works because the handle memory is managed by `kunit_kzalloc()` and remains valid until test exit — the `end` call only removes the node from the RCU list, it doesn't free the struct. This is an important property that makes the direct API more ergonomic than the scoped form for post-scope count checks. The code comment in patch 1 explains this, and the doc example demonstrates it. Good.

**Also good:** The patch fixes a missing newline at end of file in usage.rst:
```
-	}
\ No newline at end of file
+	}
```

**No issues found.**

---

**Summary:** This is a well-crafted v13 series with solid design, comprehensive tests, and clean code. The RCU + spinlock synchronization is standard kernel practice, the static branch gives zero overhead outside tests, and the `in_task()` guard prevents false suppression from interrupt context. The series has appropriate reviews from KUnit and security maintainers. No blocking issues found from a DRM subsystem perspective; the drm_rect_test changes in patch 3 are clean and correct.

---
Generated by Claude Code Patch Reviewer

[PATCH v11 0/4] kunit: Add support for suppressing warning backtraces

[Albert Esteve]

Some unit tests intentionally trigger warning backtraces by passing bad
parameters to kernel API functions. Such unit tests typically check the
return value from such calls, not the existence of the warning backtrace.

Such intentionally generated warning backtraces are neither desirable
nor useful for a number of reasons:
- They can result in overlooked real problems.
- A warning that suddenly starts to show up in unit tests needs to be
  investigated and has to be marked to be ignored, for example by
  adjusting filter scripts. Such filters are ad hoc because there is
  no real standard format for warnings. On top of that, such filter
  scripts would require constant maintenance.

One option to address the problem would be to add messages such as
"expected warning backtraces start/end here" to the kernel log.
However, that would again require filter scripts, might result in
missing real problematic warning backtraces triggered while the test
is running, and the irrelevant backtrace(s) would still clog the
kernel log.

Solve the problem by providing a means to suppress warning backtraces
originating from the current kthread while executing test code.
Since each KUnit test runs in its own kthread, this effectively scopes
suppression to the test that enabled it, without requiring any
architecture-specific code.

Overview:
Patch#1 Introduces the suppression infrastructure integrated into
        KUnit's hook mechanism.
Patch#2 Adds selftests to validate the functionality.
Patch#3 Demonstrates real-world usage in the DRM subsystem.
Patch#4 Documents the new API and usage guidelines.

Design Notes:
Suppression is integrated into the existing KUnit hooks infrastructure,
reusing the kunit_running static branch for zero overhead
when no tests are running. The implementation lives entirely in the
kunit module; only a static-inline wrapper and a function pointer
slot are added to built-in code.

Suppression is checked at three points in the warning path:
- In `warn_slowpath_fmt()` (kernel/panic.c), for architectures without
  __WARN_FLAGS. The check runs before any output, fully suppressing
  both message and backtrace.
- In `__warn_printk()` (kernel/panic.c), for architectures that define
  __WARN_FLAGS but not their own __WARN_printf (arm64, loongarch,
  parisc, powerpc, riscv, sh). The check suppresses the warning message
  text that is printed before the trap enters __report_bug().
- In `__report_bug()` (lib/bug.c), for architectures that define
  __WARN_FLAGS. The check runs before `__warn()` is called, suppressing
  the backtrace and stack dump. 

To avoid double-counting on architectures where both `__warn_printk()`
and `__report_bug()` run for the same warning, the hook takes a bool
parameter: true to increment the suppression counter, false to suppress
without counting.

The suppression state is dynamically allocated via kunit_kzalloc() and
tied to the KUnit test lifecycle via `kunit_add_action()`, ensuring
automatic cleanup at test exit. Writer-side access to the global
suppression list is serialized with a spinlock; readers use RCU.

Two API forms are provided:
- kunit_warning_suppress(test) { ... }: scoped blocks with automatic
  cleanup. The suppression handle is not accessible outside the block,
  so warning counts (if needed) must be checked inside. Multiple
  suppression blocks are allowed.
- kunit_start/end_suppress_warning(test): direct functions that return
  an explicit handle. Use when the handle needs to be retained, or passed
  across helpers. Multiple suppression blocks are allowed.

This series is based on the RFC patch and subsequent discussion at
https://patchwork.kernel.org/project/linux-kselftest/patch/02546e59-1afe-4b08-ba81-d94f3b691c9a@moroto.mountain/
and offers a more comprehensive solution of the problem discussed there.

Changes since RFC:
- Introduced CONFIG_KUNIT_SUPPRESS_BACKTRACE
- Minor cleanups and bug fixes
- Added support for all affected architectures
- Added support for counting suppressed warnings
- Added unit tests using those counters
- Added patch to suppress warning backtraces in dev_addr_lists tests

Changes since v1:
- Rebased to v6.9-rc1
- Added Tested-by:, Acked-by:, and Reviewed-by: tags
  [I retained those tags since there have been no functional changes]
- Introduced KUNIT_SUPPRESS_BACKTRACE configuration option, enabled by
  default.

Changes since v2:
- Rebased to v6.9-rc2
- Added comments to drm warning suppression explaining why it is needed.
- Added patch to move conditional code in arch/sh/include/asm/bug.h
  to avoid kerneldoc warning
- Added architecture maintainers to Cc: for architecture specific patches
- No functional changes

Changes since v3:
- Rebased to v6.14-rc6
- Dropped net: "kunit: Suppress lock warning noise at end of dev_addr_lists tests"
  since 3db3b62955cd6d73afde05a17d7e8e106695c3b9
- Added __kunit_ and KUNIT_ prefixes.
- Tested on interessed architectures.

Changes since v4:
- Rebased to v6.15-rc7
- Dropped all code in __report_bug()
- Moved all checks in WARN*() macros.
- Dropped all architecture specific code.
- Made __kunit_is_suppressed_warning nice to noinstr functions.

Changes since v5:
- Rebased to v7.0-rc3
- Added RCU protection for the suppressed warnings list.
- Added static key and branching optimization.
- Removed custom `strcmp` implementation and reworked
  __kunit_is_suppressed_warning() entrypoint function.

Changes since v6:
- Moved suppression checks from WARN*() macros to warn_slowpath_fmt()
  and __report_bug().
- Replaced stack-allocated suppression struct with kunit_kzalloc() heap
  allocation tied to the KUnit test lifecycle.
- Changed suppression strategy from function-name matching to task-scoped:
  all warnings on the current task are suppressed between START and END,
  rather than only warnings originating from a specific named function.
- Simplified macro API: removed KUNIT_DECLARE_SUPPRESSED_WARNING(),
  the START macro now takes (test) and handles allocation internally.
- Removed static key and branching optiomization, as by the time it
  was executed, callers are already in warn slowpaths.
- Link to v6: https://lore.kernel.org/r/20260317-kunit_add_support-v6-0-dd22aeb3fe5d@redhat.com

Changes since v7:
- Integrated suppression into existing KUnit hooks infrastructure
- Removed CONFIG_KUNIT_SUPPRESS_BACKTRACE
- Added suppression check in __warn_printk()
- Added spinlock for writer-side RCU protection
- Replaced explicit rcu_read_lock/unlock with guard(rcu)()
- Added scoped API (kunit_warning_suppress) using __cleanup attribute
- Updated DRM patch to use scoped API
- Expanded self-tests: incremental counting, cross-kthread isolation
- Rewrote documentation covering all three API forms with examples
- Link to v7: https://lore.kernel.org/r/20260420-kunit_add_support-v7-0-e8bc6e0f70de@redhat.com

Changes since v8:
- Rebased to v7.1-rc2
- Remove KUNIT_START/END_SUPPRESSED_WARNING() macros
- Add KUNIT_EXPECT_SUPPRESSED_WARNING_COUNT checks to drm tests
- Link to v8: https://lore.kernel.org/r/20260504-kunit_add_support-v8-0-3e5957cdd235@redhat.com

Changes since v9:
- Fix silent false-pass when kunit_start_suppress_warning() returns NULL
- Fix RCU lockdep splat for kunit_is_suppressed_warning() calls
- Move disable_trace_on_warning() in __report_bug()
- Make suppress counter atomic
- Mark helper warn functions in selftest as noinline
- Add kunit_skip() for CONFIG_BUG=n in selftests
- Fix potentially uninitialized data.was_active in kthread seltest
- Add kthread_stop() in kthread selftest early exit
- Initialize scaling_factor to INT_MIN in DRM scaling tests
- Add include for bool in test-bug.h to fix CONFIG_KUNIT=n case
- Link to v9: https://lore.kernel.org/r/20260508-kunit_add_support-v9-0-99df7aa880f6@redhat.com

Changes since v10:
- Remove synchronize_rcu() to avoid sleeping in atomic context
- Pin task_struct refcount to prevent ABA false-positive matches
- Loop in suppression selftest to prevent use-after-free on kthread exit
- Skip DRM rect tests on CONFIG_BUG=n
- Link to v10: https://lore.kernel.org/r/20260513-kunit_add_support-v10-0-e379d206c8cd@redhat.com

--
2.34.1

---
To: Brendan Higgins <brendan.higgins@linux.dev>
To: David Gow <david@davidgow.net>
To: Rae Moar <raemoar63@gmail.com>
To: Andrew Morton <akpm@linux-foundation.org>
To: Paul Walmsley <pjw@kernel.org>
To: Palmer Dabbelt <palmer@dabbelt.com>
To: Albert Ou <aou@eecs.berkeley.edu>
To: Alexandre Ghiti <alex@ghiti.fr>
To: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
To: Maxime Ripard <mripard@kernel.org>
To: Thomas Zimmermann <tzimmermann@suse.de>
To: David Airlie <airlied@gmail.com>
To: Simona Vetter <simona@ffwll.ch>
To: Jonathan Corbet <corbet@lwn.net>
To: Shuah Khan <skhan@linuxfoundation.org>
Cc: linux-kernel@vger.kernel.org
Cc: linux-kselftest@vger.kernel.org
Cc: kunit-dev@googlegroups.com
Cc: linux-riscv@lists.infradead.org
Cc: dri-devel@lists.freedesktop.org
Cc: workflows@vger.kernel.org
Cc: linux-doc@vger.kernel.org

---
Alessandro Carminati (1):
      bug/kunit: Core support for suppressing warning backtraces

Guenter Roeck (3):
      kunit: Add backtrace suppression self-tests
      drm: Suppress intentional warning backtraces in scaling unit tests
      kunit: Add documentation for warning backtrace suppression API

 Documentation/dev-tools/kunit/usage.rst |  46 +++++++-
 drivers/gpu/drm/tests/drm_rect_test.c   |  46 +++++++-
 include/kunit/test-bug.h                |  26 +++++
 include/kunit/test.h                    |  98 ++++++++++++++++
 kernel/panic.c                          |  11 ++
 lib/bug.c                               |  14 ++-
 lib/kunit/Makefile                      |   4 +-
 lib/kunit/backtrace-suppression-test.c  | 198 ++++++++++++++++++++++++++++++++
 lib/kunit/bug.c                         | 120 +++++++++++++++++++
 lib/kunit/hooks-impl.h                  |   2 +
 10 files changed, 555 insertions(+), 10 deletions(-)
---
base-commit: 74fe02ce122a6103f207d29fafc8b3a53de6abaf
change-id: 20260312-kunit_add_support-2f35806b19dd

Best regards,
-- 
Albert Esteve <aesteve@redhat.com>

[PATCH v11 4/4] kunit: Add documentation for warning backtrace suppression API

[Albert Esteve]

From: Guenter Roeck <linux@roeck-us.net>

Document API functions for suppressing warning backtraces.

Tested-by: Linux Kernel Functional Testing <lkft@linaro.org>
Acked-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Alessandro Carminati <acarmina@redhat.com>
Reviewed-by: David Gow <david@davidgow.net>
Signed-off-by: Albert Esteve <aesteve@redhat.com>
---
 Documentation/dev-tools/kunit/usage.rst | 46 ++++++++++++++++++++++++++++++++-
 1 file changed, 45 insertions(+), 1 deletion(-)

diff --git a/Documentation/dev-tools/kunit/usage.rst b/Documentation/dev-tools/kunit/usage.rst
index ebd06f5ea4550..1c78dfff94e8a 100644
--- a/Documentation/dev-tools/kunit/usage.rst
+++ b/Documentation/dev-tools/kunit/usage.rst
@@ -157,6 +157,50 @@ Alternatively, one can take full control over the error message by using
 	if (some_setup_function())
 		KUNIT_FAIL(test, "Failed to setup thing for testing");
 
+Suppressing warning backtraces
+------------------------------
+
+Some unit tests trigger warning backtraces either intentionally or as a side
+effect. Such backtraces are normally undesirable since they distract from
+the actual test and may result in the impression that there is a problem.
+
+Backtraces can be suppressed with **task-scoped suppression**: while
+suppression is active on the current task, the backtrace and stack dump from
+``WARN*()``, ``WARN_ON*()``, and related macros on that task are suppressed.
+Two API forms are available.
+
+- Scoped suppression is the simplest form. Wrap the code that triggers
+  warnings in a ``kunit_warning_suppress()`` block:
+
+.. code-block:: c
+
+	static void some_test(struct kunit *test)
+	{
+		kunit_warning_suppress(test) {
+			trigger_backtrace();
+			KUNIT_EXPECT_SUPPRESSED_WARNING_COUNT(test, 1);
+		}
+	}
+
+.. note::
+   The warning count must be checked inside the block; the suppression handle
+   is not accessible after the block exits.
+
+- Direct functions return an explicit handle pointer. Use them when the handle
+  needs to be retained or passed across helper functions:
+
+.. code-block:: c
+
+	static void some_test(struct kunit *test)
+	{
+		struct kunit_suppressed_warning *w;
+
+		w = kunit_start_suppress_warning(test);
+		trigger_backtrace();
+		kunit_end_suppress_warning(test, w);
+
+		KUNIT_EXPECT_EQ(test, kunit_suppressed_warning_count(w), 1);
+	}
 
 Test Suites
 ~~~~~~~~~~~
@@ -1211,4 +1255,4 @@ For example:
 		dev_managed_string = devm_kstrdup(fake_device, "Hello, World!");
 
 		// Everything is cleaned up automatically when the test ends.
-	}
\ No newline at end of file
+	}

-- 
2.53.0

Claude review: kunit: Add documentation for warning backtrace suppression API

[Claude Code Review Bot]

Patch Review

**Overall: Clear and correct documentation.**

The documentation covers both API forms with code examples and includes the important note about checking warning counts inside the scoped block.

**Minor: The doc shows `kunit_suppressed_warning_count(w)` used after `kunit_end_suppress_warning()`:**

```rst
+		w = kunit_start_suppress_warning(test);
+		trigger_backtrace();
+		kunit_end_suppress_warning(test, w);
+
+		KUNIT_EXPECT_EQ(test, kunit_suppressed_warning_count(w), 1);
```

This is valid because `w` remains allocated (via `kunit_kzalloc`) until test exit. But it's worth being explicit in the documentation that `w` remains readable after `kunit_end_suppress_warning` — a reader might assume end-suppress frees the handle. The test code in patch 2 (`backtrace_suppression_test_multi_scope`) relies on this same property, so it's an intentional design choice. A one-line note in the docs like "The handle remains valid for count queries after ``kunit_end_suppress_warning()``" would prevent confusion.

**Fixes missing newline at EOF:**

```diff
-	}
\ No newline at end of file
+	}
```

Nice cleanup.

**Summary:** This is a mature, well-reviewed series at v11. The core design is sound, the test coverage is thorough, and the DRM usage patch is a good practical demonstration. The issues identified are all minor style/documentation nits rather than correctness problems. From the DRM subsystem perspective, patch 3 is a clean improvement to the drm_rect tests.

---
Generated by Claude Code Patch Reviewer

[PATCH v10 0/4] kunit: Add support for suppressing warning backtraces

[Albert Esteve]

Some unit tests intentionally trigger warning backtraces by passing bad
parameters to kernel API functions. Such unit tests typically check the
return value from such calls, not the existence of the warning backtrace.

Such intentionally generated warning backtraces are neither desirable
nor useful for a number of reasons:
- They can result in overlooked real problems.
- A warning that suddenly starts to show up in unit tests needs to be
  investigated and has to be marked to be ignored, for example by
  adjusting filter scripts. Such filters are ad hoc because there is
  no real standard format for warnings. On top of that, such filter
  scripts would require constant maintenance.

One option to address the problem would be to add messages such as
"expected warning backtraces start/end here" to the kernel log.
However, that would again require filter scripts, might result in
missing real problematic warning backtraces triggered while the test
is running, and the irrelevant backtrace(s) would still clog the
kernel log.

Solve the problem by providing a means to suppress warning backtraces
originating from the current kthread while executing test code.
Since each KUnit test runs in its own kthread, this effectively scopes
suppression to the test that enabled it, without requiring any
architecture-specific code.

Overview:
Patch#1 Introduces the suppression infrastructure integrated into
        KUnit's hook mechanism.
Patch#2 Adds selftests to validate the functionality.
Patch#3 Demonstrates real-world usage in the DRM subsystem.
Patch#4 Documents the new API and usage guidelines.

Design Notes:
Suppression is integrated into the existing KUnit hooks infrastructure,
reusing the kunit_running static branch for zero overhead
when no tests are running. The implementation lives entirely in the
kunit module; only a static-inline wrapper and a function pointer
slot are added to built-in code.

Suppression is checked at three points in the warning path:
- In `warn_slowpath_fmt()` (kernel/panic.c), for architectures without
  __WARN_FLAGS. The check runs before any output, fully suppressing
  both message and backtrace.
- In `__warn_printk()` (kernel/panic.c), for architectures that define
  __WARN_FLAGS but not their own __WARN_printf (arm64, loongarch,
  parisc, powerpc, riscv, sh). The check suppresses the warning message
  text that is printed before the trap enters __report_bug().
- In `__report_bug()` (lib/bug.c), for architectures that define
  __WARN_FLAGS. The check runs before `__warn()` is called, suppressing
  the backtrace and stack dump. 

To avoid double-counting on architectures where both `__warn_printk()`
and `__report_bug()` run for the same warning, the hook takes a bool
parameter: true to increment the suppression counter, false to suppress
without counting.

The suppression state is dynamically allocated via kunit_kzalloc() and
tied to the KUnit test lifecycle via `kunit_add_action()`, ensuring
automatic cleanup at test exit. Writer-side access to the global
suppression list is serialized with a spinlock; readers use RCU.

Two API forms are provided:
- kunit_warning_suppress(test) { ... }: scoped blocks with automatic
  cleanup. The suppression handle is not accessible outside the block,
  so warning counts (if needed) must be checked inside. Multiple
  suppression blocks are allowed.
- kunit_start/end_suppress_warning(test): direct functions that return
  an explicit handle. Use when the handle needs to be retained, or passed
  across helpers. Multiple suppression blocks are allowed.

This series is based on the RFC patch and subsequent discussion at
https://patchwork.kernel.org/project/linux-kselftest/patch/02546e59-1afe-4b08-ba81-d94f3b691c9a@moroto.mountain/
and offers a more comprehensive solution of the problem discussed there.

Changes since RFC:
- Introduced CONFIG_KUNIT_SUPPRESS_BACKTRACE
- Minor cleanups and bug fixes
- Added support for all affected architectures
- Added support for counting suppressed warnings
- Added unit tests using those counters
- Added patch to suppress warning backtraces in dev_addr_lists tests

Changes since v1:
- Rebased to v6.9-rc1
- Added Tested-by:, Acked-by:, and Reviewed-by: tags
  [I retained those tags since there have been no functional changes]
- Introduced KUNIT_SUPPRESS_BACKTRACE configuration option, enabled by
  default.

Changes since v2:
- Rebased to v6.9-rc2
- Added comments to drm warning suppression explaining why it is needed.
- Added patch to move conditional code in arch/sh/include/asm/bug.h
  to avoid kerneldoc warning
- Added architecture maintainers to Cc: for architecture specific patches
- No functional changes

Changes since v3:
- Rebased to v6.14-rc6
- Dropped net: "kunit: Suppress lock warning noise at end of dev_addr_lists tests"
  since 3db3b62955cd6d73afde05a17d7e8e106695c3b9
- Added __kunit_ and KUNIT_ prefixes.
- Tested on interessed architectures.

Changes since v4:
- Rebased to v6.15-rc7
- Dropped all code in __report_bug()
- Moved all checks in WARN*() macros.
- Dropped all architecture specific code.
- Made __kunit_is_suppressed_warning nice to noinstr functions.

Changes since v5:
- Rebased to v7.0-rc3
- Added RCU protection for the suppressed warnings list.
- Added static key and branching optimization.
- Removed custom `strcmp` implementation and reworked
  __kunit_is_suppressed_warning() entrypoint function.

Changes since v6:
- Moved suppression checks from WARN*() macros to warn_slowpath_fmt()
  and __report_bug().
- Replaced stack-allocated suppression struct with kunit_kzalloc() heap
  allocation tied to the KUnit test lifecycle.
- Changed suppression strategy from function-name matching to task-scoped:
  all warnings on the current task are suppressed between START and END,
  rather than only warnings originating from a specific named function.
- Simplified macro API: removed KUNIT_DECLARE_SUPPRESSED_WARNING(),
  the START macro now takes (test) and handles allocation internally.
- Removed static key and branching optiomization, as by the time it
  was executed, callers are already in warn slowpaths.
- Link to v6: https://lore.kernel.org/r/20260317-kunit_add_support-v6-0-dd22aeb3fe5d@redhat.com

Changes since v7:
- Integrated suppression into existing KUnit hooks infrastructure
- Removed CONFIG_KUNIT_SUPPRESS_BACKTRACE
- Added suppression check in __warn_printk()
- Added spinlock for writer-side RCU protection
- Replaced explicit rcu_read_lock/unlock with guard(rcu)()
- Added scoped API (kunit_warning_suppress) using __cleanup attribute
- Updated DRM patch to use scoped API
- Expanded self-tests: incremental counting, cross-kthread isolation
- Rewrote documentation covering all three API forms with examples
- Link to v7: https://lore.kernel.org/r/20260420-kunit_add_support-v7-0-e8bc6e0f70de@redhat.com

Changes since v8:
- Rebased to v7.1-rc2
- Remove KUNIT_START/END_SUPPRESSED_WARNING() macros
- Add KUNIT_EXPECT_SUPPRESSED_WARNING_COUNT checks to drm tests
- Link to v8: https://lore.kernel.org/r/20260504-kunit_add_support-v8-0-3e5957cdd235@redhat.com

Changes since v9:
- Fix silent false-pass when kunit_start_suppress_warning() returns NULL
- Fix RCU lockdep splat for kunit_is_suppressed_warning() calls
- Move disable_trace_on_warning() in __report_bug()
- Make suppress counter atomic
- Mark helper warn functions in selftest as noinline
- Add kunit_skip() for CONFIG_BUG=n in selftests
- Fix potentially uninitialized data.was_active in kthread seltest
- Add kthread_stop() in kthread selftest early exit
- Initialize scaling_factor to INT_MIN in DRM scaling tests
- Add include for bool in test-bug.h to fix CONFIG_KUNIT=n case
- Link to v9: https://lore.kernel.org/r/20260508-kunit_add_support-v9-0-99df7aa880f6@redhat.com

--
2.34.1

---
To: Brendan Higgins <brendan.higgins@linux.dev>
To: David Gow <david@davidgow.net>
To: Rae Moar <raemoar63@gmail.com>
To: Andrew Morton <akpm@linux-foundation.org>
To: Paul Walmsley <pjw@kernel.org>
To: Palmer Dabbelt <palmer@dabbelt.com>
To: Albert Ou <aou@eecs.berkeley.edu>
To: Alexandre Ghiti <alex@ghiti.fr>
To: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
To: Maxime Ripard <mripard@kernel.org>
To: Thomas Zimmermann <tzimmermann@suse.de>
To: David Airlie <airlied@gmail.com>
To: Simona Vetter <simona@ffwll.ch>
To: Jonathan Corbet <corbet@lwn.net>
To: Shuah Khan <skhan@linuxfoundation.org>
Cc: linux-kernel@vger.kernel.org
Cc: linux-kselftest@vger.kernel.org
Cc: kunit-dev@googlegroups.com
Cc: linux-riscv@lists.infradead.org
Cc: dri-devel@lists.freedesktop.org
Cc: workflows@vger.kernel.org
Cc: linux-doc@vger.kernel.org

---
Alessandro Carminati (1):
      bug/kunit: Core support for suppressing warning backtraces

Guenter Roeck (3):
      kunit: Add backtrace suppression self-tests
      drm: Suppress intentional warning backtraces in scaling unit tests
      kunit: Add documentation for warning backtrace suppression API

 Documentation/dev-tools/kunit/usage.rst |  46 +++++++-
 drivers/gpu/drm/tests/drm_rect_test.c   |  32 +++++-
 include/kunit/test-bug.h                |  26 +++++
 include/kunit/test.h                    |  98 ++++++++++++++++
 kernel/panic.c                          |  11 ++
 lib/bug.c                               |  14 ++-
 lib/kunit/Makefile                      |   4 +-
 lib/kunit/backtrace-suppression-test.c  | 196 ++++++++++++++++++++++++++++++++
 lib/kunit/bug.c                         | 119 +++++++++++++++++++
 lib/kunit/hooks-impl.h                  |   2 +
 10 files changed, 538 insertions(+), 10 deletions(-)
---
base-commit: 74fe02ce122a6103f207d29fafc8b3a53de6abaf
change-id: 20260312-kunit_add_support-2f35806b19dd

Best regards,
-- 
Albert Esteve <aesteve@redhat.com>

[PATCH v10 4/4] kunit: Add documentation for warning backtrace suppression API

[Albert Esteve]

From: Guenter Roeck <linux@roeck-us.net>

Document API functions for suppressing warning backtraces.

Tested-by: Linux Kernel Functional Testing <lkft@linaro.org>
Acked-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Alessandro Carminati <acarmina@redhat.com>
Reviewed-by: David Gow <david@davidgow.net>
Signed-off-by: Albert Esteve <aesteve@redhat.com>
---
 Documentation/dev-tools/kunit/usage.rst | 46 ++++++++++++++++++++++++++++++++-
 1 file changed, 45 insertions(+), 1 deletion(-)

diff --git a/Documentation/dev-tools/kunit/usage.rst b/Documentation/dev-tools/kunit/usage.rst
index ebd06f5ea4550..1c78dfff94e8a 100644
--- a/Documentation/dev-tools/kunit/usage.rst
+++ b/Documentation/dev-tools/kunit/usage.rst
@@ -157,6 +157,50 @@ Alternatively, one can take full control over the error message by using
 	if (some_setup_function())
 		KUNIT_FAIL(test, "Failed to setup thing for testing");
 
+Suppressing warning backtraces
+------------------------------
+
+Some unit tests trigger warning backtraces either intentionally or as a side
+effect. Such backtraces are normally undesirable since they distract from
+the actual test and may result in the impression that there is a problem.
+
+Backtraces can be suppressed with **task-scoped suppression**: while
+suppression is active on the current task, the backtrace and stack dump from
+``WARN*()``, ``WARN_ON*()``, and related macros on that task are suppressed.
+Two API forms are available.
+
+- Scoped suppression is the simplest form. Wrap the code that triggers
+  warnings in a ``kunit_warning_suppress()`` block:
+
+.. code-block:: c
+
+	static void some_test(struct kunit *test)
+	{
+		kunit_warning_suppress(test) {
+			trigger_backtrace();
+			KUNIT_EXPECT_SUPPRESSED_WARNING_COUNT(test, 1);
+		}
+	}
+
+.. note::
+   The warning count must be checked inside the block; the suppression handle
+   is not accessible after the block exits.
+
+- Direct functions return an explicit handle pointer. Use them when the handle
+  needs to be retained or passed across helper functions:
+
+.. code-block:: c
+
+	static void some_test(struct kunit *test)
+	{
+		struct kunit_suppressed_warning *w;
+
+		w = kunit_start_suppress_warning(test);
+		trigger_backtrace();
+		kunit_end_suppress_warning(test, w);
+
+		KUNIT_EXPECT_EQ(test, kunit_suppressed_warning_count(w), 1);
+	}
 
 Test Suites
 ~~~~~~~~~~~
@@ -1211,4 +1255,4 @@ For example:
 		dev_managed_string = devm_kstrdup(fake_device, "Hello, World!");
 
 		// Everything is cleaned up automatically when the test ends.
-	}
\ No newline at end of file
+	}

-- 
2.53.0

Claude review: kunit: Add documentation for warning backtrace suppression API

[Claude Code Review Bot]

Patch Review

**Good documentation** covering both API forms with clear examples. The RST formatting looks correct.

**One minor doc issue:** The documentation shows the direct API example checking the count *after* `kunit_end_suppress_warning()`:

```c
w = kunit_start_suppress_warning(test);
trigger_backtrace();
kunit_end_suppress_warning(test, w);

KUNIT_EXPECT_EQ(test, kunit_suppressed_warning_count(w), 1);
```

This works because the `kunit_kzalloc`'d memory outlives the suppression deactivation (as confirmed by the multi_scope selftest). But it's worth noting this is a subtlety that could trip up readers — the handle remains readable after `kunit_end_suppress_warning()` because the allocation is test-scoped. The doc might benefit from a brief note about this, but it's not a blocker.

**Fix at the end:** The patch also fixes a missing newline at end of file (`\ No newline at end of file` → proper newline). Good housekeeping.

---
Generated by Claude Code Patch Reviewer

[PATCH v8 0/4] kunit: Add support for suppressing warning backtraces

[Albert Esteve]

Some unit tests intentionally trigger warning backtraces by passing bad
parameters to kernel API functions. Such unit tests typically check the
return value from such calls, not the existence of the warning backtrace.

Such intentionally generated warning backtraces are neither desirable
nor useful for a number of reasons:
- They can result in overlooked real problems.
- A warning that suddenly starts to show up in unit tests needs to be
  investigated and has to be marked to be ignored, for example by
  adjusting filter scripts. Such filters are ad hoc because there is
  no real standard format for warnings. On top of that, such filter
  scripts would require constant maintenance.

One option to address the problem would be to add messages such as
"expected warning backtraces start/end here" to the kernel log.
However, that would again require filter scripts, might result in
missing real problematic warning backtraces triggered while the test
is running, and the irrelevant backtrace(s) would still clog the
kernel log.

Solve the problem by providing a means to suppress warning backtraces
originating from the current kthread while executing test code.
Since each KUnit test runs in its own kthread, this effectively scopes
suppression to the test that enabled it, without requiring any
architecture-specific code.

Overview:
Patch#1 Introduces the suppression infrastructure integrated into
        KUnit's hook mechanism.
Patch#2 Adds selftests to validate the functionality.
Patch#3 Demonstrates real-world usage in the DRM subsystem.
Patch#4 Documents the new API and usage guidelines.

Design Notes:
Suppression is integrated into the existing KUnit hooks infrastructure,
reusing the kunit_running static branch for zero overhead
when no tests are running. The implementation lives entirely in the
kunit module; only a static-inline wrapper and a function pointer
slot are added to built-in code.

Suppression is checked at three points in the warning path:
- In `warn_slowpath_fmt()` (kernel/panic.c), for architectures without
  __WARN_FLAGS. The check runs before any output, fully suppressing
  both message and backtrace.
- In `__warn_printk()` (kernel/panic.c), for architectures that define
  __WARN_FLAGS but not their own __WARN_printf (arm64, loongarch,
  parisc, powerpc, riscv, sh). The check suppresses the warning message
  text that is printed before the trap enters __report_bug().
- In `__report_bug()` (lib/bug.c), for architectures that define
  __WARN_FLAGS. The check runs before `__warn()` is called, suppressing
  the backtrace and stack dump. 

To avoid double-counting on architectures where both `__warn_printk()`
and `__report_bug()` run for the same warning, the hook takes a bool
parameter: true to increment the suppression counter, false to suppress
without counting.

The suppression state is dynamically allocated via kunit_kzalloc() and
tied to the KUnit test lifecycle via `kunit_add_action()`, ensuring
automatic cleanup at test exit. Writer-side access to the global
suppression list is serialized with a spinlock; readers use RCU.

Three API forms are provided:
- kunit_warning_suppress(test) { ... }: scoped blocks with automatic
  cleanup. The suppression handle is not accessible outside the block,
  so warning counts (if needed) must be checked inside. Multiple
  suppression blocks are allowed.
- KUNIT_START/END_SUPPRESSED_WARNING(test): manual macros for larger
  blocks or post-suppression count checks. Limited to one pair per
  scope.
- kunit_start/end_suppress_warning(test): direct functions that return
  an explicit handle. Use when the handle needs to be retained, or passed
  across helpers. Multiple suppression blocks are allowed.

This series is based on the RFC patch and subsequent discussion at
https://patchwork.kernel.org/project/linux-kselftest/patch/02546e59-1afe-4b08-ba81-d94f3b691c9a@moroto.mountain/
and offers a more comprehensive solution of the problem discussed there.

Changes since RFC:
- Introduced CONFIG_KUNIT_SUPPRESS_BACKTRACE
- Minor cleanups and bug fixes
- Added support for all affected architectures
- Added support for counting suppressed warnings
- Added unit tests using those counters
- Added patch to suppress warning backtraces in dev_addr_lists tests

Changes since v1:
- Rebased to v6.9-rc1
- Added Tested-by:, Acked-by:, and Reviewed-by: tags
  [I retained those tags since there have been no functional changes]
- Introduced KUNIT_SUPPRESS_BACKTRACE configuration option, enabled by
  default.

Changes since v2:
- Rebased to v6.9-rc2
- Added comments to drm warning suppression explaining why it is needed.
- Added patch to move conditional code in arch/sh/include/asm/bug.h
  to avoid kerneldoc warning
- Added architecture maintainers to Cc: for architecture specific patches
- No functional changes

Changes since v3:
- Rebased to v6.14-rc6
- Dropped net: "kunit: Suppress lock warning noise at end of dev_addr_lists tests"
  since 3db3b62955cd6d73afde05a17d7e8e106695c3b9
- Added __kunit_ and KUNIT_ prefixes.
- Tested on interessed architectures.

Changes since v4:
- Rebased to v6.15-rc7
- Dropped all code in __report_bug()
- Moved all checks in WARN*() macros.
- Dropped all architecture specific code.
- Made __kunit_is_suppressed_warning nice to noinstr functions.

Changes since v5:
- Rebased to v7.0-rc3
- Added RCU protection for the suppressed warnings list.
- Added static key and branching optimization.
- Removed custom `strcmp` implementation and reworked
  __kunit_is_suppressed_warning() entrypoint function.

Changes since v6:
- Moved suppression checks from WARN*() macros to warn_slowpath_fmt()
  and __report_bug().
- Replaced stack-allocated suppression struct with kunit_kzalloc() heap
  allocation tied to the KUnit test lifecycle.
- Changed suppression strategy from function-name matching to task-scoped:
  all warnings on the current task are suppressed between START and END,
  rather than only warnings originating from a specific named function.
- Simplified macro API: removed KUNIT_DECLARE_SUPPRESSED_WARNING(),
  the START macro now takes (test) and handles allocation internally.
- Removed static key and branching optiomization, as by the time it
  was executed, callers are already in warn slowpaths.
- Link to v6: https://lore.kernel.org/r/20260317-kunit_add_support-v6-0-dd22aeb3fe5d@redhat.com

Changes since v7:
- Integrated suppression into existing KUnit hooks infrastructure
- Removed CONFIG_KUNIT_SUPPRESS_BACKTRACE
- Added suppression check in __warn_printk()
- Added spinlock for writer-side RCU protection
- Replaced explicit rcu_read_lock/unlock with guard(rcu)()
- Added scoped API (kunit_warning_suppress) using __cleanup attribute
- Updated DRM patch to use scoped API
- Expanded self-tests: incremental counting, cross-kthread isolation
- Rewrote documentation covering all three API forms with examples
- Link to v7: https://lore.kernel.org/r/20260420-kunit_add_support-v7-0-e8bc6e0f70de@redhat.com

--
2.34.1

---
Alessandro Carminati (1):
      bug/kunit: Core support for suppressing warning backtraces

Guenter Roeck (3):
      kunit: Add backtrace suppression self-tests
      drm: Suppress intentional warning backtraces in scaling unit tests
      kunit: Add documentation for warning backtrace suppression API

 Documentation/dev-tools/kunit/usage.rst |  63 ++++++++++-
 drivers/gpu/drm/tests/drm_rect_test.c   |  23 +++-
 include/kunit/test-bug.h                |  25 +++++
 include/kunit/test.h                    | 138 ++++++++++++++++++++++++
 kernel/panic.c                          |  15 ++-
 lib/bug.c                               |  10 ++
 lib/kunit/Makefile                      |   4 +-
 lib/kunit/backtrace-suppression-test.c  | 184 ++++++++++++++++++++++++++++++++
 lib/kunit/bug.c                         | 115 ++++++++++++++++++++
 lib/kunit/hooks-impl.h                  |   2 +
 10 files changed, 571 insertions(+), 8 deletions(-)
---
base-commit: 80234b5ab240f52fa45d201e899e207b9265ef91
change-id: 20260312-kunit_add_support-2f35806b19dd

Best regards,
-- 
Albert Esteve <aesteve@redhat.com>

[PATCH v8 4/4] kunit: Add documentation for warning backtrace suppression API

[Albert Esteve]

From: Guenter Roeck <linux@roeck-us.net>

Document API functions for suppressing warning backtraces.

Tested-by: Linux Kernel Functional Testing <lkft@linaro.org>
Acked-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Alessandro Carminati <acarmina@redhat.com>
Reviewed-by: David Gow <david@davidgow.net>
Signed-off-by: Albert Esteve <aesteve@redhat.com>
---
 Documentation/dev-tools/kunit/usage.rst | 63 ++++++++++++++++++++++++++++++++-
 1 file changed, 62 insertions(+), 1 deletion(-)

diff --git a/Documentation/dev-tools/kunit/usage.rst b/Documentation/dev-tools/kunit/usage.rst
index ebd06f5ea4550..25724f7e72969 100644
--- a/Documentation/dev-tools/kunit/usage.rst
+++ b/Documentation/dev-tools/kunit/usage.rst
@@ -157,6 +157,67 @@ Alternatively, one can take full control over the error message by using
 	if (some_setup_function())
 		KUNIT_FAIL(test, "Failed to setup thing for testing");
 
+Suppressing warning backtraces
+------------------------------
+
+Some unit tests trigger warning backtraces either intentionally or as a side
+effect. Such backtraces are normally undesirable since they distract from
+the actual test and may result in the impression that there is a problem.
+
+Backtraces can be suppressed with **task-scoped suppression**: while
+suppression is active on the current task, the backtrace and stack dump from
+``WARN*()``, ``WARN_ON*()``, and related macros on that task are suppressed.
+Three API forms are available, in order of convenience.
+
+- Scoped suppression is the simplest form. Wrap the code that triggers
+  warnings in a ``kunit_warning_suppress()`` block:
+
+.. code-block:: c
+
+	static void some_test(struct kunit *test)
+	{
+		kunit_warning_suppress(test) {
+			trigger_backtrace();
+			KUNIT_EXPECT_SUPPRESSED_WARNING_COUNT(test, 1);
+		}
+	}
+
+.. note::
+   The warning count must be checked inside the block; the suppression handle
+   is not accessible after the block exits.
+
+- Manual macros are useful when the suppressed region is large enough that
+  extra indentation is undesirable, or when the warning count needs to be
+  checked after suppression ends. ``KUNIT_START_SUPPRESSED_WARNING()`` must
+  appear before ``KUNIT_END_SUPPRESSED_WARNING()`` in the same scope.
+  Limited to one pair per scope.
+
+.. code-block:: c
+
+	static void some_test(struct kunit *test)
+	{
+		KUNIT_START_SUPPRESSED_WARNING(test);
+		trigger_backtrace();
+		KUNIT_END_SUPPRESSED_WARNING(test);
+
+		KUNIT_EXPECT_SUPPRESSED_WARNING_COUNT(test, 1);
+	}
+
+- Direct functions return an explicit handle pointer. Use them when the handle
+  needs to be retained or passed across helper functions:
+
+.. code-block:: c
+
+	static void some_test(struct kunit *test)
+	{
+		struct kunit_suppressed_warning *w;
+
+		w = kunit_start_suppress_warning(test);
+		trigger_backtrace();
+		kunit_end_suppress_warning(test, w);
+
+		KUNIT_EXPECT_EQ(test, kunit_suppressed_warning_count(w), 1);
+	}
 
 Test Suites
 ~~~~~~~~~~~
@@ -1211,4 +1272,4 @@ For example:
 		dev_managed_string = devm_kstrdup(fake_device, "Hello, World!");
 
 		// Everything is cleaned up automatically when the test ends.
-	}
\ No newline at end of file
+	}

-- 
2.53.0

Claude review: kunit: Add documentation for warning backtrace suppression API

[Claude Code Review Bot]

Patch Review

**Good documentation.** All three API forms are documented with clear examples and the correct ordering (scoped first as recommended, direct last as most verbose).

**Nit:** The documentation adds a trailing newline to the final line of `usage.rst`:
```diff
-	}
\ No newline at end of file
+	}
```
This is a drive-by fix for a pre-existing missing newline. Fine, but it might cause a trivial merge conflict if another patch touches the end of this file.

**Overall assessment:** This is a well-designed, well-tested series at v8 maturity. The core infrastructure is sound, the API is clean, and the test coverage is thorough. The series is ready to merge with no blocking issues. The observations above are minor and mostly informational.

---
Generated by Claude Code Patch Reviewer

[PATCH v7 0/5] kunit: Add support for suppressing warning backtraces

[Albert Esteve]

Some unit tests intentionally trigger warning backtraces by passing bad
parameters to kernel API functions. Such unit tests typically check the
return value from such calls, not the existence of the warning backtrace.

Such intentionally generated warning backtraces are neither desirable
nor useful for a number of reasons:
- They can result in overlooked real problems.
- A warning that suddenly starts to show up in unit tests needs to be
  investigated and has to be marked to be ignored, for example by
  adjusting filter scripts. Such filters are ad hoc because there is
  no real standard format for warnings. On top of that, such filter
  scripts would require constant maintenance.

One option to address the problem would be to add messages such as
"expected warning backtraces start/end here" to the kernel log.
However, that would again require filter scripts, might result in
missing real problematic warning backtraces triggered while the test
is running, and the irrelevant backtrace(s) would still clog the
kernel log.

Solve the problem by providing a means to identify and suppress specific
warning backtraces while executing test code. Support suppressing multiple
backtraces while at the same time limiting changes to generic code to the
absolute minimum.

Overview:
Patch#1 Introduces the suppression infrastructure.
Patch#2 Mitigates the impact of suppression.
Patch#3 Adds selftests to validate the functionality.
Patch#4 Demonstrates real-world usage in the DRM subsystem.
Patch#5 Documents the new API and usage guidelines.

Design Notes:
The objective is to suppress unwanted WARN*() generated messages.

Although most major architectures share common bug handling via `lib/bug.c`
and `report_bug()`, some minor or legacy architectures still rely on their
own platform-specific handling. This divergence must be considered in any
such feature. Additionally, a key challenge in implementing this feature is
the fragmentation of `WARN*()` message emission: part of the output is
produced in the macro itself (via __warn_printk()), and part in the exception
handler.

Lessons from the Previous Attempt:
In earlier iterations, suppression logic was added inside the
`__report_bug()` function to intercept WARN*() output. To implement the
check in the bug handler code, two strategies were considered:

* Strategy #1: Use `kallsyms` to infer the originating function. This
  approach proved unreliable due to compiler-induced transformations
  such as inlining, cloning, and code fragmentation.

* Strategy #2: Store function name `__func__` in `struct bug_entry` in
  the `__bug_table`. However, `__func__` is a compiler-generated symbol,
  which complicates relocation and linking in position-independent code.
  Additionally, architectures not using the unified `BUG()` path would
  still require ad-hoc handling.

A per-macro solution was also attempted (v5-v6), injecting checks
directly into the `WARN*()` macros in `include/asm-generic/bug.h`.
While this offered full control, it required modifying the generic
bug header and was considered too invasive and damaging the critical
path, and thus incorrect [1].

Current Proposal: Check in `warn_slowpath_fmt()` and `__report_bug()`.
Suppression is checked at two points in the warning path:
- In `warn_slowpath_fmt()` (kernel/panic.c), for architectures without
  __WARN_FLAGS. The check runs before any output, fully suppressing
  both message and backtrace.
- In `__report_bug()` (lib/bug.c), for architectures that define
  __WARN_FLAGS. The check runs before `__warn()` is called, suppressing
  the backtrace and stack dump. On this path, the `WARN()` format message
  may still appear in the kernel log since `__warn_printk()` executes
  before the trap.
This approach avoids modifying include/asm-generic/bug.h entirely,
requires no architecture-specific code, and limits changes to generic
code to the absolute minimum.

A helper function, `__kunit_is_suppressed_warning()`, walks an RCU-
protected list of active suppressions, matching by current task. The
suppression state is dynamically allocated via kunit_kzalloc() and
tied to the KUnit test lifecycle via kunit_add_action(), ensuring
automatic cleanup at test exit.

To minimize runtime impact when no suppressions are active, an atomic
counter tracks the number of active suppressions.
`__kunit_is_suppressed_warning()` checks this counter first and returns
immediately when it is zero, avoiding the RCU-protected list traversal
in the common case.

This series is based on the RFC patch and subsequent discussion at
https://patchwork.kernel.org/project/linux-kselftest/patch/02546e59-1afe-4b08-ba81-d94f3b691c9a@moroto.mountain/
and offers a more comprehensive solution of the problem discussed there.

[1] https://lore.kernel.org/all/CAGegRW76X8Fk_5qqOBw_aqBwAkQTsc8kXKHEuu9ECeXzdJwMSw@mail.gmail.com/

Changes since RFC:
- Introduced CONFIG_KUNIT_SUPPRESS_BACKTRACE
- Minor cleanups and bug fixes
- Added support for all affected architectures
- Added support for counting suppressed warnings
- Added unit tests using those counters
- Added patch to suppress warning backtraces in dev_addr_lists tests

Changes since v1:
- Rebased to v6.9-rc1
- Added Tested-by:, Acked-by:, and Reviewed-by: tags
  [I retained those tags since there have been no functional changes]
- Introduced KUNIT_SUPPRESS_BACKTRACE configuration option, enabled by
  default.

Changes since v2:
- Rebased to v6.9-rc2
- Added comments to drm warning suppression explaining why it is needed.
- Added patch to move conditional code in arch/sh/include/asm/bug.h
  to avoid kerneldoc warning
- Added architecture maintainers to Cc: for architecture specific patches
- No functional changes

Changes since v3:
- Rebased to v6.14-rc6
- Dropped net: "kunit: Suppress lock warning noise at end of dev_addr_lists tests"
  since 3db3b62955cd6d73afde05a17d7e8e106695c3b9
- Added __kunit_ and KUNIT_ prefixes.
- Tested on interessed architectures.

Changes since v4:
- Rebased to v6.15-rc7
- Dropped all code in __report_bug()
- Moved all checks in WARN*() macros.
- Dropped all architecture specific code.
- Made __kunit_is_suppressed_warning nice to noinstr functions.

Changes since v5:
- Rebased to v7.0-rc3
- Added RCU protection for the suppressed warnings list.
- Added static key and branching optimization.
- Removed custom `strcmp` implementation and reworked
  __kunit_is_suppressed_warning() entrypoint function.

Changes since v6:
- Moved suppression checks from WARN*() macros to warn_slowpath_fmt()
  and __report_bug().
- Replaced stack-allocated suppression struct with kunit_kzalloc() heap
  allocation tied to the KUnit test lifecycle.
- Changed suppression strategy from function-name matching to task-scoped:
  all warnings on the current task are suppressed between START and END,
  rather than only warnings originating from a specific named function.
- Simplified macro API: removed KUNIT_DECLARE_SUPPRESSED_WARNING(),
  the START macro now takes (test) and handles allocation internally.
- Removed static key and branching optiomization, as by the time it
  was executed, callers are already in warn slowpaths.
- Link to v6: https://lore.kernel.org/r/20260317-kunit_add_support-v6-0-dd22aeb3fe5d@redhat.com

Alessandro Carminati (2):
  bug/kunit: Core support for suppressing warning backtraces
  bug/kunit: Suppressing warning backtraces reduced impact on WARN*()
    sites

Guenter Roeck (3):
  Add unit tests to verify that warning backtrace suppression works.
  drm: Suppress intentional warning backtraces in scaling unit tests
  kunit: Add documentation for warning backtrace suppression API

 Documentation/dev-tools/kunit/usage.rst |  30 ++++++-
 drivers/gpu/drm/tests/drm_rect_test.c   |  16 ++++
 include/asm-generic/bug.h               |  48 +++++++----
 include/kunit/bug.h                     |  62 ++++++++++++++
 include/kunit/test.h                    |   1 +
 lib/kunit/Kconfig                       |   9 ++
 lib/kunit/Makefile                      |   9 +-
 lib/kunit/backtrace-suppression-test.c  | 105 ++++++++++++++++++++++++
 lib/kunit/bug.c                         |  54 ++++++++++++
 9 files changed, 316 insertions(+), 18 deletions(-)
 create mode 100644 include/kunit/bug.h
 create mode 100644 lib/kunit/backtrace-suppression-test.c
 create mode 100644 lib/kunit/bug.c

--
2.34.1

---
Alessandro Carminati (2):
      bug/kunit: Core support for suppressing warning backtraces
      bug/kunit: Reduce runtime impact of warning backtrace suppression

Guenter Roeck (3):
      kunit: Add backtrace suppression self-tests
      drm: Suppress intentional warning backtraces in scaling unit tests
      kunit: Add documentation for warning backtrace suppression API

 Documentation/dev-tools/kunit/usage.rst | 30 ++++++++++-
 drivers/gpu/drm/tests/drm_rect_test.c   | 14 +++++
 include/kunit/bug.h                     | 56 ++++++++++++++++++++
 include/kunit/test.h                    |  1 +
 kernel/panic.c                          |  8 ++-
 lib/bug.c                               |  8 +++
 lib/kunit/Kconfig                       |  9 ++++
 lib/kunit/Makefile                      |  9 +++-
 lib/kunit/backtrace-suppression-test.c  | 90 ++++++++++++++++++++++++++++++++
 lib/kunit/bug.c                         | 91 +++++++++++++++++++++++++++++++++
 10 files changed, 312 insertions(+), 4 deletions(-)
---
base-commit: 80234b5ab240f52fa45d201e899e207b9265ef91
change-id: 20260312-kunit_add_support-2f35806b19dd

Best regards,
-- 
Albert Esteve <aesteve@redhat.com>

[PATCH v7 5/5] kunit: Add documentation for warning backtrace suppression API

[Albert Esteve]

From: Guenter Roeck <linux@roeck-us.net>

Document API functions for suppressing warning backtraces.

Tested-by: Linux Kernel Functional Testing <lkft@linaro.org>
Acked-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Alessandro Carminati <acarmina@redhat.com>
Signed-off-by: Albert Esteve <aesteve@redhat.com>
---
 Documentation/dev-tools/kunit/usage.rst | 30 +++++++++++++++++++++++++++++-
 1 file changed, 29 insertions(+), 1 deletion(-)

diff --git a/Documentation/dev-tools/kunit/usage.rst b/Documentation/dev-tools/kunit/usage.rst
index ebd06f5ea4550..76e85412f240e 100644
--- a/Documentation/dev-tools/kunit/usage.rst
+++ b/Documentation/dev-tools/kunit/usage.rst
@@ -157,6 +157,34 @@ Alternatively, one can take full control over the error message by using
 	if (some_setup_function())
 		KUNIT_FAIL(test, "Failed to setup thing for testing");
 
+Suppressing warning backtraces
+------------------------------
+
+Some unit tests trigger warning backtraces either intentionally or as side
+effect. Such backtraces are normally undesirable since they distract from
+the actual test and may result in the impression that there is a problem.
+
+Such backtraces can be suppressed with **task scope suppression**: while
+``START`` / ``END`` is active on the current task, the backtrace and stack
+dump from warnings on that task are suppressed. Wrap the call from your test
+in that window, like shown in the following code.
+
+.. code-block:: c
+
+	static void some_test(struct kunit *test)
+	{
+		KUNIT_START_SUPPRESSED_WARNING(test);
+		trigger_backtrace();
+		KUNIT_END_SUPPRESSED_WARNING(test);
+	}
+
+``KUNIT_SUPPRESSED_WARNING_COUNT()`` returns the number of suppressed backtraces.
+If the suppressed backtrace was triggered on purpose, this can be used to check
+if the backtrace was actually triggered.
+
+.. code-block:: c
+
+	KUNIT_EXPECT_EQ(test, KUNIT_SUPPRESSED_WARNING_COUNT(), 1);
 
 Test Suites
 ~~~~~~~~~~~
@@ -1211,4 +1239,4 @@ For example:
 		dev_managed_string = devm_kstrdup(fake_device, "Hello, World!");
 
 		// Everything is cleaned up automatically when the test ends.
-	}
\ No newline at end of file
+	}

-- 
2.52.0

Claude review: kunit: Add documentation for warning backtrace suppression API

[Claude Code Review Bot]

Patch Review

**Documentation is clear and provides a good example**

The task-scoped suppression model is well-explained. The example showing `KUNIT_SUPPRESSED_WARNING_COUNT()` after `END` is correct because the struct memory remains valid (kunit_kzalloc'd) even after `kunit_release_action` removes it from the list.

**Minor: fixes missing newline at end of file**

```c
-	}
\ No newline at end of file
+	}
```

This is a drive-by fix, which is fine but could be noted in the commit message.

**Overall series verdict**: The concept and implementation are solid. The missing write-side spinlock for the RCU list is the main issue that should be fixed before merging. The other observations are minor.

---
Generated by Claude Code Patch Reviewer