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

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

[Claude Code Review Bot]

Patch Review

**Missing write-side lock for RCU list (bug)**

The `suppressed_warnings` list is read under `rcu_read_lock()` correctly, but writes have no mutual exclusion:

```c
+static LIST_HEAD(suppressed_warnings);
...
+	list_add_rcu(&warning->node, &suppressed_warnings);
...
+	list_del_rcu(&warning->node);
```

The standard RCU list pattern requires a spinlock (or equivalent) protecting all write-side operations. Two concurrent calls to `__kunit_start_suppress_warning()` (e.g., from parallel test suites, or concurrent tests with `--run_concurrently`) would corrupt the list. This should be:

```c
static DEFINE_SPINLOCK(suppressed_warnings_lock);

// in __kunit_start_suppress_warning():
spin_lock(&suppressed_warnings_lock);
list_add_rcu(&warning->node, &suppressed_warnings);
spin_unlock(&suppressed_warnings_lock);

// in __kunit_suppress_warning_remove():
spin_lock(&suppressed_warnings_lock);
list_del_rcu(&warning->node);
spin_unlock(&suppressed_warnings_lock);
synchronize_rcu();
```

**`disable_trace_on_warning()` side effect for suppressed warnings**

In `__report_bug()`, the suppression check is placed after `disable_trace_on_warning()`:

```c
 	disable_trace_on_warning();
 
 	bug_get_file_line(bug, &file, &line);
 	fmt = bug_get_format(bug);
 
 	warning  = bug->flags & BUGFLAG_WARNING;
 ...
+	if (warning && __kunit_is_suppressed_warning())
+		return BUG_TRAP_TYPE_WARN;
```

A suppressed warning will still call `disable_trace_on_warning()` and decode the bug table entry, which is wasted work and potentially disruptive. Consider moving the suppression check earlier, or at least before `disable_trace_on_warning()`. That said, this would require restructuring since `warning` is derived from bug flags parsed afterward, so this may be intentional given the "minimum changes to generic code" goal.

**WARN_ON_ONCE behavior is correct**

The placement of the suppression check before the `once` logic is well-considered:

```c
+	if (warning && __kunit_is_suppressed_warning())
+		return BUG_TRAP_TYPE_WARN;
+
 	if (warning && once) {
 		if (done)
 			return BUG_TRAP_TYPE_WARN;
 		bug->flags |= BUGFLAG_DONE;
 	}
```

This correctly prevents suppressed warnings from consuming the single-fire budget of `WARN_ON_ONCE()`, and the comment explaining this is helpful.

**No failure reporting on allocation failure**

In `__kunit_start_suppress_warning()`, if `kunit_kzalloc()` fails, the function silently returns NULL and suppression is disabled. The macros handle NULL gracefully, but the test would proceed without suppression (potentially spewing backtraces) with no indication to the user. Consider adding a `kunit_warn()` or `KUNIT_ASSERT_NOT_NULL()` so that allocation failure is visible.

**Header design is clean**

The `#ifdef CONFIG_KUNIT_SUPPRESS_BACKTRACE` / `#else` stub pattern in `include/kunit/bug.h` is well-done, with the disabled path providing no-op macros and an inline false-returning function.

---
Generated by Claude Code Patch Reviewer

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

[Claude Code Review Bot]

Patch Review

**Architecture and design: Good.**

The hook integration is clean — one new function pointer in `kunit_hooks_table`, one static-inline wrapper in `test-bug.h`, implementation in `lib/kunit/bug.c`.

**Issue 1 (minor): `w->counter` increment is not atomic under concurrent warnings.**

In `__kunit_is_suppressed_warning_impl()`:
```c
	guard(rcu)();
	list_for_each_entry_rcu(w, &suppressed_warnings, node) {
		if (w->task == current) {
			if (count)
				w->counter++;
			return true;
		}
	}
```

The `w->counter++` is a non-atomic increment under RCU read-side lock. Since suppression is task-scoped (`w->task == current`), two different CPUs cannot match the same entry simultaneously for the same `current`, so this is actually safe. Worth noting that the cover letter's claim that "each KUnit test runs in its own kthread" is what makes this correct — if two threads ever shared a suppression entry, this would race. The current design correctly prevents that.

**Issue 2 (minor): `synchronize_rcu()` in `kunit_suppress_warning_remove()` may be called from kunit_release_action context.**

```c
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);
	synchronize_rcu(); /* Wait for readers to finish */
}
```

`synchronize_rcu()` can sleep. This is called via `kunit_release_action()` which calls the action function directly. In normal KUnit test teardown context this is fine (process context). However, confirm that `kunit_release_action` is never called from atomic context. Given that the existing kunit action infrastructure is designed for process context, this should be safe, but it's worth noting in a comment.

**Issue 3 (question): `kunit_has_active_suppress_warning()` is a public API but uses the implementation function.**

```c
bool kunit_has_active_suppress_warning(void)
{
	return __kunit_is_suppressed_warning_impl(false);
}
```

This calls the implementation function directly rather than going through the hooks table (`kunit_is_suppressed_warning(false)`). This is correct because `kunit_has_active_suppress_warning()` is only callable from within the kunit module itself (it's in `lib/kunit/bug.c`), so the hook indirection is unnecessary. Fine as-is.

**Issue 4 (minor): No nesting support — is this too restrictive?**

```c
	if (kunit_has_active_suppress_warning()) {
		KUNIT_FAIL(test, "Another suppression block is already active");
		return NULL;
	}
```

The series explicitly disallows nesting. This makes sense for simplicity, but the `backtrace_suppression_test_multi_scope` test in patch 2 uses sequential (not nested) suppression blocks via the direct API. Consider whether a test helper that itself needs to suppress warnings while the caller also has suppression active would be a valid use case. If so, this restriction may need revisiting. For v8, this is an acceptable simplification — it can always be relaxed later.

**Issue 5 (correctness): `__report_bug()` suppression check placement is good.**

```c
+#ifdef CONFIG_KUNIT
+	/*
+	 * 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;
+#endif
```

This is placed before the `BUGFLAG_ONCE` check, which is correct — a suppressed warning should not consume the once-budget. The `#ifdef CONFIG_KUNIT` guard is appropriate here (matching the style of `kunit_fail_current_test` usage elsewhere in kernel code), though the `kunit_is_suppressed_warning()` stub already returns false when CONFIG_KUNIT is disabled, so the `#ifdef` is belt-and-suspenders. Not a problem, just noting it's slightly redundant.

**Issue 6 (nit): The `__cleanup` attribute usage in the scoped API is well done.**

```c
#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)
```

This is a standard pattern. The `__cleanup` attribute fires if the loop exits via `break`, `return`, or `goto`. The for-loop increment handles normal exit. `kunit_add_action()` handles the case where the kthread exits abnormally (e.g., `kthread_exit` from a failed assertion). All three cleanup paths are covered. The `__kunit_suppress = NULL` in the increment expression ensures `__cleanup` doesn't double-free. Clean.

**Issue 7 (minor): `kunit_end_suppress_warning` does not check that suppression is actually active for that specific handle.**

```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);
}
```

Calling this with a stale or already-ended handle would hit `kunit_release_action` which would fail to find the action. This is fine — `kunit_release_action` handles the not-found case gracefully. No issue.

---
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 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 kunit_kzalloc() and
tied to the KUnit test lifecycle via kunit_add_action(), ensuring
automatic cleanup at test exit. On cleanup, the node is removed with
list_del_rcu() followed by synchronize_rcu() to wait for any concurrent
RCU readers to finish. Because kunit_end_suppress_warning() (and the
__cleanup wrapper) always runs from process context, synchronize_rcu()
is safe. The handle memory remains valid until the test exits, so the
suppression count can be read after the scope closes. Writer-side
access to the global suppression list is serialized with a spinlock;
readers use RCU. To avoid false suppression of warnings fired from
hardware interrupt handlers (where current still points to the test
task), the check exits early when not in task context.

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          | 127 +++++++++++++++++++++++++++++++++++++++++++++++
 lib/kunit/hooks-impl.h   |   2 +
 7 files changed, 276 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..6752b497aeefe
--- /dev/null
+++ b/lib/kunit/bug.c
@@ -0,0 +1,127 @@
+// 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/spinlock.h>
+
+#include "hooks-impl.h"
+
+struct kunit_suppressed_warning {
+	struct list_head node;
+	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_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);
+	synchronize_rcu(); /* Wait for readers to finish */
+}
+
+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 = kunit_kzalloc(test, sizeof(*w), GFP_KERNEL);
+	if (!w) {
+		KUNIT_FAIL(test, "Failed to allocate suppression handle.");
+		return NULL;
+	}
+
+	/*
+	 * Store current without taking a reference. The test task cannot
+	 * exit before kunit tears down the test, so the pointer is stable
+	 * for the lifetime of this handle.
+	 */
+	w->task = 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;
+
+	if (!in_task())
+		return false;
+
+	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

[PATCH v13 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

[PATCH v13 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

[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

Re: [PATCH v13 1/4] bug/kunit: Core support for suppressing warning backtraces

[Albert Esteve]

On Fri, May 15, 2026 at 2:29 PM Albert Esteve <aesteve@redhat.com> wrote:
>
> 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 kunit_kzalloc() and
> tied to the KUnit test lifecycle via kunit_add_action(), ensuring
> automatic cleanup at test exit. On cleanup, the node is removed with
> list_del_rcu() followed by synchronize_rcu() to wait for any concurrent
> RCU readers to finish. Because kunit_end_suppress_warning() (and the
> __cleanup wrapper) always runs from process context, synchronize_rcu()
> is safe. The handle memory remains valid until the test exits, so the
> suppression count can be read after the scope closes. Writer-side
> access to the global suppression list is serialized with a spinlock;
> readers use RCU. To avoid false suppression of warnings fired from
> hardware interrupt handlers (where current still points to the test
> task), the check exits early when not in task context.
>
> 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.

Let me address sashiko's comments for
https://sashiko.dev/#/patchset/20260515-kunit_add_support-v13-0-18ee42f96e7b%40redhat.com?part=1
here:

1. "Is this assumption always accurate? Tests frequently acquire
spinlocks or RCU read locks."
The assumption is accurate because kunit_end_suppress_warning() and
the __cleanup wrapper fire at the closing brace of the
kunit_warning_suppress() scope. If a developer holds a spinlock or RCU
read lock when that scope closes, their test is structurally incorrect
regardless of this API. The API documentation notes that process
context is required.

2. "If kunit_start_suppress_warning() fails and returns NULL, will
this skip the entire loop body?"
Yes, intentionally. KUNIT_FAIL() is called before returning NULL, so
the test is already marked as failed at that point. Skipping the body
of a failed test is expected KUnit behavior. In patch 3,
scaling_factor is initialized to INT_MIN precisely for this reason.

3. "Does this mean the single-fire budget is consumed anyway on
non-CONFIG_GENERIC_BUG architectures?"
Yes, true, but it should only affect non-__WARN_FLAGS architectures.
If there is demand, it can be addressed in a follow-up series. It does
not affect current API users.

4. "Would GFP_KERNEL / synchronize_rcu() cause a sleep-in-atomic bug
if used in atomic context?"
Yes, and that would be a test design error. kunit_warning_suppress()
is a KUnit test API; KUnit tests run in process context by design. As
stated in `Documentation/core-api/memory-allocation.rst`, GFP_KERNEL
implies GFP_RECLAIM, which requires the calling context to be allowed
to sleep. Using it in an atomic context is incorrect regardless of
which API calls it. `kunit_kzalloc(test, ..., GFP_KERNEL)` is the
standard allocation pattern throughout KUnit itself (e.g.,
lib/kunit/assert_test.c, platform-test.c, ...), so this API follows
the same convention.

5. "Could a child kthread's task_struct be freed and reused, causing
false suppression?"
The API is designed to be called from the test task only. w->task =
current stores the caller's task_struct, and the inline comment
explains the stability guarantee: the test task cannot exit before
KUnit tears down the test. The correct pattern for child kthread is
demonstrated in backtrace_suppression_test_cross_kthread: the test
task opens and closes the suppression scope; child threads only read
the suppression state. Otherwise it is an API misuse.

>
> 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          | 127 +++++++++++++++++++++++++++++++++++++++++++++++
>  lib/kunit/hooks-impl.h   |   2 +
>  7 files changed, 276 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..6752b497aeefe
> --- /dev/null
> +++ b/lib/kunit/bug.c
> @@ -0,0 +1,127 @@
> +// 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/spinlock.h>
> +
> +#include "hooks-impl.h"
> +
> +struct kunit_suppressed_warning {
> +       struct list_head node;
> +       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_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);
> +       synchronize_rcu(); /* Wait for readers to finish */
> +}
> +
> +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 = kunit_kzalloc(test, sizeof(*w), GFP_KERNEL);
> +       if (!w) {
> +               KUNIT_FAIL(test, "Failed to allocate suppression handle.");
> +               return NULL;
> +       }
> +
> +       /*
> +        * Store current without taking a reference. The test task cannot
> +        * exit before kunit tears down the test, so the pointer is stable
> +        * for the lifetime of this handle.
> +        */
> +       w->task = 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;
> +
> +       if (!in_task())
> +               return false;
> +
> +       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
>

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

[Guenter Roeck]

Hi Albert,

On 5/15/26 05:29, Albert Esteve wrote:
...

> 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
> 


How much of that is from me at this point ? Wouldn't it make sense to drop me
as "author" of those patches ?

I would not mind. I had the idea, but others like you are doing the hard work
of pushing it through.

Thanks,
Guenter

Re: [PATCH v13 2/4] kunit: Add backtrace suppression self-tests

[Albert Esteve]

On Fri, May 15, 2026 at 2:30 PM Albert Esteve <aesteve@redhat.com> wrote:
>
> 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.
>

Another set of sashiko comments for this patch
https://sashiko.dev/#/patchset/20260515-kunit_add_support-v13-0-18ee42f96e7b%40redhat.com?part=2
here:

1. CPU spike from while (!kthread_should_stop()) schedule()
Ha! I expected this one because I saw it in a previous review from the
bot. schedule() from TASK_RUNNING yields the CPU; it does not
spin-wait. The thread is rescheduled only when the scheduler gives it
time, not in a tight loop. But the important thing is that the window
where this loop actually runs is negligible: the parent calls
kthread_stop() immediately after wait_for_completion() returns. Using
set_current_state(TASK_INTERRUPTIBLE) would be slightly more
CPU-friendly, but for a test that probably runs and exits in
microseconds, it makes no practical difference. And it unnecessarily
adds complexity.

2. Orphaned kthread on early abort
This cannot happen in this test. The only KUNIT_ASSERT_* that could
abort early is KUNIT_ASSERT_FALSE(test, IS_ERR(task)). If that
assertion fails, it means kthread_run() itself returned an error,
therefore, the kthread was never started and there is nothing to
orphan. If kthread_run() succeeds, the assertion passes, and execution
continues sequentially to kthread_stop(). No code path allows a live
kthread to exist while bypassing kthread_stop().

> 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
>

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

[Albert Esteve]

On Fri, May 15, 2026 at 3:51 PM Guenter Roeck <linux@roeck-us.net> wrote:
>
> Hi Albert,
>
> On 5/15/26 05:29, Albert Esteve wrote:
> ...
>
> > 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
> >
>
>
> How much of that is from me at this point ? Wouldn't it make sense to drop me
> as "author" of those patches ?

Hi Guenter,

I do not mind authorship either. Signed-off-by lines already attest to
everyone's contribution.

In principle I am done with Sashiko's reviews. I addressed all
comments in their respective patches. Some lead to the rabbit holes I
put myself into for previous versions.

I do not plan to send a new version unless a human review requires a change.

If that happens, I may update the authorship. Otherwise, I'd keep it
as is, since you said you do not mind :)

Thanks!

BR,
Albert.

>
> I would not mind. I had the idea, but others like you are doing the hard work
> of pushing it through.
>
> Thanks,
> Guenter
>

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: 9
Reviewed: 2026-05-16T09:18:00.257637

---

This is a well-evolved v13 series (originally RFC by Alessandro Carminati, carried forward by Guenter Roeck, now stewarded by Albert Esteve) adding KUnit warning backtrace suppression. The design is solid: task-scoped suppression integrated into the existing KUnit hooks infrastructure, with zero overhead when no tests are running (via the `kunit_running` static branch). The series is cleanly split into infrastructure, tests, a real-world user, and documentation.

**Strengths:**
- Clean integration into existing KUnit hooks — no new config options, no arch-specific code
- Three suppression points covering both `__WARN_FLAGS` and non-`__WARN_FLAGS` architectures
- The `count` parameter to `kunit_is_suppressed_warning()` elegantly solves the double-counting problem
- The `in_task()` guard prevents false suppression from hardirq context
- Good test coverage including cross-kthread isolation
- Scoped API using `__cleanup` attribute is ergonomic

**Concerns (minor):**
- The `synchronize_rcu()` in cleanup path could add measurable latency in test suites with many suppression scopes, though it's correct
- The nesting prohibition (only one active suppression per task) may be limiting for complex test helpers, but is reasonable for v1

Overall this looks ready to merge with the reviews it already has (David Gow, Kees Cook). A few minor observations follow.

---

---
Generated by Claude Code Patch Reviewer

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

[Claude Code Review Bot]

Patch Review

**Architecture & correctness: Good**

The hook integration pattern follows the existing `fail_current_test` / `get_static_stub_address` approach exactly, which is the right way to extend this.

**`__report_bug()` changes (lib/bug.c):**

The placement of the suppression check *before* the `WARN_ON_ONCE` single-fire logic is a critical design choice and is correct:

```c
+	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;
 		bug->flags |= BUGFLAG_DONE;
 	}
```

This ensures suppressed warnings don't consume the single-fire budget of `WARN_ON_ONCE()`, which is the right behavior. The movement of `disable_trace_on_warning()` below the suppression check is also correct — no need to disable tracing for a suppressed warning.

**`kunit_is_suppressed_warning()` inline (test-bug.h):**

```c
+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);
+}
```

The NULL check on `is_suppressed_warning` is belt-and-suspenders since `kunit_install_hooks()` sets all pointers at module init, but it's harmless and consistent with defensive coding. Fine.

**`__kunit_is_suppressed_warning_impl()` (lib/kunit/bug.c):**

```c
+bool __kunit_is_suppressed_warning_impl(bool count)
+{
+	if (!in_task())
+		return false;
+
+	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;
+}
```

The `in_task()` check at the top is essential — a hardirq on the test task's CPU would see `current` pointing to the test task and could falsely suppress a real warning. Good.

**Lifecycle management:**

```c
+static void kunit_suppress_warning_remove(struct kunit_suppressed_warning *w)
+{
+	...
+	list_del_rcu(&w->node);
+	...
+	synchronize_rcu();
+}
```

The `synchronize_rcu()` after `list_del_rcu()` is the standard RCU removal pattern and is correct here since cleanup always runs from process context (kunit test teardown). The cover letter notes v12 tried `call_rcu()` but reverted to `synchronize_rcu()` — this is the simpler and safer choice.

**Minor observation:** `kunit_start_suppress_warning()` calls `kunit_has_active_suppress_warning()` which internally calls `__kunit_is_suppressed_warning_impl(false)`, walking the RCU list. This means every `kunit_start_suppress_warning()` does a list walk to check for nesting. For the expected use case (one suppression per test, short list) this is negligible.

**Potential issue — `kunit_add_action_or_reset` failure path:**

```c
+	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;
+	}
```

If `kunit_add_action_or_reset()` fails, the `_or_reset` variant calls the cleanup action itself, which does `list_del_rcu` + `synchronize_rcu`, so the node is properly removed from the list. The memory is managed by `kunit_kzalloc` so it will be freed at test exit. This is correct.

**Scoped macro:**

```c
+#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)
```

This is a well-constructed scoped cleanup pattern. The `for` loop body executes once (while `__kunit_suppress` is non-NULL), then the increment expression calls `kunit_end_suppress_warning` and sets the pointer to NULL, terminating the loop. If the body exits via `break`/`return`/`goto`, `__cleanup` fires `__kunit_suppress_auto_cleanup`. If `kunit_start_suppress_warning` returns NULL (failure), the body is skipped entirely, which silently swallows the failure — but `KUNIT_FAIL` was already called inside `kunit_start_suppress_warning`, so the test is already marked failed. Correct.

**No issues found in this patch.**

---

---
Generated by Claude Code Patch Reviewer

Claude review: kunit: Add backtrace suppression self-tests

[Claude Code Review Bot]

Patch Review

Good test coverage. Tests exercise:
- Direct WARN with message (`backtrace_suppression_test_warn_direct`)
- Indirect WARN through a helper (`backtrace_suppression_test_warn_indirect`)
- Multiple warnings in one scope (`backtrace_suppression_test_warn_multi`)
- WARN_ON (no format string) (`backtrace_suppression_test_warn_on_direct/indirect`)
- Incremental counting (`backtrace_suppression_test_count`)
- Active state transitions (`backtrace_suppression_test_active_state`)
- Sequential independent scopes via direct API (`backtrace_suppression_test_multi_scope`)
- Cross-kthread isolation (`backtrace_suppression_test_cross_kthread`)

**Cross-kthread test:**

```c
+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);
+}
```

This is well-structured. The `kthread_stop()` is inside the suppress block so the kthread is guaranteed to be stopped before the suppression scope exits. The `while (!kthread_should_stop()) schedule();` loop in the kthread function ensures the kthread stays alive until `kthread_stop()` is called, avoiding use-after-free.

**Minor observation:** The helper functions `trigger_backtrace_warn()` and `trigger_backtrace_warn_on()` are marked `noinline`, which is important — without this, the compiler could inline them into the test function, and on some architectures the WARN path might behave differently with regard to the return address. Good.

**No issues found.**

---

---
Generated by Claude Code Patch Reviewer

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

[Claude Code Review Bot]

Patch Review

**Good real-world demonstration of the API.**

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

The `INT_MIN` initialization is defensive — if the `kunit_warning_suppress` block fails to set up (returns NULL due to allocation failure), `scaling_factor` retains a sentinel value that won't accidentally match the expected value. The `#include <linux/limits.h>` is added for `INT_MIN`.

The `expected_warnings` computation is clever: on `CONFIG_BUG=n`, WARN_ON is a no-op, so the counter stays zero. On `CONFIG_BUG=y`, warnings fire only for the `-EINVAL` test cases (bad parameters). The boolean-to-int conversion `(params->expected_scaling_factor == -EINVAL)` yields 0 or 1, which is correct.

```c
+	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);
```

The `KUNIT_EXPECT_EQ` check on `scaling_factor` is correctly placed *outside* the suppress block — it doesn't need suppression and this keeps the suppress scope as narrow as possible. The warning count check is inside the block as required by the API.

**No issues found.**

---

---
Generated by Claude Code Patch Reviewer

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

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

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

[Claude Code Review Bot]

Patch Review

**Overall: Good.** This is the core infrastructure patch and is well-designed.

**`disable_trace_on_warning()` move in `lib/bug.c`:**

The patch moves `disable_trace_on_warning()` to after the suppression check and after the flag extraction block:

```c
+#ifdef CONFIG_KUNIT
+	if (warning && kunit_is_suppressed_warning(true))
+		return BUG_TRAP_TYPE_WARN;
+#endif
+
+	disable_trace_on_warning();
+
 	if (warning && once) {
```

This is correct: suppressed warnings should not disable tracing, and importantly the comment says the check is placed "before the once logic so suppressed warnings do not consume the single-fire budget of WARN_ON_ONCE()." Good design choice.

However, the `disable_trace_on_warning()` move also affects non-KUnit code paths. Previously it ran unconditionally early; now it runs after the flag extraction. This is a minor semantic change for BUG (non-warning) traps — `disable_trace_on_warning()` now runs slightly later but still before any output. Should be harmless but worth noting.

**`#ifdef CONFIG_KUNIT` in `lib/bug.c`:**

```c
+#ifdef CONFIG_KUNIT
+	if (warning && kunit_is_suppressed_warning(true))
+		return BUG_TRAP_TYPE_WARN;
+#endif
```

Using `#ifdef CONFIG_KUNIT` rather than `IS_ENABLED(CONFIG_KUNIT)` is slightly inconsistent with the inline in `test-bug.h` which uses `IS_ENABLED`. However, since `kunit_is_suppressed_warning()` isn't available without the header, and `test-bug.h` provides a stub when `!CONFIG_KUNIT`, this is fine — the `#ifdef` avoids the header include cost for the `!CONFIG_KUNIT` case. But the `#include <kunit/test-bug.h>` is already added unconditionally at the top of `lib/bug.c`, so the `#ifdef` guard is redundant — you could just use the inline stub. This is a style nit, not a bug.

**Race in `kunit_start_suppress_warning`:**

```c
+	if (kunit_has_active_suppress_warning()) {
+		KUNIT_FAIL(test, "Another suppression block is already active");
+		return NULL;
+	}
```

The check-then-act between `kunit_has_active_suppress_warning()` and `list_add_rcu()` is not atomic, but this is fine in practice because KUnit tests run in their own kthread and nesting is explicitly disallowed — two concurrent calls for the same task can't happen.

**`kunit_has_active_suppress_warning()` is exported but not declared in `test.h`:**

The function is declared in `test.h` (line 597 in the patch). Confirmed, this is fine.

**Lifetime management is correct:** `get_task_struct()` pins the task to prevent ABA reuse, and `kunit_add_action_or_reset` ensures cleanup. The `kunit_end_suppress_warning` -> `kunit_release_action` path removes from the list and `put_task_struct`. Good.

**`__kunit_suppress_auto_cleanup` accessing `(*wp)->test`:**

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

This works because `w->test` is set during `kunit_start_suppress_warning` and remains valid — the test struct outlives the suppression handle. Clean.

**No `EXPORT_SYMBOL_GPL` for `__kunit_is_suppressed_warning_impl`:** This function is not exported, which is correct since it's only called through the hook table indirection. The hook pointer is set in `kunit_install_hooks()` which runs at module init.

---
Generated by Claude Code Patch Reviewer

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

[Claude Code Review Bot]

Patch Review

**test-bug.h changes — Good:**

The `kunit_is_suppressed_warning()` inline correctly gates on the existing `kunit_running` static branch before dereferencing the hook pointer:

```c
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);
}
```

The NULL-check on `kunit_hooks.is_suppressed_warning` before calling is important for the window between kunit_running being enabled and hooks being installed. The `#include <linux/types.h>` for `bool` is the right fix for `CONFIG_KUNIT=n` builds.

**kernel/panic.c changes — Good:**

```c
if (kunit_is_suppressed_warning(true)) {
	warn_rcu_exit(rcu);
	return;
}
```

In `warn_slowpath_fmt()`, the check with `count=true` is placed after `warn_rcu_enter()` and correctly exits via `warn_rcu_exit()` before returning. Same pattern in `__warn_printk()` but with `count=false` to avoid double-counting. This is correct — on architectures with `__WARN_FLAGS`, both `__warn_printk()` and `__report_bug()` fire for the same warning; counting only in `__report_bug()` ensures one increment per warning.

**lib/bug.c changes — Correct but worth noting:**

```c
#ifdef CONFIG_KUNIT
	if (warning && kunit_is_suppressed_warning(true))
		return BUG_TRAP_TYPE_WARN;
#endif

	disable_trace_on_warning();
```

The `disable_trace_on_warning()` is moved after the suppression check, which means suppressed warnings won't disable tracing. This is the right behavior — suppressed warnings should have zero side effects. The `#ifdef CONFIG_KUNIT` guard avoids a function call in non-KUnit builds.

Placing the check before the `once` logic is correct: suppressed WARN_ON_ONCE() calls don't consume the single-fire budget, so they remain armed for non-test contexts.

**bug.c (new file) — Minor observations:**

The `synchronize_rcu()` in `kunit_suppress_warning_remove()` is correct to ensure no readers see a stale pointer, but it means removal is a blocking call. This is fine for test teardown paths but means `kunit_end_suppress_warning()` (which calls `kunit_release_action()` → `kunit_suppress_warning_remove()`) blocks on a grace period. In test contexts this is acceptable.

The `__kunit_is_suppressed_warning_impl()` function:
```c
bool __kunit_is_suppressed_warning_impl(bool count)
{
	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;
}
```

This is called from `__warn_printk()` which can be in `noinstr` context on some architectures. The cover letter mentions this was addressed ("Made __kunit_is_suppressed_warning nice to noinstr functions"), but the static branch check in the inline wrapper ensures this function is only reached when KUnit is running, which won't happen in early boot noinstr paths. The `guard(rcu)()` use is clean.

**One subtle issue:** `kunit_start_suppress_warning()` calls `kunit_has_active_suppress_warning()` which calls `__kunit_is_suppressed_warning_impl(false)`. This does an RCU-protected list walk to check if suppression is active for `current`. But the caller hasn't taken any locks for the list yet — this is fine because the check is inherently racy (the list could change between check and `list_add_rcu()`), but since all calls are from the same kthread and suppression is per-task, no other thread would be adding entries for our task. So the check-then-act pattern is safe here.

---
Generated by Claude Code Patch Reviewer