[PATCH v7] drm/amdgpu: replace PASID IDR with XArray

[Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>]

Replace the PASID IDR + spinlock with XArray as noted in the TODO
left by commit dccd79bb1c7f ("drm/amdgpu: fix the idr allocation
flags").

The IDR conversion still has an IRQ safety issue:
amdgpu_pasid_free() can be called from hardirq context via the fence
signal path, but amdgpu_pasid_idr_lock is taken with plain spin_lock()
in process context, creating a potential deadlock:

     CPU0
     ----
     spin_lock(&amdgpu_pasid_idr_lock)   // process context, IRQs on
     <Interrupt>
       spin_lock(&amdgpu_pasid_idr_lock) // deadlock

   The hardirq call chain is:

     sdma_v6_0_process_trap_irq
      -> amdgpu_fence_process
       -> dma_fence_signal
        -> drm_sched_job_done
         -> dma_fence_signal
          -> amdgpu_pasid_free_cb
           -> amdgpu_pasid_free

Use XArray with XA_FLAGS_LOCK_IRQ (all xa operations use IRQ-safe
locking internally) and XA_FLAGS_ALLOC1 (zero is not a valid PASID).
Both xa_alloc_cyclic() and xa_erase() then handle locking
consistently, fixing the IRQ safety issue and removing the need for
an explicit spinlock.

Suggested-by: Lijo Lazar <lijo.lazar@amd.com>
Fixes: e6d765de3d6b ("drm/amdgpu: prevent immediate PASID reuse case")
Signed-off-by: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>
---

v7: Rebased on amd-staging-drm-next which already includes
    dccd79bb1c7f ("drm/amdgpu: fix the idr allocation flags").
    Updated commit message to reflect that sleeping-under-spinlock
    is already fixed and the xarray conversion now addresses the
    remaining IRQ safety issue.  Inverted error check to
    if (r < 0) return r; per Christian König.
v6: Use DEFINE_XARRAY_FLAGS with XA_FLAGS_LOCK_IRQ | XA_FLAGS_ALLOC1
    so all xa operations use IRQ-safe locking internally.  Drop
    Cc: stable since the regression was never released to any stable
    kernel. (Christian König)
    https://lore.kernel.org/all/20260331111733.118553-1-mikhail.v.gavrilov@gmail.com/
v5: Use explicit xa_lock_irqsave/__xa_erase for amdgpu_pasid_free()
    since xa_erase() only uses plain xa_lock() which is not safe from
    hardirq context.
    https://lore.kernel.org/all/20260330191120.105065-1-mikhail.v.gavrilov@gmail.com/
v4: Use xa_alloc_cyclic/xa_erase directly instead of explicit
    xa_lock_irqsave, as suggested by Lijo Lazar.
    https://lore.kernel.org/all/20260330162038.25073-1-mikhail.v.gavrilov@gmail.com/
v3: Replace IDR with XArray instead of fixing the spinlock, as
    suggested by Lijo Lazar.
    https://lore.kernel.org/all/20260330110346.16548-1-mikhail.v.gavrilov@gmail.com/
v2: Added second patch fixing the {HARDIRQ-ON-W} -> {IN-HARDIRQ-W}
    lock inconsistency (spin_lock -> spin_lock_irqsave).
    https://lore.kernel.org/all/20260330053025.19203-1-mikhail.v.gavrilov@gmail.com/
v1: Fixed sleeping-under-spinlock (idr_alloc_cyclic with GFP_KERNEL)
    using idr_preload/GFP_NOWAIT.
    https://lore.kernel.org/all/20260328213900.19255-1-mikhail.v.gavrilov@gmail.com/

 drivers/gpu/drm/amd/amdgpu/amdgpu_ids.c | 34 ++++++++++---------------
 1 file changed, 13 insertions(+), 21 deletions(-)

diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_ids.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_ids.c
index e495a8fa13fd..a6ac3b4ce0df 100644
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_ids.c
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_ids.c
@@ -22,7 +22,7 @@
  */
 #include "amdgpu_ids.h"
 
-#include <linux/idr.h>
+#include <linux/xarray.h>
 #include <linux/dma-fence-array.h>
 
 
@@ -40,8 +40,8 @@
  * VMs are looked up from the PASID per amdgpu_device.
  */
 
-static DEFINE_IDR(amdgpu_pasid_idr);
-static DEFINE_SPINLOCK(amdgpu_pasid_idr_lock);
+static DEFINE_XARRAY_FLAGS(amdgpu_pasid_xa, XA_FLAGS_LOCK_IRQ | XA_FLAGS_ALLOC1);
+static u32 amdgpu_pasid_xa_next;
 
 /* Helper to free pasid from a fence callback */
 struct amdgpu_pasid_cb {
@@ -62,22 +62,19 @@ struct amdgpu_pasid_cb {
  */
 int amdgpu_pasid_alloc(unsigned int bits)
 {
-	int pasid;
+	u32 pasid;
+	int r;
 
 	if (bits == 0)
 		return -EINVAL;
 
-	spin_lock(&amdgpu_pasid_idr_lock);
-	/* TODO: Need to replace the idr with an xarry, and then
-	 * handle the internal locking with ATOMIC safe paths.
-	 */
-	pasid = idr_alloc_cyclic(&amdgpu_pasid_idr, NULL, 1,
-				 1U << bits, GFP_ATOMIC);
-	spin_unlock(&amdgpu_pasid_idr_lock);
-
-	if (pasid >= 0)
-		trace_amdgpu_pasid_allocated(pasid);
+	r = xa_alloc_cyclic(&amdgpu_pasid_xa, &pasid, xa_mk_value(0),
+			    XA_LIMIT(1, (1U << bits) - 1),
+			    &amdgpu_pasid_xa_next, GFP_KERNEL);
+	if (r < 0)
+		return r;
 
+	trace_amdgpu_pasid_allocated(pasid);
 	return pasid;
 }
 
@@ -88,10 +85,7 @@ int amdgpu_pasid_alloc(unsigned int bits)
 void amdgpu_pasid_free(u32 pasid)
 {
 	trace_amdgpu_pasid_freed(pasid);
-
-	spin_lock(&amdgpu_pasid_idr_lock);
-	idr_remove(&amdgpu_pasid_idr, pasid);
-	spin_unlock(&amdgpu_pasid_idr_lock);
+	xa_erase(&amdgpu_pasid_xa, pasid);
 }
 
 static void amdgpu_pasid_free_cb(struct dma_fence *fence,
@@ -634,7 +628,5 @@ void amdgpu_vmid_mgr_fini(struct amdgpu_device *adev)
  */
 void amdgpu_pasid_mgr_cleanup(void)
 {
-	spin_lock(&amdgpu_pasid_idr_lock);
-	idr_destroy(&amdgpu_pasid_idr);
-	spin_unlock(&amdgpu_pasid_idr_lock);
+	xa_destroy(&amdgpu_pasid_xa);
 }
-- 
2.53.0