[PATCH v5] drm/amdgpu: replace PASID IDR with XArray

[PATCH v5] drm/amdgpu: replace PASID IDR with XArray

[Mikhail Gavrilov]

Commit 8f1de51f49be ("drm/amdgpu: prevent immediate PASID reuse case")
converted the global PASID allocator from IDA to IDR with a spinlock
for cyclic allocation, but introduced two locking bugs:

1) idr_alloc_cyclic() is called with GFP_KERNEL under spin_lock(),
   which can sleep.

2) amdgpu_pasid_free() can be called from hardirq context via the
   fence signal path (amdgpu_pasid_free_cb), but the 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

   This was observed on an RX 7900 XTX when exiting a Vulkan game
   running under Proton/Wine, which triggers the fence callback path
   during VM teardown.

Replace the IDR + spinlock with XArray.  xa_alloc_cyclic() handles
GFP_KERNEL pre-allocation and IRQ-safe locking internally, so it is
used directly in amdgpu_pasid_alloc().  For amdgpu_pasid_free(), which
can be called from hardirq context, use explicit xa_lock_irqsave()
with __xa_erase() since xa_erase() only uses plain xa_lock() which
is not IRQ-safe.

Suggested-by: Lijo Lazar <lijo.lazar@amd.com>
Fixes: 8f1de51f49be ("drm/amdgpu: prevent immediate PASID reuse case")
Cc: stable@vger.kernel.org
Signed-off-by: Mikhail Gavrilov <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. Keep xa_alloc_cyclic() for amdgpu_pasid_alloc()
    as it handles locking internally. (Lijo Lazar)
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 | 47 ++++++++++++-------------
 1 file changed, 23 insertions(+), 24 deletions(-)

diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_ids.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_ids.c
index d88523568b62..3fbf631e67c7 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>
 
 
@@ -35,13 +35,13 @@
  * PASIDs are global address space identifiers that can be shared
  * between the GPU, an IOMMU and the driver. VMs on different devices
  * may use the same PASID if they share the same address
- * space. Therefore PASIDs are allocated using IDR cyclic allocator
- * (similar to kernel PID allocation) which naturally delays reuse.
- * VMs are looked up from the PASID per amdgpu_device.
+ * space. Therefore PASIDs are allocated using an XArray cyclic
+ * allocator (similar to kernel PID allocation) which naturally delays
+ * reuse. 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_ALLOC(amdgpu_pasid_xa);
+static u32 amdgpu_pasid_xa_next;
 
 /* Helper to free pasid from a fence callback */
 struct amdgpu_pasid_cb {
@@ -53,8 +53,7 @@ struct amdgpu_pasid_cb {
  * amdgpu_pasid_alloc - Allocate a PASID
  * @bits: Maximum width of the PASID in bits, must be at least 1
  *
- * Uses kernel's IDR cyclic allocator (same as PID allocation).
- * Allocates sequentially with automatic wrap-around.
+ * Uses XArray cyclic allocator for sequential allocation with wrap-around.
  *
  * Returns a positive integer on success. Returns %-EINVAL if bits==0.
  * Returns %-ENOSPC if no PASID was available. Returns %-ENOMEM on
@@ -62,20 +61,22 @@ 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);
-	pasid = idr_alloc_cyclic(&amdgpu_pasid_idr, NULL, 1,
-				 1U << bits, GFP_KERNEL);
-	spin_unlock(&amdgpu_pasid_idr_lock);
+	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 (pasid >= 0)
+	if (r >= 0) {
 		trace_amdgpu_pasid_allocated(pasid);
+		return pasid;
+	}
 
-	return pasid;
+	return r;
 }
 
 /**
@@ -84,11 +85,13 @@ int amdgpu_pasid_alloc(unsigned int bits)
  */
 void amdgpu_pasid_free(u32 pasid)
 {
+	unsigned long flags;
+
 	trace_amdgpu_pasid_freed(pasid);
 
-	spin_lock(&amdgpu_pasid_idr_lock);
-	idr_remove(&amdgpu_pasid_idr, pasid);
-	spin_unlock(&amdgpu_pasid_idr_lock);
+	xa_lock_irqsave(&amdgpu_pasid_xa, flags);
+	__xa_erase(&amdgpu_pasid_xa, pasid);
+	xa_unlock_irqrestore(&amdgpu_pasid_xa, flags);
 }
 
 static void amdgpu_pasid_free_cb(struct dma_fence *fence,
@@ -625,13 +628,9 @@ void amdgpu_vmid_mgr_fini(struct amdgpu_device *adev)
 }
 
 /**
- * amdgpu_pasid_mgr_cleanup - cleanup PASID manager
- *
- * Cleanup the IDR allocator.
+ * amdgpu_pasid_mgr_cleanup - Cleanup PASID manager
  */
 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

Re: [PATCH v5] drm/amdgpu: replace PASID IDR with XArray

[Eric Huang]

It looks good to me.

Reviewed-by: Eric Huang <jinhuieric.huang@amd.com>

On 2026-03-30 15:11, Mikhail Gavrilov wrote:
> Commit 8f1de51f49be ("drm/amdgpu: prevent immediate PASID reuse case")
> converted the global PASID allocator from IDA to IDR with a spinlock
> for cyclic allocation, but introduced two locking bugs:
>
> 1) idr_alloc_cyclic() is called with GFP_KERNEL under spin_lock(),
>     which can sleep.
>
> 2) amdgpu_pasid_free() can be called from hardirq context via the
>     fence signal path (amdgpu_pasid_free_cb), but the 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
>
>     This was observed on an RX 7900 XTX when exiting a Vulkan game
>     running under Proton/Wine, which triggers the fence callback path
>     during VM teardown.
>
> Replace the IDR + spinlock with XArray.  xa_alloc_cyclic() handles
> GFP_KERNEL pre-allocation and IRQ-safe locking internally, so it is
> used directly in amdgpu_pasid_alloc().  For amdgpu_pasid_free(), which
> can be called from hardirq context, use explicit xa_lock_irqsave()
> with __xa_erase() since xa_erase() only uses plain xa_lock() which
> is not IRQ-safe.
>
> Suggested-by: Lijo Lazar <lijo.lazar@amd.com>
> Fixes: 8f1de51f49be ("drm/amdgpu: prevent immediate PASID reuse case")
> Cc: stable@vger.kernel.org
> Signed-off-by: Mikhail Gavrilov <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. Keep xa_alloc_cyclic() for amdgpu_pasid_alloc()
>      as it handles locking internally. (Lijo Lazar)
> 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 | 47 ++++++++++++-------------
>   1 file changed, 23 insertions(+), 24 deletions(-)
>
> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_ids.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_ids.c
> index d88523568b62..3fbf631e67c7 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>
>   
>   
> @@ -35,13 +35,13 @@
>    * PASIDs are global address space identifiers that can be shared
>    * between the GPU, an IOMMU and the driver. VMs on different devices
>    * may use the same PASID if they share the same address
> - * space. Therefore PASIDs are allocated using IDR cyclic allocator
> - * (similar to kernel PID allocation) which naturally delays reuse.
> - * VMs are looked up from the PASID per amdgpu_device.
> + * space. Therefore PASIDs are allocated using an XArray cyclic
> + * allocator (similar to kernel PID allocation) which naturally delays
> + * reuse. 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_ALLOC(amdgpu_pasid_xa);
> +static u32 amdgpu_pasid_xa_next;
>   
>   /* Helper to free pasid from a fence callback */
>   struct amdgpu_pasid_cb {
> @@ -53,8 +53,7 @@ struct amdgpu_pasid_cb {
>    * amdgpu_pasid_alloc - Allocate a PASID
>    * @bits: Maximum width of the PASID in bits, must be at least 1
>    *
> - * Uses kernel's IDR cyclic allocator (same as PID allocation).
> - * Allocates sequentially with automatic wrap-around.
> + * Uses XArray cyclic allocator for sequential allocation with wrap-around.
>    *
>    * Returns a positive integer on success. Returns %-EINVAL if bits==0.
>    * Returns %-ENOSPC if no PASID was available. Returns %-ENOMEM on
> @@ -62,20 +61,22 @@ 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);
> -	pasid = idr_alloc_cyclic(&amdgpu_pasid_idr, NULL, 1,
> -				 1U << bits, GFP_KERNEL);
> -	spin_unlock(&amdgpu_pasid_idr_lock);
> +	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 (pasid >= 0)
> +	if (r >= 0) {
>   		trace_amdgpu_pasid_allocated(pasid);
> +		return pasid;
> +	}
>   
> -	return pasid;
> +	return r;
>   }
>   
>   /**
> @@ -84,11 +85,13 @@ int amdgpu_pasid_alloc(unsigned int bits)
>    */
>   void amdgpu_pasid_free(u32 pasid)
>   {
> +	unsigned long flags;
> +
>   	trace_amdgpu_pasid_freed(pasid);
>   
> -	spin_lock(&amdgpu_pasid_idr_lock);
> -	idr_remove(&amdgpu_pasid_idr, pasid);
> -	spin_unlock(&amdgpu_pasid_idr_lock);
> +	xa_lock_irqsave(&amdgpu_pasid_xa, flags);
> +	__xa_erase(&amdgpu_pasid_xa, pasid);
> +	xa_unlock_irqrestore(&amdgpu_pasid_xa, flags);
>   }
>   
>   static void amdgpu_pasid_free_cb(struct dma_fence *fence,
> @@ -625,13 +628,9 @@ void amdgpu_vmid_mgr_fini(struct amdgpu_device *adev)
>   }
>   
>   /**
> - * amdgpu_pasid_mgr_cleanup - cleanup PASID manager
> - *
> - * Cleanup the IDR allocator.
> + * amdgpu_pasid_mgr_cleanup - Cleanup PASID manager
>    */
>   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);
>   }

Re: [PATCH v5] drm/amdgpu: replace PASID IDR with XArray

[Lazar, Lijo]

On 31-Mar-26 12:41 AM, Mikhail Gavrilov wrote:
> Commit 8f1de51f49be ("drm/amdgpu: prevent immediate PASID reuse case")
> converted the global PASID allocator from IDA to IDR with a spinlock
> for cyclic allocation, but introduced two locking bugs:
> 
> 1) idr_alloc_cyclic() is called with GFP_KERNEL under spin_lock(),
>     which can sleep.
> 
> 2) amdgpu_pasid_free() can be called from hardirq context via the
>     fence signal path (amdgpu_pasid_free_cb), but the 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
> 
>     This was observed on an RX 7900 XTX when exiting a Vulkan game
>     running under Proton/Wine, which triggers the fence callback path
>     during VM teardown.
> 
> Replace the IDR + spinlock with XArray.  xa_alloc_cyclic() handles
> GFP_KERNEL pre-allocation and IRQ-safe locking internally, so it is
> used directly in amdgpu_pasid_alloc().  For amdgpu_pasid_free(), which
> can be called from hardirq context, use explicit xa_lock_irqsave()
> with __xa_erase() since xa_erase() only uses plain xa_lock() which
> is not IRQ-safe.
> 
> Suggested-by: Lijo Lazar <lijo.lazar@amd.com>
> Fixes: 8f1de51f49be ("drm/amdgpu: prevent immediate PASID reuse case")
> Cc: stable@vger.kernel.org
> Signed-off-by: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>

Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>

Thanks,
Lijo

> ---
> 
> 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. Keep xa_alloc_cyclic() for amdgpu_pasid_alloc()
>      as it handles locking internally. (Lijo Lazar)
> 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 | 47 ++++++++++++-------------
>   1 file changed, 23 insertions(+), 24 deletions(-)
> 
> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_ids.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_ids.c
> index d88523568b62..3fbf631e67c7 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>
>   
>   
> @@ -35,13 +35,13 @@
>    * PASIDs are global address space identifiers that can be shared
>    * between the GPU, an IOMMU and the driver. VMs on different devices
>    * may use the same PASID if they share the same address
> - * space. Therefore PASIDs are allocated using IDR cyclic allocator
> - * (similar to kernel PID allocation) which naturally delays reuse.
> - * VMs are looked up from the PASID per amdgpu_device.
> + * space. Therefore PASIDs are allocated using an XArray cyclic
> + * allocator (similar to kernel PID allocation) which naturally delays
> + * reuse. 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_ALLOC(amdgpu_pasid_xa);
> +static u32 amdgpu_pasid_xa_next;
>   
>   /* Helper to free pasid from a fence callback */
>   struct amdgpu_pasid_cb {
> @@ -53,8 +53,7 @@ struct amdgpu_pasid_cb {
>    * amdgpu_pasid_alloc - Allocate a PASID
>    * @bits: Maximum width of the PASID in bits, must be at least 1
>    *
> - * Uses kernel's IDR cyclic allocator (same as PID allocation).
> - * Allocates sequentially with automatic wrap-around.
> + * Uses XArray cyclic allocator for sequential allocation with wrap-around.
>    *
>    * Returns a positive integer on success. Returns %-EINVAL if bits==0.
>    * Returns %-ENOSPC if no PASID was available. Returns %-ENOMEM on
> @@ -62,20 +61,22 @@ 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);
> -	pasid = idr_alloc_cyclic(&amdgpu_pasid_idr, NULL, 1,
> -				 1U << bits, GFP_KERNEL);
> -	spin_unlock(&amdgpu_pasid_idr_lock);
> +	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 (pasid >= 0)
> +	if (r >= 0) {
>   		trace_amdgpu_pasid_allocated(pasid);
> +		return pasid;
> +	}
>   
> -	return pasid;
> +	return r;
>   }
>   
>   /**
> @@ -84,11 +85,13 @@ int amdgpu_pasid_alloc(unsigned int bits)
>    */
>   void amdgpu_pasid_free(u32 pasid)
>   {
> +	unsigned long flags;
> +
>   	trace_amdgpu_pasid_freed(pasid);
>   
> -	spin_lock(&amdgpu_pasid_idr_lock);
> -	idr_remove(&amdgpu_pasid_idr, pasid);
> -	spin_unlock(&amdgpu_pasid_idr_lock);
> +	xa_lock_irqsave(&amdgpu_pasid_xa, flags);
> +	__xa_erase(&amdgpu_pasid_xa, pasid);
> +	xa_unlock_irqrestore(&amdgpu_pasid_xa, flags);
>   }
>   
>   static void amdgpu_pasid_free_cb(struct dma_fence *fence,
> @@ -625,13 +628,9 @@ void amdgpu_vmid_mgr_fini(struct amdgpu_device *adev)
>   }
>   
>   /**
> - * amdgpu_pasid_mgr_cleanup - cleanup PASID manager
> - *
> - * Cleanup the IDR allocator.
> + * amdgpu_pasid_mgr_cleanup - Cleanup PASID manager
>    */
>   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);
>   }

Claude review: drm/amdgpu: replace PASID IDR with XArray

[Claude Code Review Bot]

Overall Series Review

Subject: drm/amdgpu: replace PASID IDR with XArray
Author: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>
Patches: 3
Reviewed: 2026-03-31T16:57:38.374656

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This is a single v5 patch converting amdgpu's PASID allocator from IDR+spinlock to XArray, fixing two real bugs introduced by commit 8f1de51f49be: sleeping under spinlock and a process/hardirq lock deadlock. The conversion is clean and well-motivated, but **the patch has a remaining deadlock vulnerability** — the same class of bug it's trying to fix.

The core issue: `xa_alloc_cyclic()` uses plain `xa_lock()` internally (IRQs remain enabled), while `amdgpu_pasid_free()` correctly uses `xa_lock_irqsave()`. If a hardirq fires on the same CPU while `xa_alloc_cyclic()` holds the xa_lock, and the IRQ handler calls `amdgpu_pasid_free()` → `xa_lock_irqsave()`, it will deadlock on the already-held lock — exactly the same pattern as the original bug.

**The fix is straightforward**: use `xa_alloc_cyclic_irq()` instead of `xa_alloc_cyclic()`. This variant (at `include/linux/xarray.h:1054`) takes and releases the xa_lock while disabling interrupts, which prevents the deadlock.

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Claude review: drm/amdgpu: replace PASID IDR with XArray

[Claude Code Review Bot]

Patch Review

**Commit message**: Excellent. The bug description is thorough, with clear lockdep-style annotation and a concrete reproduction scenario (RX 7900 XTX, Vulkan game under Proton). The changelog across v1-v5 is well documented.

**Bug — xa_alloc_cyclic() is not IRQ-safe** (line 70-72 of patched file):

```c
r = xa_alloc_cyclic(&amdgpu_pasid_xa, &pasid, xa_mk_value(0),
		    XA_LIMIT(1, (1U << bits) - 1),
		    &amdgpu_pasid_xa_next, GFP_KERNEL);
```

`xa_alloc_cyclic()` uses plain `xa_lock()` (see `include/linux/xarray.h:982`):
```c
xa_lock(xa);
err = __xa_alloc_cyclic(xa, id, entry, limit, next, gfp);
xa_unlock(xa);
```

Since `amdgpu_pasid_free()` can be called from hardirq context (the entire reason for this patch), an IRQ arriving while this lock is held will deadlock. This must be `xa_alloc_cyclic_irq()` which disables IRQs around the lock (`include/linux/xarray.h:1054`).

**Return value change** (lines 74-79): The original `idr_alloc_cyclic()` returned the allocated ID on success (positive) or negative errno. `xa_alloc_cyclic()` returns 0 on success and writes the ID to `*pasid`. The patch correctly handles this — checking `r >= 0` and returning `pasid` rather than `r`. Note that `xa_alloc_cyclic()` actually swallows the wrap-around indicator (returns 0 even on wrap), but `__xa_alloc_cyclic()` returns 1 on wrap. Since this code doesn't care about wrap notification, this is fine.

**Return value for -EBUSY** (line 79): The original IDR code returned `-ENOSPC` on exhaustion (per IDR semantics), while `xa_alloc_cyclic()` returns `-EBUSY`. The docstring at line 59 still says "Returns %-ENOSPC if no PASID was available" — this should be updated to `-EBUSY` to match reality. Minor documentation nit.

**amdgpu_pasid_free() locking** (lines 92-94): Correct. Using `xa_lock_irqsave()`/`__xa_erase()`/`xa_unlock_irqrestore()` is the right approach for hardirq context.

**xa_mk_value(0) as stored entry** (line 70): Storing `xa_mk_value(0)` works — XArray treats NULL as "empty", so an explicit non-NULL value is needed to mark the slot as occupied. `xa_mk_value(0)` is the lightest-weight way to do this. Fine.

**Cleanup** (line 261): `xa_destroy()` without locking is correct — this is called at module teardown when no concurrent access is possible.

**Recommendation**: NAK as-is due to the `xa_alloc_cyclic()` deadlock. The one-line fix is to change it to `xa_alloc_cyclic_irq()`. With that change, the patch would be correct and ready.

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