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

[Eric Huang <jinhuieric.huang@amd.com>]

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);
>   }