Re: [PATCH v3 20/36] drm/xe/vf: Avoid indefinite blocking in preempt rebind worker for VFs supporting migration

["Lis, Tomasz" <tomasz.lis@intel.com>]

On 9/29/2025 4:55 AM, Matthew Brost wrote:
> Blocking in work queues on a hardware action that may never occur —
> especially when it depends on a software fixup also scheduled on the
> awork queue — is a recipe for deadlock. This situation arises with
> the preempt rebind worker and VF post-migration recovery. To prevent
> potential deadlocks, avoid indefinite blocking in the preempt rebind
> worker for VFs that support migration.

Some would say the timeout value is a magic number here, but I don't 
have anything better to propose.

And we do not have obligation to match each tracepoint _enter() with 
_exit(), that's ok as well.

So, all good:

Reviewed-by: Tomasz Lis <tomasz.lis@intel.com>


> Signed-off-by: Matthew Brost <matthew.brost@intel.com>
> ---
>   drivers/gpu/drm/xe/xe_vm.c | 29 ++++++++++++++++++++++++++++-
>   1 file changed, 28 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/xe/xe_vm.c b/drivers/gpu/drm/xe/xe_vm.c
> index 80b7f13ecd80..b527ee2a5da5 100644
> --- a/drivers/gpu/drm/xe/xe_vm.c
> +++ b/drivers/gpu/drm/xe/xe_vm.c
> @@ -35,6 +35,7 @@
>   #include "xe_pt.h"
>   #include "xe_pxp.h"
>   #include "xe_res_cursor.h"
> +#include "xe_sriov_vf.h"
>   #include "xe_svm.h"
>   #include "xe_sync.h"
>   #include "xe_tile.h"
> @@ -111,12 +112,25 @@ static int alloc_preempt_fences(struct xe_vm *vm, struct list_head *list,
>   static int wait_for_existing_preempt_fences(struct xe_vm *vm)
>   {
>   	struct xe_exec_queue *q;
> +	bool vf_migration = IS_SRIOV_VF(vm->xe) &&
> +		xe_sriov_vf_migration_supported(vm->xe);
>   
>   	xe_vm_assert_held(vm);
>   
>   	list_for_each_entry(q, &vm->preempt.exec_queues, lr.link) {
>   		if (q->lr.pfence) {
> -			long timeout = dma_fence_wait(q->lr.pfence, false);
> +			long timeout;
> +
> +			if (vf_migration)
> +				timeout = dma_fence_wait_timeout(q->lr.pfence,
> +								 false, HZ / 5);
> +			else
> +				timeout = dma_fence_wait(q->lr.pfence, false);
> +
> +			if (!timeout) {
> +				xe_assert(vm->xe, vf_migration);
> +				return -EAGAIN;
> +			}
>   
>   			/* Only -ETIME on fence indicates VM needs to be killed */
>   			if (timeout < 0 || q->lr.pfence->error == -ETIME)
> @@ -541,6 +555,19 @@ static void preempt_rebind_work_func(struct work_struct *w)
>   out_unlock_outer:
>   	if (err == -EAGAIN) {
>   		trace_xe_vm_rebind_worker_retry(vm);
> +
> +		/*
> +		 * We can't block in workers on a VF which supports migration
> +		 * given this can block the VF post-migration workers from
> +		 * getting scheduled.
> +		 */
> +		if (IS_SRIOV_VF(vm->xe) &&
> +		    xe_sriov_vf_migration_supported(vm->xe)) {
> +			up_write(&vm->lock);
> +			xe_vm_queue_rebind_worker(vm);
> +			return;
> +		}
> +
>   		goto retry;
>   	}
>