Re: [PATCH v3] drm/sched: Call drm_sched_fence_set_parent() from drm_sched_fence_scheduled()

[Boris Brezillon <boris.brezillon@collabora.com>]

On Fri, 23 Jun 2023 09:52:04 +0200
Boris Brezillon <boris.brezillon@collabora.com> wrote:

> Drivers that can delegate waits to the firmware/GPU pass the scheduled
> fence to drm_sched_job_add_dependency(), and issue wait commands to
> the firmware/GPU at job submission time. For this to be possible, they
> need all their 'native' dependencies to have a valid parent since this
> is where the actual HW fence information are encoded.
> 
> In drm_sched_main(), we currently call drm_sched_fence_set_parent()
> after drm_sched_fence_scheduled(), leaving a short period of time
> during which the job depending on this fence can be submitted.
> 
> Since setting parent and signaling the fence are two things that are
> kinda related (you can't have a parent if the job hasn't been scheduled),
> it probably makes sense to pass the parent fence to
> drm_sched_fence_scheduled() and let it call drm_sched_fence_set_parent()
> before it signals the scheduled fence.
> 
> Here is a detailed description of the race we are fixing here:
> 
> Thread A				Thread B
> 
> - calls drm_sched_fence_scheduled()
> - signals s_fence->scheduled which
>   wakes up thread B
> 
> 					- entity dep signaled, checking
> 					  the next dep
> 					- no more deps waiting
> 					- entity is picked for job
> 					  submission by drm_gpu_scheduler
> 					- run_job() is called
> 					- run_job() tries to
> 					  collect native fence info from
> 					  s_fence->parent, but it's
> 					  NULL =>
> 					  BOOM, we can't do our native
> 					  wait
> 
> - calls drm_sched_fence_set_parent()
> 
> v2:
> * Fix commit message
> 
> v3:
> * Add a detailed description of the race to the commit message
> * Add Luben's R-b
> 

FYI, I didn't put a Fixes tag because the various moves/modifications
that happened on this file will make it hard to backport anyway, and no
one complained about it so far. But if we want to have one, it would
probably be:

Fixes: 754ce0fa55c4 ("drm/amd: add parent for sched fence")

> Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
> Cc: Frank Binns <frank.binns@imgtec.com>
> Cc: Sarah Walker <sarah.walker@imgtec.com>
> Cc: Donald Robson <donald.robson@imgtec.com>
> Cc: Luben Tuikov <luben.tuikov@amd.com>
> Cc: David Airlie <airlied@gmail.com>
> Cc: Daniel Vetter <daniel@ffwll.ch>
> Cc: Sumit Semwal <sumit.semwal@linaro.org>
> Cc: "Christian König" <christian.koenig@amd.com>
> Reviewed-by: Luben Tuikov <luben.tuikov@amd.com>
> ---
>  drivers/gpu/drm/scheduler/sched_fence.c | 40 +++++++++++++++----------
>  drivers/gpu/drm/scheduler/sched_main.c  |  3 +-
>  include/drm/gpu_scheduler.h             |  5 ++--
>  3 files changed, 28 insertions(+), 20 deletions(-)
> 
> diff --git a/drivers/gpu/drm/scheduler/sched_fence.c b/drivers/gpu/drm/scheduler/sched_fence.c
> index fe9c6468e440..b6e70ddb4ee5 100644
> --- a/drivers/gpu/drm/scheduler/sched_fence.c
> +++ b/drivers/gpu/drm/scheduler/sched_fence.c
> @@ -48,8 +48,32 @@ static void __exit drm_sched_fence_slab_fini(void)
>  	kmem_cache_destroy(sched_fence_slab);
>  }
>  
> -void drm_sched_fence_scheduled(struct drm_sched_fence *fence)
> +static void drm_sched_fence_set_parent(struct drm_sched_fence *s_fence,
> +				       struct dma_fence *fence)
>  {
> +	/*
> +	 * smp_store_release() to ensure another thread racing us
> +	 * in drm_sched_fence_set_deadline_finished() sees the
> +	 * fence's parent set before test_bit()
> +	 */
> +	smp_store_release(&s_fence->parent, dma_fence_get(fence));
> +	if (test_bit(DRM_SCHED_FENCE_FLAG_HAS_DEADLINE_BIT,
> +		     &s_fence->finished.flags))
> +		dma_fence_set_deadline(fence, s_fence->deadline);
> +}
> +
> +void drm_sched_fence_scheduled(struct drm_sched_fence *fence,
> +			       struct dma_fence *parent)
> +{
> +	/* Set the parent before signaling the scheduled fence, such that,
> +	 * any waiter expecting the parent to be filled after the job has
> +	 * been scheduled (which is the case for drivers delegating waits
> +	 * to some firmware) doesn't have to busy wait for parent to show
> +	 * up.
> +	 */
> +	if (!IS_ERR_OR_NULL(parent))
> +		drm_sched_fence_set_parent(fence, parent);
> +
>  	dma_fence_signal(&fence->scheduled);
>  }
>  
> @@ -179,20 +203,6 @@ struct drm_sched_fence *to_drm_sched_fence(struct dma_fence *f)
>  }
>  EXPORT_SYMBOL(to_drm_sched_fence);
>  
> -void drm_sched_fence_set_parent(struct drm_sched_fence *s_fence,
> -				struct dma_fence *fence)
> -{
> -	/*
> -	 * smp_store_release() to ensure another thread racing us
> -	 * in drm_sched_fence_set_deadline_finished() sees the
> -	 * fence's parent set before test_bit()
> -	 */
> -	smp_store_release(&s_fence->parent, dma_fence_get(fence));
> -	if (test_bit(DRM_SCHED_FENCE_FLAG_HAS_DEADLINE_BIT,
> -		     &s_fence->finished.flags))
> -		dma_fence_set_deadline(fence, s_fence->deadline);
> -}
> -
>  struct drm_sched_fence *drm_sched_fence_alloc(struct drm_sched_entity *entity,
>  					      void *owner)
>  {
> diff --git a/drivers/gpu/drm/scheduler/sched_main.c b/drivers/gpu/drm/scheduler/sched_main.c
> index aea5a90ff98b..eff0a7f42f69 100644
> --- a/drivers/gpu/drm/scheduler/sched_main.c
> +++ b/drivers/gpu/drm/scheduler/sched_main.c
> @@ -1040,10 +1040,9 @@ static int drm_sched_main(void *param)
>  		trace_drm_run_job(sched_job, entity);
>  		fence = sched->ops->run_job(sched_job);
>  		complete_all(&entity->entity_idle);
> -		drm_sched_fence_scheduled(s_fence);
> +		drm_sched_fence_scheduled(s_fence, fence);
>  
>  		if (!IS_ERR_OR_NULL(fence)) {
> -			drm_sched_fence_set_parent(s_fence, fence);
>  			/* Drop for original kref_init of the fence */
>  			dma_fence_put(fence);
>  
> diff --git a/include/drm/gpu_scheduler.h b/include/drm/gpu_scheduler.h
> index c0586d832260..b29e347b10a9 100644
> --- a/include/drm/gpu_scheduler.h
> +++ b/include/drm/gpu_scheduler.h
> @@ -582,15 +582,14 @@ void drm_sched_entity_set_priority(struct drm_sched_entity *entity,
>  				   enum drm_sched_priority priority);
>  bool drm_sched_entity_is_ready(struct drm_sched_entity *entity);
>  
> -void drm_sched_fence_set_parent(struct drm_sched_fence *s_fence,
> -				struct dma_fence *fence);
>  struct drm_sched_fence *drm_sched_fence_alloc(
>  	struct drm_sched_entity *s_entity, void *owner);
>  void drm_sched_fence_init(struct drm_sched_fence *fence,
>  			  struct drm_sched_entity *entity);
>  void drm_sched_fence_free(struct drm_sched_fence *fence);
>  
> -void drm_sched_fence_scheduled(struct drm_sched_fence *fence);
> +void drm_sched_fence_scheduled(struct drm_sched_fence *fence,
> +			       struct dma_fence *parent);
>  void drm_sched_fence_finished(struct drm_sched_fence *fence);
>  
>  unsigned long drm_sched_suspend_timeout(struct drm_gpu_scheduler *sched);