Re: [RFC 2/4] drm/sched: Always wake up correct scheduler in drm_sched_entity_push_job

[Philipp Stanner <pstanner@redhat.com>]

On Fri, 2024-09-06 at 19:06 +0100, Tvrtko Ursulin wrote:
> From: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
> 
> Since drm_sched_entity_modify_sched() can modify the entities run
> queue
> lets make sure to only derefernce the pointer once so both adding and
> waking up are guaranteed to be consistent.
> 
> Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
> Fixes: b37aced31eb0 ("drm/scheduler: implement a function to modify
> sched list")
> Cc: Christian König <christian.koenig@amd.com>
> Cc: Alex Deucher <alexander.deucher@amd.com>
> Cc: Luben Tuikov <ltuikov89@gmail.com>
> Cc: Matthew Brost <matthew.brost@intel.com>
> Cc: David Airlie <airlied@gmail.com>
> Cc: Daniel Vetter <daniel@ffwll.ch>
> Cc: dri-devel@lists.freedesktop.org
> Cc: <stable@vger.kernel.org> # v5.7+
> ---
>  drivers/gpu/drm/scheduler/sched_entity.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/scheduler/sched_entity.c
> b/drivers/gpu/drm/scheduler/sched_entity.c
> index ae8be30472cd..62b07ef7630a 100644
> --- a/drivers/gpu/drm/scheduler/sched_entity.c
> +++ b/drivers/gpu/drm/scheduler/sched_entity.c
> @@ -599,6 +599,8 @@ void drm_sched_entity_push_job(struct
> drm_sched_job *sched_job)
>  
>  	/* first job wakes up scheduler */
>  	if (first) {
> +		struct drm_sched_rq *rq;
> +
>  		/* Add the entity to the run queue */
>  		spin_lock(&entity->rq_lock);
>  		if (entity->stopped) {
> @@ -608,13 +610,15 @@ void drm_sched_entity_push_job(struct
> drm_sched_job *sched_job)
>  			return;
>  		}
>  
> -		drm_sched_rq_add_entity(entity->rq, entity);
> +		rq = entity->rq;
> +
> +		drm_sched_rq_add_entity(rq, entity);
>  		spin_unlock(&entity->rq_lock);
>  
>  		if (drm_sched_policy == DRM_SCHED_POLICY_FIFO)
>  			drm_sched_rq_update_fifo(entity, submit_ts);
>  
> -		drm_sched_wakeup(entity->rq->sched, entity);
> +		drm_sched_wakeup(rq->sched, entity);

OK, I think that makes sense. But I'd mention that the more readable
solution of moving the spin_unlock() down here cannot be done because
drm_sched_rq_update_fifo() needs that same lock.

P.

>  	}
>  }
>  EXPORT_SYMBOL(drm_sched_entity_push_job);