[PATCH] drm/sched: Fix dynamic job-flow control race

[Philipp Stanner]

On Fri, 2024-09-13 at 09:53 -0700, Rob Clark wrote:
> From: Rob Clark <robdclark at chromium.org>
> 
> Fixes a race condition reported here:
> https://github.com/AsahiLinux/linux/issues/309#issuecomment-2238968609

As Danilo suggested before, I'd put this in a Fixes: section at the
bottom and instead have a sentence here detailing what the race
consists of, i.e., who is racing with whom.

P.

> 
> The whole premise of lockless access to a single-producer-single-
> consumer queue is that there is just a single producer and single
> consumer.  That means we can't call drm_sched_can_queue() (which is
> about queueing more work to the hw, not to the spsc queue) from
> anywhere other than the consumer (wq).
> 
> This call in the producer is just an optimization to avoid scheduling
> the consuming worker if it cannot yet queue more work to the hw.  It
> is safe to drop this optimization to avoid the race condition.
> 
> Suggested-by: Asahi Lina <lina at asahilina.net>
> Fixes: a78422e9dff3 ("drm/sched: implement dynamic job-flow control")
> Signed-off-by: Rob Clark <robdclark at chromium.org>
> ---
>  drivers/gpu/drm/scheduler/sched_main.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/scheduler/sched_main.c
> b/drivers/gpu/drm/scheduler/sched_main.c
> index ab53ab486fe6..1af1dbe757d5 100644
> --- a/drivers/gpu/drm/scheduler/sched_main.c
> +++ b/drivers/gpu/drm/scheduler/sched_main.c
> @@ -1020,8 +1020,7 @@ EXPORT_SYMBOL(drm_sched_job_cleanup);
>  void drm_sched_wakeup(struct drm_gpu_scheduler *sched,
>  		      struct drm_sched_entity *entity)
>  {
> -	if (drm_sched_can_queue(sched, entity))
> -		drm_sched_run_job_queue(sched);
> +	drm_sched_run_job_queue(sched);
>  }
>  
>  /**