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

[Danilo Krummrich]

On Mon, Sep 16, 2024 at 10:21:21AM +0200, Philipp Stanner wrote:
> 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

I think you mean 'Closes'.

Please note that there's a v2 [1] already, which does that.

[1] https://lore.kernel.org/dri-devel/20240913202301.16772-1-robdclark@gmail.com/

> bottom and instead have a sentence here detailing what the race
> consists of, i.e., who is racing with whom.

That's written right below, isn't it?

> 
> 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);
> >  }
> >  
> >  /**
>