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

[Rob Clark]

On Fri, Sep 13, 2024 at 10:03 AM Michel Dänzer
<michel.daenzer at mailbox.org> wrote:
>
> On 2024-09-13 18:53, 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
> >
> > 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);
> >  }
> >
> >  /**
>
> The entity parameter is unused now.

Right.. and we probably should collapse drm_sched_wakeup() and
drm_sched_run_job_queue()..

But this fix needs to be cherry picked back to a bunch of release
branches, so I intentionally avoided refactoring as part of the fix.

BR,
-R

>
>
> --
> Earthling Michel Dänzer       \        GNOME / Xwayland / Mesa developer
> https://redhat.com             \               Libre software enthusiast