Re: [PATCH v2] drm/sched: Document race condition in drm_sched_fini()

[Philipp Stanner <phasta@mailbox.org>]

On Wed, 2025-08-13 at 14:58 +0200, Danilo Krummrich wrote:
> On Wed Aug 13, 2025 at 10:56 AM CEST, Philipp Stanner wrote:
> > In drm_sched_fini() all entities are marked as stopped - without taking
> > the appropriate lock, because that would deadlock. That means that
> > drm_sched_fini() and drm_sched_entity_push_job() can race against each
> > other.
> > 
> > This should most likely be fixed by establishing the rule that all
> > entities associated with a scheduler must be torn down first. Then,
> > however, the locking should be removed from drm_sched_fini() alltogether
> > with an appropriate comment.
> > 
> > Reported-by: James Flowers <bold.zone2373@fastmail.com>
> > Link: https://lore.kernel.org/dri-devel/20250720235748.2798-1-bold.zone2373@fastmail.com/
> > Signed-off-by: Philipp Stanner <phasta@kernel.org>

Applied to drm-misc-next

> > ---
> > Changes in v2:
> >   - Fix typo.
> > ---
> >  drivers/gpu/drm/scheduler/sched_main.c | 16 ++++++++++++++++
> >  1 file changed, 16 insertions(+)
> > 
> > diff --git a/drivers/gpu/drm/scheduler/sched_main.c b/drivers/gpu/drm/scheduler/sched_main.c
> > index 5a550fd76bf0..46119aacb809 100644
> > --- a/drivers/gpu/drm/scheduler/sched_main.c
> > +++ b/drivers/gpu/drm/scheduler/sched_main.c
> > @@ -1424,6 +1424,22 @@ void drm_sched_fini(struct drm_gpu_scheduler *sched)
> >  			 * Prevents reinsertion and marks job_queue as idle,
> >  			 * it will be removed from the rq in drm_sched_entity_fini()
> >  			 * eventually
> > +			 *
> > +			 * FIXME:
> > +			 * This lacks the proper spin_lock(&s_entity->lock) and
> > +			 * is, therefore, a race condition. Most notably, it
> > +			 * can race with drm_sched_entity_push_job(). The lock
> > +			 * cannot be taken here, however, because this would
> > +			 * lead to lock inversion -> deadlock.
> > +			 *
> > +			 * The best solution probably is to enforce the life
> > +			 * time rule of all entities having to be torn down
> > +			 * before their scheduler. Then, however, locking could
> > +			 * be dropped alltogether from this function.
> 
> "Enforce the rule" is correct, since factually it's there, as a dependency in
> the code.
> 
> Do we know which drivers violate this lifetime rule?

I've got no idea :(

> 
> @Christian: What about amdgpu (for which the below was added to begin with)?

+1

P.

> 
> > +			 * For now, this remains a potential race in all
> > +			 * drivers that keep entities alive for longer than
> > +			 * the scheduler.
> >  			 */
> >  			s_entity->stopped = true;
> >  		spin_unlock(&rq->lock);
> > -- 
> > 2.49.0
>