drm/sched: Document race condition in drm_sched_fini()

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 <[e-mail]>
Link: https://lore.kernel.org/dri-devel/[e-mail]/
Reviewed-by: Christian König <[e-mail]>
Signed-off-by: Philipp Stanner <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

drivers/gpu/drm/scheduler/sched_main.c

···
 			 * 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.
+			 *
+			 * 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);