[PATCH] drm/sched: Document race condition in drm_sched_fini()
Philipp Stanner
phasta at kernel.org
Thu Jul 31 09:30:09 UTC 2025
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 at fastmail.com>
Link: https://lore.kernel.org/dri-devel/20250720235748.2798-1-bold.zone2373@fastmail.com/
Signed-off-by: Philipp Stanner <phasta at kernel.org>
---
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..738aefed727c 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.
+ *
+ * For now, this remains a potential race in all
+ * drivers that keep entities aliver for longer than
+ * the scheduler.
*/
s_entity->stopped = true;
spin_unlock(&rq->lock);
--
2.49.0
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