On Fri, 2024-09-06 at 19:06 +0100, Tvrtko Ursulin wrote:
From: Tvrtko Ursulin tvrtko.ursulin@igalia.com
Without the locking amdgpu currently can race amdgpu_ctx_set_entity_priority() and drm_sched_job_arm(),
I would explicitly say "amdgpu's amdgpu_ctx_set_entity_priority() races through drm_sched_entity_modify_sched() with drm_sched_job_arm()".
The actual issue then seems to be drm_sched_job_arm() calling drm_sched_entity_select_rq(). I would mention that, too.
leading to the latter accesing potentially inconsitent entity->sched_list and entity->num_sched_list pair.
The comment on drm_sched_entity_modify_sched() however says:
""" * Note that this must be called under the same common lock for @entity as * drm_sched_job_arm() and drm_sched_entity_push_job(), or the driver needs to * guarantee through some other means that this is never called while new jobs * can be pushed to @entity. """
It is unclear if that is referring to this race or something else.
That comment is indeed a bit awkward. Both drm_sched_entity_push_job() and drm_sched_job_arm() take rq_lock. But drm_sched_entity_modify_sched() doesn't.
The comment was written in 981b04d968561. Interestingly, in drm_sched_entity_push_job(), this "common lock" is mentioned with the soft requirement word "should" and apparently is more about keeping sequence numbers in order when inserting.
I tend to think that the issue discovered by you is unrelated to that comment. But if no one can make sense of the comment, should it maybe be removed? Confusing comment is arguably worse than no comment
P.
Signed-off-by: Tvrtko Ursulin tvrtko.ursulin@igalia.com Fixes: b37aced31eb0 ("drm/scheduler: implement a function to modify sched list") Cc: Christian König christian.koenig@amd.com Cc: Alex Deucher alexander.deucher@amd.com Cc: Luben Tuikov ltuikov89@gmail.com Cc: Matthew Brost matthew.brost@intel.com Cc: David Airlie airlied@gmail.com Cc: Daniel Vetter daniel@ffwll.ch Cc: dri-devel@lists.freedesktop.org Cc: stable@vger.kernel.org # v5.7+
drivers/gpu/drm/scheduler/sched_entity.c | 2 ++ 1 file changed, 2 insertions(+)
diff --git a/drivers/gpu/drm/scheduler/sched_entity.c b/drivers/gpu/drm/scheduler/sched_entity.c index 58c8161289fe..ae8be30472cd 100644 --- a/drivers/gpu/drm/scheduler/sched_entity.c +++ b/drivers/gpu/drm/scheduler/sched_entity.c @@ -133,8 +133,10 @@ void drm_sched_entity_modify_sched(struct drm_sched_entity *entity, { WARN_ON(!num_sched_list || !sched_list);
- spin_lock(&entity->rq_lock);
entity->sched_list = sched_list; entity->num_sched_list = num_sched_list;
- spin_unlock(&entity->rq_lock);
} EXPORT_SYMBOL(drm_sched_entity_modify_sched);