[RFC 1/4] drm/sched: Add locking to drm_sched_entity_modify_sched

[Tvrtko Ursulin <tursulin@igalia.com>]

From: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>

Without the locking amdgpu currently can race
amdgpu_ctx_set_entity_priority() and drm_sched_job_arm(), 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.

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);
 
-- 
2.46.0