[PATCH V4] drm/sched: Fix fence reference count leak

[Qianyi Liu <liuqianyi125@gmail.com>]

From: qianyi liu <liuqianyi125@gmail.com>

The last_scheduled fence leaks when an entity is being killed and adding
the cleanup callback fails.

Decrement the reference count of prev when dma_fence_add_callback()
fails, ensuring proper balance.

Cc: stable@vger.kernel.org
Fixes: 2fdb8a8f07c2 ("drm/scheduler: rework entity flush, kill and fini")
Signed-off-by: qianyi liu <liuqianyi125@gmail.com>
---
v3 -> v4: Improve commit message and add code comments (Philipp)
v2 -> v3: Rework commit message (Markus)
v1 -> v2: Added 'Fixes:' tag and clarified commit message (Philipp and Matthew)
---
 drivers/gpu/drm/scheduler/sched_entity.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/scheduler/sched_entity.c b/drivers/gpu/drm/scheduler/sched_entity.c
index 69bcf0e99d57..da00572d7d42 100644
--- a/drivers/gpu/drm/scheduler/sched_entity.c
+++ b/drivers/gpu/drm/scheduler/sched_entity.c
@@ -259,9 +259,16 @@ static void drm_sched_entity_kill(struct drm_sched_entity *entity)
 		struct drm_sched_fence *s_fence = job->s_fence;
 
 		dma_fence_get(&s_fence->finished);
-		if (!prev || dma_fence_add_callback(prev, &job->finish_cb,
-					   drm_sched_entity_kill_jobs_cb))
+		if (!prev ||
+		    dma_fence_add_callback(prev, &job->finish_cb,
+					   drm_sched_entity_kill_jobs_cb)) {
+			/*
+			 * Adding callback above failed.
+			 * dma_fence_put() checks for NULL.
+			 */
+			dma_fence_put(prev);
 			drm_sched_entity_kill_jobs_cb(NULL, &job->finish_cb);
+		}
 
 		prev = &s_fence->finished;
 	}
-- 
2.25.1