[PATCH V2] drm/sched: fix fence reference count leak

[Qianyi Liu <liuqianyi125@gmail.com>]

From: qianyi liu <liuqianyi125@gmail.com>

We leaked last_scheduled fences when the entity was being killed and the
fence callback add fails.

To fix this, we should decrement the reference count of prev when
dma_fence_add_callback() fails, ensuring proper balance.

v2:
 * Make commit message more clearly. (Philipp and Matt)
 * Add "Fixes: " tag and put the stable kernel on Cc. (Philipp)
 * Correct subject line from "drm/scheduler" to "drm/sched". (Philipp)

Cc: stable@vger.kernel.org
Fixes: 2fdb8a8f07c2 ("drm/scheduler: rework entity flush, kill and fini")

Signed-off-by: qianyi liu <liuqianyi125@gmail.com>
---
 drivers/gpu/drm/scheduler/sched_entity.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/scheduler/sched_entity.c b/drivers/gpu/drm/scheduler/sched_entity.c
index 69bcf0e99d57..1c0c14bcf726 100644
--- a/drivers/gpu/drm/scheduler/sched_entity.c
+++ b/drivers/gpu/drm/scheduler/sched_entity.c
@@ -259,9 +259,12 @@ 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)) {
+			dma_fence_put(prev);
 			drm_sched_entity_kill_jobs_cb(NULL, &job->finish_cb);
+		}
 
 		prev = &s_fence->finished;
 	}
-- 
2.25.1