[Intel-gfx] [PATCH] drm/i915: do not dereference dangling engine pointer on fence release

[Andrzej Hajda <andrzej.hajda@intel.com>]

rq->engine can be a dangling pointer if rq->execution_mask has more than
one bit set, ie it could be already freed virtual engine. Changing check
order prevents dereferncing it in intel_engine_is_virtual(rq->engine).
Full description of possible scenarios at the inline comment before
the change.

Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/7926
Signed-off-by: Andrzej Hajda <andrzej.hajda@intel.com>
---
 drivers/gpu/drm/i915/i915_request.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_request.c b/drivers/gpu/drm/i915/i915_request.c
index 630a732aaecca8..8775952f5c1bbd 100644
--- a/drivers/gpu/drm/i915/i915_request.c
+++ b/drivers/gpu/drm/i915/i915_request.c
@@ -166,8 +166,8 @@ static void i915_fence_release(struct dma_fence *fence)
 	 * know that if the rq->execution_mask is a single bit, rq->engine
 	 * can be a physical engine with the exact corresponding mask.
 	 */
-	if (!intel_engine_is_virtual(rq->engine) &&
-	    is_power_of_2(rq->execution_mask) &&
+	if (is_power_of_2(rq->execution_mask) &&
+	    !intel_engine_is_virtual(rq->engine) &&
 	    !cmpxchg(&rq->engine->request_pool, NULL, rq))
 		return;
 
-- 
2.34.1