drm/i915/gt: Check the virtual still matches upon locking

If another sibling is able to claim the virtual request, by the time we
inspect the request under the lock it may no longer match the local
engine.

Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/2877Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Andi Shyti <andi.shyti@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210104115145.24460-4-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/gt/intel_execlists_submission.c

@@ -1016,6 +1016,9 @@ static bool virtual_matches(const struct virtual_engine *ve,
 {
 	const struct intel_engine_cs *inflight;
 
+	if (!rq)
+		return false;
+
 	if (!(rq->execution_mask & engine->mask)) /* We peeked too soon! */
 		return false;
 
@@ -1423,8 +1426,8 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
 		spin_lock(&ve->base.active.lock);
 
 		rq = ve->request;
-		if (unlikely(!rq)) /* lost the race to a sibling */
-			goto unlock;
+		if (unlikely(!virtual_matches(ve, rq, engine)))
+			goto unlock; /* lost the race to a sibling */
 
 		GEM_BUG_ON(rq->engine != &ve->base);
 		GEM_BUG_ON(rq->context != &ve->context);
@@ -1434,8 +1437,6 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
 			break;
 		}
 
-		GEM_BUG_ON(!virtual_matches(ve, rq, engine));
-
 		if (last && !can_merge_rq(last, rq)) {
 			spin_unlock(&ve->base.active.lock);
 			spin_unlock(&engine->active.lock);