dma-buf: fix busy wait for new shared fences

When reservation_object_add_shared_fence is replacing an old fence with a new
one we should not drop the old one before the new one is in place.

Otherwise other cores can busy wait for the new one to appear.
Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/322030/

drivers/dma-buf/reservation.c

@@ -196,6 +196,7 @@ void reservation_object_add_shared_fence(struct reservation_object *obj,
 					 struct dma_fence *fence)
 {
 	struct reservation_object_list *fobj;
+	struct dma_fence *old;
 	unsigned int i, count;
 
 	dma_fence_get(fence);
@@ -209,18 +210,16 @@ void reservation_object_add_shared_fence(struct reservation_object *obj,
 	write_seqcount_begin(&obj->seq);
 
 	for (i = 0; i < count; ++i) {
-		struct dma_fence *old_fence;
 
-		old_fence = rcu_dereference_protected(fobj->shared[i],
+		old = rcu_dereference_protected(fobj->shared[i],
 						reservation_object_held(obj));
-		if (old_fence->context == fence->context ||
-		    dma_fence_is_signaled(old_fence)) {
-			dma_fence_put(old_fence);
+		if (old->context == fence->context ||
+		    dma_fence_is_signaled(old))
 			goto replace;
 	}
-	}
 
 	BUG_ON(fobj->shared_count >= fobj->shared_max);
+	old = NULL;
 	count++;
 
 replace:
@@ -230,6 +229,7 @@ void reservation_object_add_shared_fence(struct reservation_object *obj,
 
 	write_seqcount_end(&obj->seq);
 	preempt_enable();
+	dma_fence_put(old);
 }
 EXPORT_SYMBOL(reservation_object_add_shared_fence);