drm/nouveau: add more fine-grained locking to channel list + structures

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nouveau_fence.c

@@ -393,8 +393,18 @@ nouveau_fence_sync(struct nouveau_fence *fence,
 		return nouveau_fence_wait(fence, NULL, false, false);
 	}
 
+	/* try to take wchan's mutex, if we can't take it right away
+	 * we have to fallback to software sync to prevent locking
+	 * order issues
+	 */
+	if (!mutex_trylock(&wchan->mutex)) {
+		free_semaphore(&sema->ref);
+		return nouveau_fence_wait(fence, NULL, false, false);
+	}
+
 	/* Make wchan wait until it gets signalled */
 	ret = emit_semaphore(wchan, NV_SW_SEMAPHORE_ACQUIRE, sema);
+	mutex_unlock(&wchan->mutex);
 	if (ret)
 		goto out;