diff --git a/fs/btrfs/tree-log.c b/fs/btrfs/tree-log.c
index 8f23a94..2009cea 100644
--- a/fs/btrfs/tree-log.c
+++ b/fs/btrfs/tree-log.c
@@ -3116,8 +3116,11 @@ int btrfs_sync_log(struct btrfs_trans_handle *trans,
 	mutex_unlock(&log_root_tree->log_mutex);
 
 	/*
-	 * The barrier before waitqueue_active is implied by mutex_unlock
+	 * The barrier before waitqueue_active is needed so all the updates
+	 * above are seen by the woken threads. It might not be necessary, but
+	 * proving that seems to be hard.
 	 */
+	smp_mb();
 	if (waitqueue_active(&log_root_tree->log_commit_wait[index2]))
 		wake_up(&log_root_tree->log_commit_wait[index2]);
 out:
@@ -3128,8 +3131,11 @@ int btrfs_sync_log(struct btrfs_trans_handle *trans,
 	mutex_unlock(&root->log_mutex);
 
 	/*
-	 * The barrier before waitqueue_active is implied by mutex_unlock
+	 * The barrier before waitqueue_active is needed so all the updates
+	 * above are seen by the woken threads. It might not be necessary, but
+	 * proving that seems to be hard.
 	 */
+	smp_mb();
 	if (waitqueue_active(&root->log_commit_wait[index1]))
 		wake_up(&root->log_commit_wait[index1]);
 	return ret;