[PATCH] JBD: b_committed_data locking fix

The locking rules say that b_committed_data is covered by
jbd_lock_bh_state(), so implement that during the start of commit, while
throwing away unused shadow buffers.

I don't expect that there is really a race here, but them's the rules.

fs/jbd/commit.c

@@ -177,8 +177,14 @@ void journal_commit_transaction(journal_t *journal)
 		 * leave undo-committed data.
 		 */
 		if (jh->b_committed_data) {
-			kfree(jh->b_committed_data);
-			jh->b_committed_data = NULL;
+			struct buffer_head *bh = jh2bh(jh);
+
+			jbd_lock_bh_state(bh);
+			if (jh->b_committed_data) {
+				kfree(jh->b_committed_data);
+				jh->b_committed_data = NULL;
+			}
+			jbd_unlock_bh_state(bh);
 		}
 		journal_refile_buffer(journal, jh);
 	}