[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

@@ -176,10 +176,16 @@ void journal_commit_transaction(journal_t *journal)
 		 * A journal_get_undo_access()+journal_release_buffer() may
 		 * leave undo-committed data.
 		 */
+		if (jh->b_committed_data) {
+			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);
 	}