[PATCH] ext3 scheduling latency fix

Sometimes kjournald has to refile a huge number of buffers, because someone
else wrote them out beforehand - they are all clean.

This happens under a lock and scheduling latencies of 88 milliseconds on a
2.7GHx CPU were observed.

The patch forward-ports a little bit of the 2.4 low-latency patch to fix this
problem.

Worst-case on ext3 is now sub-half-millisecond, except for when the RCU
dentry reaping softirq cuts in :(

fs/jbd/commit.c

@@ -264,6 +264,16 @@ void journal_commit_transaction(journal_t *journal)
 				jbd_unlock_bh_state(bh);
 				journal_remove_journal_head(bh);
 				__brelse(bh);
+				if (need_resched() && commit_transaction->
+							t_sync_datalist) {
+					commit_transaction->t_sync_datalist =
+								next_jh;
+					if (bufs)
+						break;
+					spin_unlock(&journal->j_list_lock);
+					cond_resched();
+					goto write_out_data;
+				}
 			}
 		}
 		if (bufs == ARRAY_SIZE(wbuf)) {
@@ -284,7 +294,6 @@ void journal_commit_transaction(journal_t *journal)
 		cond_resched();
 		journal_brelse_array(wbuf, bufs);
 		spin_lock(&journal->j_list_lock);
-		if (bufs)
 		goto write_out_data_locked;
 	}