[PATCH] cfq-iosched: fix bug in timer handling for the idle class

There's a small window from when the timer is entered and we grab
the queue lock, where cfq_set_active_queue() could be rearming the
timer for us. Seen in the wild on a 12-way ppc box. Fix this by
just using mod_timer(), which will do the right thing for us.
Signed-off-by: Jens Axboe <axboe@suse.de>

block/cfq-iosched.c

@@ -2193,10 +2193,9 @@ static void cfq_idle_class_timer(unsigned long data)
 	 * race with a non-idle queue, reset timer
 	 */
 	end = cfqd->last_end_request + CFQ_IDLE_GRACE;
-	if (!time_after_eq(jiffies, end)) {
-		cfqd->idle_class_timer.expires = end;
-		add_timer(&cfqd->idle_class_timer);
-	} else
+	if (!time_after_eq(jiffies, end))
+		mod_timer(&cfqd->idle_class_timer, end);
+	else
 		cfq_schedule_dispatch(cfqd);
 
 	spin_unlock_irqrestore(cfqd->queue->queue_lock, flags);