[PATCH] md: fix a plug/unplug race in raid5

When a device is unplugged, requests are moved from one or two (depending on
whether a bitmap is in use) queues to the main request queue.

So whenever requests are put on either of those queues, we should make sure
the raid5 array is 'plugged'.  However we don't.  We currently plug the raid5
queue just before putting requests on queues, so there is room for a race.  If
something unplugs the queue at just the wrong time, requests will be left on
the queue and nothing will want to unplug them.  Normally something else will
plug and unplug the queue fairly soon, but there is a risk that nothing will.

Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

drivers/md/raid5.c

@@ -88,12 +88,14 @@ static void __release_stripe(raid5_conf_t *conf, struct stripe_head *sh)
 		BUG_ON(!list_empty(&sh->lru));
 		BUG_ON(atomic_read(&conf->active_stripes)==0);
 		if (test_bit(STRIPE_HANDLE, &sh->state)) {
-			if (test_bit(STRIPE_DELAYED, &sh->state))
+			if (test_bit(STRIPE_DELAYED, &sh->state)) {
 				list_add_tail(&sh->lru, &conf->delayed_list);
-			else if (test_bit(STRIPE_BIT_DELAY, &sh->state) &&
-				 conf->seq_write == sh->bm_seq)
+				blk_plug_device(conf->mddev->queue);
+			} else if (test_bit(STRIPE_BIT_DELAY, &sh->state) &&
+				   conf->seq_write == sh->bm_seq) {
 				list_add_tail(&sh->lru, &conf->bitmap_list);
-			else {
+				blk_plug_device(conf->mddev->queue);
+			} else {
 				clear_bit(STRIPE_BIT_DELAY, &sh->state);
 				list_add_tail(&sh->lru, &conf->handle_list);
 			}
@@ -2555,13 +2557,6 @@ static int raid5_issue_flush(request_queue_t *q, struct gendisk *disk,
 	return ret;
 }
 
-static inline void raid5_plug_device(raid5_conf_t *conf)
-{
-	spin_lock_irq(&conf->device_lock);
-	blk_plug_device(conf->mddev->queue);
-	spin_unlock_irq(&conf->device_lock);
-}
-
 static int make_request(request_queue_t *q, struct bio * bi)
 {
 	mddev_t *mddev = q->queuedata;
@@ -2671,7 +2666,6 @@ static int make_request(request_queue_t *q, struct bio * bi)
 				goto retry;
 			}
 			finish_wait(&conf->wait_for_overlap, &w);
-			raid5_plug_device(conf);
 			handle_stripe(sh, NULL);
 			release_stripe(sh);
 		} else {