[PATCH 008 of 9] md: Fix possible raid1/raid10 deadlock on read error during resync.

[NeilBrown]

Thanks to K.Tanaka and the scsi fault injection framework, here is
a fix for another possible deadlock in raid1/raid10 error handing.

If a read request returns an error while a resync is happening and
a resync request is pending, the attempt to fix the error will block
until the resync progresses, and the resync will block until the
read request completes.  Thus a deadlock.

This patch fixes the problem.

Cc: "K.Tanaka" <k-tanaka@xxxxxxxxxxxxx>
Signed-off-by: Neil Brown <neilb@xxxxxxx>

### Diffstat output
 ./drivers/md/raid1.c  |   11 +++++++++--
 ./drivers/md/raid10.c |   11 +++++++++--
 2 files changed, 18 insertions(+), 4 deletions(-)

diff .prev/drivers/md/raid10.c ./drivers/md/raid10.c
--- .prev/drivers/md/raid10.c	2008-03-03 11:03:39.000000000 +1100
+++ ./drivers/md/raid10.c	2008-03-03 09:56:53.000000000 +1100
@@ -747,13 +747,20 @@ static void freeze_array(conf_t *conf)
 	/* stop syncio and normal IO and wait for everything to
 	 * go quiet.
 	 * We increment barrier and nr_waiting, and then
-	 * wait until barrier+nr_pending match nr_queued+2
+	 * wait until nr_pending match nr_queued+1
+	 * This is called in the context of one normal IO request
+	 * that has failed. Thus any sync request that might be pending
+	 * will be blocked by nr_pending, and we need to wait for
+	 * pending IO requests to complete or be queued for re-try.
+	 * Thus the number queued (nr_queued) plus this request (1)
+	 * must match the number of pending IOs (nr_pending) before
+	 * we continue.
 	 */
 	spin_lock_irq(&conf->resync_lock);
 	conf->barrier++;
 	conf->nr_waiting++;
 	wait_event_lock_irq(conf->wait_barrier,
-			    conf->barrier+conf->nr_pending == conf->nr_queued+2,
+			    conf->nr_pending == conf->nr_queued+1,
 			    conf->resync_lock,
 			    ({ flush_pending_writes(conf);
 			       raid10_unplug(conf->mddev->queue); }));

diff .prev/drivers/md/raid1.c ./drivers/md/raid1.c
--- .prev/drivers/md/raid1.c	2008-03-03 11:03:39.000000000 +1100
+++ ./drivers/md/raid1.c	2008-03-03 09:56:52.000000000 +1100
@@ -704,13 +704,20 @@ static void freeze_array(conf_t *conf)
 	/* stop syncio and normal IO and wait for everything to
 	 * go quite.
 	 * We increment barrier and nr_waiting, and then
-	 * wait until barrier+nr_pending match nr_queued+2
+	 * wait until nr_pending match nr_queued+1
+	 * This is called in the context of one normal IO request
+	 * that has failed. Thus any sync request that might be pending
+	 * will be blocked by nr_pending, and we need to wait for
+	 * pending IO requests to complete or be queued for re-try.
+	 * Thus the number queued (nr_queued) plus this request (1)
+	 * must match the number of pending IOs (nr_pending) before
+	 * we continue.
 	 */
 	spin_lock_irq(&conf->resync_lock);
 	conf->barrier++;
 	conf->nr_waiting++;
 	wait_event_lock_irq(conf->wait_barrier,
-			    conf->barrier+conf->nr_pending == conf->nr_queued+2,
+			    conf->nr_pending == conf->nr_queued+1,
 			    conf->resync_lock,
 			    ({ flush_pending_writes(conf);
 			       raid1_unplug(conf->mddev->queue); }));
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