md: fix possible raid1/raid10 deadlock on read error during resync

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@ce.jp.nec.com>
Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 9 insertions, 2 deletions

diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
index 6c486d839c99..8e5671d2f3d3 100644
--- a/drivers/md/raid10.c
+++ b/drivers/md/raid10.c
@@ -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); }));