md/raid1,raid10: fix deadlock with freeze_array()

When raid1/raid10 needs to fix a read error, it first drains
all pending requests by calling freeze_array().
This calls flush_pending_writes() if it needs to sleep,
but some writes may be pending in a per-process plug rather
than in the per-array request queue.

When raid1{,0}_unplug() moves the request from the per-process
plug to the per-array request queue (from which
flush_pending_writes() can flush them), it needs to wake up
freeze_array(), or freeze_array() will never flush them and so
it will block forever.

So add the requires wake_up() calls.

This bug was introduced by commit
   f54a9d0e59c4bea3db733921ca9147612a6f292c
for raid1 and a similar commit for RAID10, and so has been present
since linux-3.6.  As the bug causes a deadlock I believe this fix is
suitable for -stable.

Cc: stable@vger.kernel.org (3.6.y 3.7.y 3.8.y)
Reported-by: Tregaron Bayly <tbayly@bluehost.com>
Tested-by: Tregaron Bayly <tbayly@bluehost.com>
Signed-off-by: NeilBrown <neilb@suse.de>

drivers/md/raid1.c

@@ -967,6 +967,7 @@ static void raid1_unplug(struct blk_plug_cb *cb, bool from_schedule)
 		bio_list_merge(&conf->pending_bio_list, &plug->pending);
 		conf->pending_count += plug->pending_cnt;
 		spin_unlock_irq(&conf->device_lock);
+		wake_up(&conf->wait_barrier);
 		md_wakeup_thread(mddev->thread);
 		kfree(plug);
 		return;

drivers/md/raid10.c

@@ -1119,6 +1119,7 @@ static void raid10_unplug(struct blk_plug_cb *cb, bool from_schedule)
 		bio_list_merge(&conf->pending_bio_list, &plug->pending);
 		conf->pending_count += plug->pending_cnt;
 		spin_unlock_irq(&conf->device_lock);
+		wake_up(&conf->wait_barrier);
 		md_wakeup_thread(mddev->thread);
 		kfree(plug);
 		return;