From: Jonathan Brassow <jbrassow@redhat.com>
Date: Mon, 17 Dec 2007 16:02:09 -0600
Subject: [md] mirror presuspend causing cluster mirror hang
Message-id: 1197928929.28090.5.camel@hydrogen
O-Subject: [RHEL 5.2 PATCH] Incorrect order of mirror presuspend ops causes cluster mirror hang
Bugzilla: 358871

Bug #358871

Cluster mirrors do not (and can not) allow writes to mirror regions which have
not recovered yet.

When a mirror suspends it:
1) stops recovery
2) flushes the work queue (finishing outstanding writes)
3) calls log presuspend

The log has no way of knowing that the mirror is suspending until #3, so it
blocks any outstanding writes to regions that are not-in-sync.  This could hang
the mirror because recovery has been stopped and the not-in-sync regions will
never be cleared - delaying the writes indefinitely.

The solution is to switch the order of operations to:
1) stop recovery
2) call log presuspend
3) flush work queue

By doing this, the log knows that recovery has been stopped and that there will
be no collisions with writes.  Therefore, it can allow writes to regions which
have not yet been recovered.

There are no ill effects of this patch on single machine mirroring.  The core
log implementation does not define a presuspend, so there is nothing called.
The disk log implementation simply returns '0' for presuspend.  So, for
single machine mirroring, we are effectively moving a no-op.

Acked-by: Alasdair G Kergon <agk@redhat.com>

diff --git a/drivers/md/dm-raid1.c b/drivers/md/dm-raid1.c
index 515ff4e..d4ea9cb 100644
--- a/drivers/md/dm-raid1.c
+++ b/drivers/md/dm-raid1.c
@@ -1686,6 +1686,10 @@ static void mirror_presuspend(struct dm_target *ti)
 	wait_event(recovery_stopped_event,
 		   !atomic_read(&ms->rh.recovery_in_flight));
 
+	if (log->type->presuspend && log->type->presuspend(log))
+		/* FIXME: need better error handling */
+		DMWARN("log presuspend failed");
+
 	/*
 	 * Now that recovery is complete/stopped and the
 	 * delayed bios are queued, we need to wait for
@@ -1693,10 +1697,6 @@ static void mirror_presuspend(struct dm_target *ti)
 	 * we know that all of our I/O has been pushed.
 	 */
 	flush_workqueue(ms->kmirrord_wq);
-
-	if (log->type->presuspend && log->type->presuspend(log))
-		/* FIXME: need better error handling */
-		DMWARN("log presuspend failed");
 }
 
 static void mirror_postsuspend(struct dm_target *ti)