[PATCH] dm: work around mempool_alloc, bio_alloc_bioset deadlocks

This patch works around a complex dm-related deadlock/livelock down in the
mempool allocator.

Alasdair said:

Several dm targets suffer from this.

Mempools are not yet used correctly everywhere in device-mapper: they can
get shared when devices are stacked, and some targets share them across
multiple instances. I made fixing this one of the prerequisites for this
patch:

md-dm-reduce-stack-usage-with-stacked-block-devices.patch

which in some cases makes people more likely to hit the problem.

There's been some progress on this recently with (unfinished) dm-crypt
patches at:

http://www.kernel.org/pub/linux/kernel/people/agk/patches/2.6/editing/
(dm-crypt-move-io-to-workqueue.patch plus dependencies)

and:

I've no problems with a temporary workaround like that, but Milan Broz (a
new Redhat developer in the Czech Republic) has started reviewing all the
mempool usage in device-mapper so I'm expecting we'll soon have a proper fix
for this associated problems. [He's back from holiday at the start of next
week.]

For now, this sad-but-safe little patch will allow the machine to recover.

[[e-mail]: rewrote changelog]
Cc: Alasdair G Kergon <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/mempool.c

···
 	init_wait(&wait);
 	prepare_to_wait(&pool->wait, &wait, TASK_UNINTERRUPTIBLE);
 	smp_mb();
-	if (!pool->curr_nr)
-		io_schedule();
+	if (!pool->curr_nr) {
+		/*
+		 * FIXME: this should be io_schedule().  The timeout is there
+		 * as a workaround for some DM problems in 2.6.18.
+		 */
+		io_schedule_timeout(5*HZ);
+	}
 	finish_wait(&pool->wait, &wait);
 
 	goto repeat_alloc;