[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.

[akpm@osdl.org: rewrote changelog]
Cc: Alasdair G Kergon <agk@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 7 insertions, 2 deletions

diff --git a/mm/mempool.c b/mm/mempool.c
index fe6e05289cc5..ccd8cb8cd41f 100644
--- a/mm/mempool.c
+++ b/mm/mempool.c
@@ -238,8 +238,13 @@ repeat_alloc:
         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;