[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>