Fix possible cpuset_sem ABBA deadlock if 'notify_on_release' set.

For a particular usage pattern, creating and destroying cpusets fairly
frequently using notify_on_release, on a very large system, this deadlock
can be seen every few days.  If you are not using the cpuset
notify_on_release feature, you will never see this deadlock.

The existing code, on task exit (or cpuset deletion) did:

  get cpuset_sem
  if cpuset marked notify_on_release and is ready to release:
    compute cpuset path relative to /dev/cpuset mount point
    call_usermodehelper() forks /sbin/cpuset_release_agent with path
  drop cpuset_sem

Unfortunately, the fork in call_usermodehelper can allocate memory, and
allocating memory can require cpuset_sem, if the mems_generation values
changed in the interim.  This results in an ABBA deadlock, trying to obtain
cpuset_sem when it is already held by the current task.

To fix this, I put the cpuset path (which must be computed while holding
cpuset_sem) in a temporary buffer, to be used in the call_usermodehelper
call of /sbin/cpuset_release_agent only _after_ dropping cpuset_sem.

So the new logic is:

  get cpuset_sem
  if cpuset marked notify_on_release and is ready to release:
    compute cpuset path relative to /dev/cpuset mount point
    stash path in kmalloc'd buffer
  drop cpuset_sem
  call_usermodehelper() forks /sbin/cpuset_release_agent with path
  free path

The sharp eyed reader might notice that this patch does not contain any
calls to kmalloc.  The existing code in the check_for_release() routine was
already kmalloc'ing a buffer to hold the cpuset path.  In the old code, it
just held the buffer for a few lines, over the cpuset_release_agent() call
that in turn invoked call_usermodehelper().  In the new code, with the
application of this patch, it returns that buffer via the new char
**ppathbuf parameter, for later use and freeing in cpuset_release_agent(),
which is called after cpuset_sem is dropped.  Whereas the old code has just
one call to cpuset_release_agent(), right in the check_for_release()
routine, the new code has three calls to cpuset_release_agent(), from the
various places that a cpuset can be released.

This patch has been build and booted on SN2, and passed a stress test that
previously hit the deadlock within a few seconds.

Signed-off-by: Paul Jackson <pj@sgi.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>