There's a race between mm->owner assignment and swapoff, more easily
seen when task slab poisoning is turned on.  The condition occurs when
try_to_unuse() runs in parallel with an exiting task.  A similar race
can occur with callers of get_task_mm(), such as /proc/<pid>/<mmstats>
or ptrace or page migration.

CPU0                                    CPU1
                                        try_to_unuse
                                        looks at mm = task0->mm
                                        increments mm->mm_users
task 0 exits
mm->owner needs to be updated, but no
new owner is found (mm_users > 1, but
no other task has task->mm = task0->mm)
mm_update_next_owner() leaves
                                        mmput(mm) decrements mm->mm_users
task0 freed
                                        dereferencing mm->owner fails

The fix is to notify the subsystem via mm_owner_changed callback(),
if no new owner is found, by specifying the new task as NULL.

Jiri Slaby:
mm->owner was set to NULL prior to calling cgroup_mm_owner_callbacks(), but
must be set after that, so as not to pass NULL as old owner causing oops.

Daisuke Nishimura:
mm_update_next_owner() may set mm->owner to NULL, but mem_cgroup_from_task()
and its callers need to take account of this situation to avoid oops.

Hugh Dickins:
Lockdep warning and hang below exec_mmap() when testing these patches.
exit_mm() up_reads mmap_sem before calling mm_update_next_owner(),
so exec_mmap() now needs to do the same.  And with that repositioning,
there's now no point in mm_need_new_owner() allowing for NULL mm.

Reported-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Balbir Singh <balbir@linux.vnet.ibm.com>
Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Signed-off-by: Hugh Dickins <hugh@veritas.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Paul Menage <menage@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

* 'x86-fixes-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/linux-2.6-tip:
  x86: disable apm on the olpc

* git://git.kernel.org/pub/scm/linux/kernel/git/bart/ide-2.6:
  cdrom: update ioctl documentation
  ide: note that IDE generic may prevent other drivers from attaching
  ide-tape: fix vendor strings
  Swarm: Fix crash due to missing initialization

* 'upstream' of git://ftp.linux-mips.org/pub/scm/upstream-linus:
  [SSB] Initialise dma_mask for SSB_BUSTYPE_SSB devices
  [MIPS] BCM47xx: Fix build error due to missing PCI functions
  [MIPS] IP27: Switch to dynamic interrupt routing avoding panic on error.
  [MIPS] au1000: Make sure GPIO value is zero or one

* 'linux-m32r' of git://www.linux-m32r.org/git/takata/linux-2.6_dev:
  m32r/kernel/: cleanups
  m32r: export __ndelay
  m32r: export empty_zero_page
  m32r: don't offer CONFIG_ISA
  m32r: remove the unused NOHIGHMEM option

* 'for_linus' of git://git.kernel.org/pub/scm/linux/kernel/git/jwessel/linux-2.6-kgdb:
  kgdboc,tty: Fix tty polling search to use name correctly
  kgdb, x86_64: fix PS CS SS registers in gdb serial
  kgdb, x86_64: gdb serial has BX and DX reversed
  kgdb, x86, arm, mips, powerpc: ignore user space single stepping
  kgdb: could not write to the last of valid memory with kgdb

* 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tiwai/sound-2.6:
  ALSA: ASoC: Fix another cs4270 error path
  ALSA: make the CS4270 driver a new-style I2C driver

* git://git.kernel.org/pub/scm/linux/kernel/git/jejb/scsi-rc-fixes-2.6:
  [SCSI] qlogicpti: fix sg list traversal error in continuation entries
  [SCSI] Fix hang with split requests
  [SCSI] qla2xxx: Defer enablement of RISC interrupts until ISP initialization completes.

* 'for-linus' of git://git.kernel.dk/linux-2.6-block:
  scsi: fix fall out of sg-chaining patch in qlogicpti