cpusets-stall-when-updating-mems_allowed-for-mempolicy-or-disjoint-nodemask-fix

stupid temporary hack to make it build with CONFIG_NUMA=n

Cc: David Rientjes <rientjes@google.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Miao Xie <miaox@cn.fujitsu.com>
Cc: Paul Menage <paul@paulmenage.org>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

cpusets: stall when updating mems_allowed for mempolicy or disjoint nodemask

Kernels where MAX_NUMNODES > BITS_PER_LONG may temporarily see an empty
nodemask in a tsk's mempolicy if its previous nodemask is remapped onto a
new set of allowed cpuset nodes where the two nodemasks, as a result of
the remap, are now disjoint.

c0ff7453bb5c ("cpuset,mm: fix no node to alloc memory when changing
cpuset's mems") adds get_mems_allowed() to prevent the set of allowed
nodes from changing for a thread.  This causes any update to a set of
allowed nodes to stall until put_mems_allowed() is called.

This stall is unncessary, however, if at least one node remains unchanged
in the update to the set of allowed nodes.  This was addressed by
89e8a244b97e ("cpusets: avoid looping when storing to mems_allowed if one
node remains set"), but it's still possible that an empty nodemask may be
read from a mempolicy because the old nodemask may be remapped to the new
nodemask during rebind.  To prevent this, only avoid the stall if there is
no mempolicy for the thread being changed.

This is a temporary solution until all reads from mempolicy nodemasks can
be guaranteed to not be empty without the get_mems_allowed()
synchronization.

Also moves the check for nodemask intersection inside task_lock() so that
tsk->mems_allowed cannot change.  This ensures that nothing can set this
tsk's mems_allowed out from under us and also protects tsk->mempolicy.

Reported-by: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: David Rientjes <rientjes@google.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Paul Menage <paul@paulmenage.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Merge remote-tracking branch 'uapi/for-next'

Merge remote-tracking branch 'kvmtool/master'

Conflicts:
include/net/9p/9p.h
scripts/kconfig/Makefile

Merge remote-tracking branch 'remoteproc/for-next'

Merge remote-tracking branch 'memblock/memblock-kill-early_node_map'

Conflicts:
arch/arm/mm/init.c
arch/score/Kconfig

Merge remote-tracking branch 'xshm/xshm-for-next'

Merge remote-tracking branch 'kmap_atomic/kmap_atomic'

Conflicts:
Documentation/feature-removal-schedule.txt

Merge remote-tracking branch 'vhost/linux-next'

Conflicts:
arch/hexagon/Kconfig
arch/m68k/Kconfig

Merge remote-tracking branch 'pinctrl/for-next'

Merge remote-tracking branch 'writeback/writeback-for-next'

Merge remote-tracking branch 'tmem/tmem'

Conflicts:
mm/swapfile.c

Merge remote-tracking branch 'char-misc/char-misc-next'

Merge remote-tracking branch 'staging/staging-next'

Conflicts:
drivers/hid/hid-hyperv.c
drivers/staging/hv/Kconfig
drivers/staging/hv/Makefile
drivers/staging/iio/adc/ad799x_core.c

Merge remote-tracking branch 'usb/usb-next'

Merge remote-tracking branch 'tty/tty-next'

Conflicts:
drivers/tty/serial/Kconfig
drivers/tty/serial/Makefile

Merge remote-tracking branch 'driver-core/driver-core-next'

Merge remote-tracking branch 'hsi/for-next'

Merge commit 'refs/next/20111214/regmap'

Conflicts:
drivers/base/regmap/regcache.c
drivers/base/regmap/regmap.c

Merge remote-tracking branch 'namespace/master'

Merge remote-tracking branch 'sysctl/master'

Merge remote-tracking branch 'percpu/for-next'

Merge remote-tracking branch 'xen-two/linux-next'

Conflicts:
arch/x86/xen/Kconfig

Merge remote-tracking branch 'xen/upstream/xen'

Conflicts:
arch/x86/xen/Kconfig

Merge remote-tracking branch 'oprofile/for-next'

Merge remote-tracking branch 'kmemleak/kmemleak'

Merge remote-tracking branch 'cgroup/for-next'

Merge remote-tracking branch 'uprobes/for-next'

Merge remote-tracking branch 'tip/auto-latest'

Conflicts:
drivers/cpufreq/cpufreq_conservative.c
drivers/cpufreq/cpufreq_ondemand.c
drivers/macintosh/rack-meter.c
fs/proc/stat.c
fs/proc/uptime.c
kernel/sched/core.c
kernel/sched/fair.c

Merge remote-tracking branch 'gpio/gpio/next'

Merge remote-tracking branch 'edac-amd/for-next'

Merge remote-tracking branch 'fsnotify/for-next'

Merge remote-tracking branch 'apm/for-next'

Merge remote-tracking branch 'pm/linux-next'

Conflicts:
fs/xfs/xfs_buf.c

Merge remote-tracking branch 'trivial/for-next'

Conflicts:
arch/powerpc/platforms/40x/Kconfig

Merge remote-tracking branch 'osd/linux-next'

Merge remote-tracking branch 'cputime/cputime'

Merge remote-tracking branch 'iommu/next'

Conflicts:
drivers/pci/hotplug/acpiphp_glue.c
include/linux/pci_regs.h

Merge remote-tracking branch 'watchdog/linux-next'

Merge remote-tracking branch 'security/next'

Conflicts:
lib/Makefile

Merge remote-tracking branch 'regulator/for-next'

Merge remote-tracking branch 'fbdev/fbdev-next'

Merge remote-tracking branch 'drm/drm-next'

Conflicts:
drivers/gpu/drm/nouveau/nouveau_sgdma.c

Merge remote-tracking branch 'mfd/for-next'

Merge remote-tracking branch 'md/for-next'

Merge remote-tracking branch 'slab/for-next'

Merge remote-tracking branch 'kgdb/kgdb-next'

Merge remote-tracking branch 'mmc/mmc-next'

Conflicts:
drivers/mmc/card/block.c

Merge remote-tracking branch 'battery/master'

Merge commit 'refs/next/20111215/block'

Merge remote-tracking branch 'input/next'

Conflicts:
drivers/input/keyboard/samsung-keypad.c

Merge branch 'quilt/rr'

Merge remote-tracking branch 'cpufreq/next'

Merge remote-tracking branch 'sound-asoc/for-next'

Merge remote-tracking branch 'sound/for-next'

Conflicts:
arch/arm/mach-s3c64xx/mach-crag6410.c

Merge remote-tracking branch 'crypto/master'

Merge remote-tracking branch 'l2-mtd/master'

Conflicts:
drivers/mtd/maps/bcm963xx-flash.c

Merge remote-tracking branch 'mtd/master'

Merge remote-tracking branch 'bluetooth/master'

Merge remote-tracking branch 'wireless-next/master'

Conflicts:
drivers/net/wireless/iwlwifi/iwl-agn.c

Merge remote-tracking branch 'net-next/master'

Conflicts:
drivers/net/wireless/iwlwifi/iwl-agn.c

Merge remote-tracking branch 'slave-dma/next'

Conflicts:
drivers/dma/pl330.c

Merge remote-tracking branch 'target-merge/for-next-merge'

Merge remote-tracking branch 'target-updates/for-next'

Merge remote-tracking branch 'scsi/master'

Merge remote-tracking branch 'dlm/next'

Merge remote-tracking branch 'ubi/linux-next'

Merge remote-tracking branch 'libata/NEXT'

Merge remote-tracking branch 'kconfig/for-next'

Merge remote-tracking branch 'kbuild/for-next'

Merge remote-tracking branch 'v4l-dvb/master'

Conflicts:
drivers/staging/media/as102/as102_drv.h

Merge remote-tracking branch 'hwmon-staging/hwmon-next'

Merge branch 'quilt/jdelvare-hwmon'

Merge remote-tracking branch 'bjdooks-i2c/next-i2c'

Conflicts:
drivers/i2c/busses/i2c-designware-platdrv.c

Merge branch 'quilt/i2c'

Merge commit 'refs/next/20111216/hid'

Merge remote-tracking branch 'pci/linux-next'

Merge remote-tracking branch 'xfs/master'

Merge remote-tracking branch 'ubifs/linux-next'

Merge remote-tracking branch 'nfsd/nfsd-next'

Merge remote-tracking branch 'nfs/linux-next'

Merge remote-tracking branch 'logfs/master'

Conflicts:
fs/logfs/file.c

Merge remote-tracking branch 'gfs2/master'

Merge remote-tracking branch 'fuse/for-next'

Merge remote-tracking branch 'ecryptfs/next'

Merge remote-tracking branch 'xtensa/master'

Merge remote-tracking branch 's390/features'

Merge remote-tracking branch '52xx-and-virtex/powerpc/next'

Merge remote-tracking branch '4xx/next'

Conflicts:
arch/powerpc/platforms/40x/ppc40x_simple.c

Merge remote-tracking branch 'powerpc/next'

Merge remote-tracking branch 'parisc/for-next'

Merge remote-tracking branch 'mips/mips-for-linux-next'

Conflicts:
arch/mips/kernel/perf_event_mipsxx.c

Merge remote-tracking branch 'm68knommu/for-next'

Conflicts:
arch/m68k/Kconfig.debug

Merge remote-tracking branch 'm68k/for-next'

Merge remote-tracking branch 'ia64/next'

Merge branch 'quilt/hexagon'

Merge remote-tracking branch 'cris/for-next'

Merge remote-tracking branch 'c6x/for-linux-next'

Merge remote-tracking branch 'blackfin/for-linus'

Merge remote-tracking branch 'tegra/for-next'

Conflicts:
arch/arm/mach-tegra/board-dt-tegra20.c
arch/arm/mach-tegra/common.c