cpuset: PF_SPREAD_PAGE and PF_SPREAD_SLAB should be atomic flags

cpuset: PF_SPREAD_PAGE and PF_SPREAD_SLAB should be atomic flags

When we change cpuset.memory_spread_{page,slab}, cpuset will flip
PF_SPREAD_{PAGE,SLAB} bit of tsk->flags for each task in that cpuset.
This should be done using atomic bitops, but currently we don't,
which is broken.

Tetsuo reported a hard-to-reproduce kernel crash on RHEL6, which happened
when one thread tried to clear PF_USED_MATH while at the same time another
thread tried to flip PF_SPREAD_PAGE/PF_SPREAD_SLAB. They both operate on
the same task.

Here's the full report:
https://lkml.org/lkml/2014/9/19/230

To fix this, we make PF_SPREAD_PAGE and PF_SPREAD_SLAB atomic flags.

v4:
- updated mm/slab.c. (Fengguang Wu)
- updated Documentation.

Cc: Peter Zijlstra <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Miao Xie <[email protected]>
Cc: Kees Cook <[email protected]>
Fixes: 950592f7b991 ("cpusets: update tasks' page/slab spread flags in time")
Cc: <[email protected]> # 2.6.31+
Reported-by: Tetsuo Handa <[email protected]>
Signed-off-by: Zefan Li <[email protected]>
Signed-off-by: Tejun Heo <[email protected]>