UPSTREAM: sched: Fix a race between __kthread_bind() and sched_setaffinity()

UPSTREAM: sched: Fix a race between __kthread_bind() and sched_setaffinity()

Because sched_setscheduler() checks p->flags & PF_NO_SETAFFINITY
without locks, a caller might observe an old value and race with the
set_cpus_allowed_ptr() call from __kthread_bind() and effectively undo
it:

	__kthread_bind()
	  do_set_cpus_allowed()
						<SYSCALL>
						  sched_setaffinity()
						    if (p->flags & PF_NO_SETAFFINITIY)
						    set_cpus_allowed_ptr()
	  p->flags |= PF_NO_SETAFFINITY

Fix the bug by putting everything under the regular scheduler locks.

This also closes a hole in the serialization of task_struct::{nr_,}cpus_allowed.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Tejun Heo <tj@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: dedekind1@gmail.com
Cc: juri.lelli@arm.com
Cc: mgorman@suse.de
Cc: riel@redhat.com
Cc: rostedt@goodmis.org
Link: http://lkml.kernel.org/r/20150515154833.545640346@infradead.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
(cherry picked from commit 25834c73f93af7f0712c98ca4593691592e6b360)
Signed-off-by: Punit Agrawal <punit.agrawal@arm.com>
BUG=chrome-os-partner:44828
TEST=Boot kernel on Oak.
TEST=smaug-release and strago-release trybots.

Change-Id: Id3c898c5ee1a22ed704e83f2ecf5f78199280d38
Reviewed-on: https://chromium-review.googlesource.com/321264
Commit-Ready: Ricky Liang <jcliang@chromium.org>
Tested-by: Ricky Liang <jcliang@chromium.org>
Reviewed-by: Ricky Liang <jcliang@chromium.org>
Conflicts:
	kernel/sched/core.c