Merge branch 'for-3.15-fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/percpu

Pull percpu fix from Tejun Heo:
"It is very late but this is an important percpu-refcount fix from
Sebastian Ott.

The problem is that percpu_ref_*() used __this_cpu_*() instead of
this_cpu_*(). The difference between the two is that the latter is
atomic on the local cpu while the former is not. this_cpu_inc() is
guaranteed to increment the percpu counter on the cpu that the
operation is executed on without any synchronization; however,
__this_cpu_inc() doesn't and if the local cpu invokes the function
from different contexts (e.g. process and irq) of the same CPU, it's
not guaranteed to actually increment as it may be implemented as rmw.

This bug existed from the get-go but it hasn't been noticed earlier
probably because on x86 __this_cpu_inc() is equivalent to
this_cpu_inc() as both get translated into single instruction;
however, s390 uses the generic rmw implementation and gets affected by
the bug. Kudos to Sebastian and Heiko for diagnosing it.

The change is very low risk and fixes a critical issue on the affected
architectures, so I think it's a good candidate for inclusion although
it's very late in the devel cycle. On the other hand, this has been
broken since v3.11, so backporting it through -stable post -rc1 won't
be the end of the world.

I'll ping Christoph whether __this_cpu_*() ops can be better annotated
so that it can trigger lockdep warning when used from multiple
contexts"

* 'for-3.15-fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/percpu:
percpu-refcount: fix usage of this_cpu_ops