Pull s390 lockref enablement from Heiko Carstens:
"Enabling the new lockless lockref variant on s390 would have been
trivial until Tony Luck added a cpu_relax() call into the
CMPXCHG_LOOP(), with commit d472d9d98b ("lockref: Relax in cmpxchg
loop")
As already mentioned cpu_relax() is very expensive on s390 since it
yields() the current virtual cpu. So we are talking of several
thousand cycles. Considering this enabling the lockless lockref
variant would contradict the intention of the new semantics. And also
some quick measurements show performance regressions of 50% and more.
Simply removing the cpu_relax() call again seems also not very
desireable since Waiman Long reported that for some workloads the call
improved performance by 5%."
* 'lockref' of git://git.kernel.org/pub/scm/linux/kernel/git/s390/linux:
s390: enable ARCH_USE_CMPXCHG_LOCKREF
lockref: use arch_mutex_cpu_relax() in CMPXCHG_LOOP()
mutex: replace CONFIG_HAVE_ARCH_MUTEX_CPU_RELAX with simple ifdef
aeebc26457