[PATCH] s390/locking: Reenable optimistic spinning

[Christian Borntraeger <borntraeger@de.ibm.com>]

commit 4badad352a6bb202ec68afa7a574c0bb961e5ebc (locking/mutex: Disable
optimistic spinning on some architectures) fenced spinning for
architectures without proper cmpxchg.
There is no need to disable mutex spinning on s390, though:
The instructions CS,CSG and friends provide the proper guarantees.
(We dont implement cmpxchg with locks).

Please note: This is a quick fix for s390, suitable for stable
as 3.16 unintenionally disabled optimistic spinning.

The proper fix is probably to use ARCH_HAVE_NMI_SAFE_CMPXCHG
instead of introducing ARCH_SUPPORTS_ATOMIC_RMW.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
CC: <mingo@kernel.org>
CC: Peter Zijlstra <peterz@infradead.org>
CC: stable@vger.kernel.org
---
 arch/s390/Kconfig | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/s390/Kconfig b/arch/s390/Kconfig
index 9a0ea87..eab3c32 100644
--- a/arch/s390/Kconfig
+++ b/arch/s390/Kconfig
@@ -63,6 +63,7 @@ config S390
 	select ARCH_HAS_ATOMIC64_DEC_IF_POSITIVE
 	select ARCH_HAS_DEBUG_STRICT_USER_COPY_CHECKS
 	select ARCH_HAVE_NMI_SAFE_CMPXCHG
+	select ARCH_SUPPORTS_ATOMIC_RMW
 	select ARCH_INLINE_READ_LOCK
 	select ARCH_INLINE_READ_LOCK_BH
 	select ARCH_INLINE_READ_LOCK_IRQ
-- 
1.8.4.2