[PATCH] ARM: mutex: use generic atomic_dec-based implementation for ARMv6+

Commit a76d7bd96d65 ("ARM: 7467/1: mutex: use generic xchg-based
implementation for ARMv6+") removed the barrier-less, ARM-specific
mutex implementation in favour of the generic xchg-based code.

Since then, a bug was uncovered in the xchg code when running on SMP
platforms, due to interactions between the locking paths and the
MUTEX_SPIN_ON_OWNER code. This was fixed in 0bce9c46bf3b ("mutex: place
lock in contended state after fastpath_lock failure"), however, the
atomic_dec-based mutex algorithm is now marginally more efficient for
ARM (~0.5% improvement in hackbench scores on dual A15).

This patch moves ARMv6+ platforms to the atomic_dec-based mutex code.

Cc: Nicolas Pitre <nico@fluxnic.net>
Signed-off-by: Will Deacon <redacted>
---
 arch/arm/include/asm/mutex.h |    8 ++++++--
 1 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/arch/arm/include/asm/mutex.h b/arch/arm/include/asm/mutex.h
index b1479fd..d69ebc7 100644
--- a/arch/arm/include/asm/mutex.h
+++ b/arch/arm/include/asm/mutex.h

@@ -9,8 +9,12 @@
 #define _ASM_MUTEX_H
 /*
  * On pre-ARMv6 hardware this results in a swp-based implementation,
- * which is the most efficient. For ARMv6+, we emit a pair of exclusive
- * accesses instead.
+ * which is the most efficient. For ARMv6+, we have exclusive memory
+ * accessors and use atomic_dec to avoid the extra xchg operations
+ * on the locking slowpaths.
  */
+#if __LINUX_ARM_ARCH < 6
 #include <asm-generic/mutex-xchg.h>
+#else
+#include <asm-generic/mutex-dec.h>
 #endif

-- 
1.7.4.1

On Mon, 13 Aug 2012, Will Deacon wrote:

Commit a76d7bd96d65 ("ARM: 7467/1: mutex: use generic xchg-based
implementation for ARMv6+") removed the barrier-less, ARM-specific
mutex implementation in favour of the generic xchg-based code.

Since then, a bug was uncovered in the xchg code when running on SMP
platforms, due to interactions between the locking paths and the
MUTEX_SPIN_ON_OWNER code. This was fixed in 0bce9c46bf3b ("mutex: place
lock in contended state after fastpath_lock failure"), however, the
atomic_dec-based mutex algorithm is now marginally more efficient for
ARM (~0.5% improvement in hackbench scores on dual A15).

This patch moves ARMv6+ platforms to the atomic_dec-based mutex code.

Cc: Nicolas Pitre <nico@fluxnic.net>
Signed-off-by: Will Deacon <redacted>

Acked-by: Nicolas Pitre <redacted>