[patch 07/10] mutex subsystem, switch ARM to use the xchg based implementation

[Ingo Molnar <mingo@elte.hu>]

as noted by Nicolas Pitre, atomic_[inc/dec]_call_if_[nonpositive/negative]()
atomic methods are slow on ARM, because they can only be implemented via
disabling interrupts. So tell the mutex code that we prefer atomic_xchg().

[ we still pull in asm-generic/atomic-call-if.h, so that they remain
  generally available primitives - even though unused on ARM at the
  moment. ]

Signed-off-by: Ingo Molnar <mingo@elte.hu>

----

 arch/arm/Kconfig         |    4 ++++
 include/asm-arm/atomic.h |    7 +------
 2 files changed, 5 insertions(+), 6 deletions(-)

Index: linux/arch/arm/Kconfig
===================================================================
--- linux.orig/arch/arm/Kconfig
+++ linux/arch/arm/Kconfig
@@ -50,6 +50,10 @@ config UID16
 	bool
 	default y
 
+config MUTEX_XCHG_ALGORITHM
+	bool
+	default y
+
 config RWSEM_GENERIC_SPINLOCK
 	bool
 	default y
Index: linux/include/asm-arm/atomic.h
===================================================================
--- linux.orig/include/asm-arm/atomic.h
+++ linux/include/asm-arm/atomic.h
@@ -179,12 +179,7 @@ static inline void atomic_clear_mask(uns
 
 /*
  * Pull in the generic wrappers for atomic_dec_call_if_negative() and
- * atomic_inc_call_if_nonpositive().
- *
- * TODO: implement optimized primitives instead, or leave the generic
- * implementation in place, or enable CONFIG_MUTEX_XCHG_ALGORITHM
- * to tell the generic mutex code to use the atomic_xchg() based
- * fastpath implementation.
+ * atomic_inc_call_if_nonpositive():
  */
 #include <asm-generic/atomic-call-if.h>
 
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