From a8ddac7e53e89cb877965097d05adfeb1c91def3 Mon Sep 17 00:00:00 2001 From: Nick Piggin Date: Tue, 21 Oct 2008 10:59:15 +0200 Subject: [PATCH] mutex: speed up generic mutex implementations - atomic operations which both modify the variable and return something imply full smp memory barriers before and after the memory operations involved (failing atomic_cmpxchg, atomic_add_unless, etc don't imply a barrier because they don't modify the target). See Documentation/atomic_ops.txt. So remove extra barriers and branches. - All architectures support atomic_cmpxchg. This has no relation to __HAVE_ARCH_CMPXCHG. We can just take the atomic_cmpxchg path unconditionally This reduces a simple single threaded fastpath lock+unlock test from 590 cycles to 203 cycles on a ppc970 system. Signed-off-by: Nick Piggin Signed-off-by: Linus Torvalds --- include/asm-generic/mutex-dec.h | 26 ++------------------------ include/asm-generic/mutex-xchg.h | 9 +-------- 2 files changed, 3 insertions(+), 32 deletions(-) diff --git a/include/asm-generic/mutex-dec.h b/include/asm-generic/mutex-dec.h index ed108be..f104af7 100644 --- a/include/asm-generic/mutex-dec.h +++ b/include/asm-generic/mutex-dec.h @@ -22,8 +22,6 @@ __mutex_fastpath_lock(atomic_t *count, void (*fail_fn)(atomic_t *)) { if (unlikely(atomic_dec_return(count) < 0)) fail_fn(count); - else - smp_mb(); } /** @@ -41,10 +39,7 @@ __mutex_fastpath_lock_retval(atomic_t *count, int (*fail_fn)(atomic_t *)) { if (unlikely(atomic_dec_return(count) < 0)) return fail_fn(count); - else { - smp_mb(); - return 0; - } + return 0; } /** @@ -63,7 +58,6 @@ __mutex_fastpath_lock_retval(atomic_t *count, int (*fail_fn)(atomic_t *)) static inline void __mutex_fastpath_unlock(atomic_t *count, void (*fail_fn)(atomic_t *)) { - smp_mb(); if (unlikely(atomic_inc_return(count) <= 0)) fail_fn(count); } @@ -88,25 +82,9 @@ __mutex_fastpath_unlock(atomic_t *count, void (*fail_fn)(atomic_t *)) static inline int __mutex_fastpath_trylock(atomic_t *count, int (*fail_fn)(atomic_t *)) { - /* - * We have two variants here. The cmpxchg based one is the best one - * because it never induce a false contention state. It is included - * here because architectures using the inc/dec algorithms over the - * xchg ones are much more likely to support cmpxchg natively. - * - * If not we fall back to the spinlock based variant - that is - * just as efficient (and simpler) as a 'destructive' probing of - * the mutex state would be. - */ -#ifdef __HAVE_ARCH_CMPXCHG - if (likely(atomic_cmpxchg(count, 1, 0) == 1)) { - smp_mb(); + if (likely(atomic_cmpxchg(count, 1, 0) == 1)) return 1; - } return 0; -#else - return fail_fn(count); -#endif } #endif diff --git a/include/asm-generic/mutex-xchg.h b/include/asm-generic/mutex-xchg.h index 7b9cd2c..580a6d3 100644 --- a/include/asm-generic/mutex-xchg.h +++ b/include/asm-generic/mutex-xchg.h @@ -27,8 +27,6 @@ __mutex_fastpath_lock(atomic_t *count, void (*fail_fn)(atomic_t *)) { if (unlikely(atomic_xchg(count, 0) != 1)) fail_fn(count); - else - smp_mb(); } /** @@ -46,10 +44,7 @@ __mutex_fastpath_lock_retval(atomic_t *count, int (*fail_fn)(atomic_t *)) { if (unlikely(atomic_xchg(count, 0) != 1)) return fail_fn(count); - else { - smp_mb(); - return 0; - } + return 0; } /** @@ -67,7 +62,6 @@ __mutex_fastpath_lock_retval(atomic_t *count, int (*fail_fn)(atomic_t *)) static inline void __mutex_fastpath_unlock(atomic_t *count, void (*fail_fn)(atomic_t *)) { - smp_mb(); if (unlikely(atomic_xchg(count, 1) != 0)) fail_fn(count); } @@ -110,7 +104,6 @@ __mutex_fastpath_trylock(atomic_t *count, int (*fail_fn)(atomic_t *)) if (prev < 0) prev = 0; } - smp_mb(); return prev; } -- 1.8.2.3