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 <npiggin@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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 @@
{
if (unlikely(atomic_dec_return(count) < 0))
fail_fn(count);
- else
- smp_mb();
}
/**
@@ -41,10 +39,7 @@
{
if (unlikely(atomic_dec_return(count) < 0))
return fail_fn(count);
- else {
- smp_mb();
- return 0;
- }
+ return 0;
}
/**
@@ -63,7 +58,6 @@
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 @@
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