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@xxxxxxx>
---
Index: linux-2.6/include/asm-generic/mutex-dec.h
===================================================================
--- linux-2.6.orig/include/asm-generic/mutex-dec.h
+++ linux-2.6/include/asm-generic/mutex-dec.h
@@ -22,8 +22,6 @@ __mutex_fastpath_lock(atomic_t *count, v
{
if (unlikely(atomic_dec_return(count) < 0))
fail_fn(count);
- else
- smp_mb();
}
/**
@@ -41,10 +39,7 @@ __mutex_fastpath_lock_retval(atomic_t *c
{
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 *c
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);
}
@@ -98,15 +92,9 @@ __mutex_fastpath_trylock(atomic_t *count
* 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
Index: linux-2.6/include/asm-generic/mutex-xchg.h
===================================================================
--- linux-2.6.orig/include/asm-generic/mutex-xchg.h
+++ linux-2.6/include/asm-generic/mutex-xchg.h
@@ -27,8 +27,6 @@ __mutex_fastpath_lock(atomic_t *count, v
{
if (unlikely(atomic_xchg(count, 0) != 1))
fail_fn(count);
- else
- smp_mb();
}
/**
@@ -46,10 +44,7 @@ __mutex_fastpath_lock_retval(atomic_t *c
{
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 *c
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
if (prev < 0)
prev = 0;
}
- smp_mb();
return prev;
}
--
To unsubscribe from this list: send the line "unsubscribe linux-arch" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
