From: Nick Piggin <npiggin@suse.de>
To: Linus Torvalds <torvalds@osdl.org>, Andrew Morton <akpm@osdl.org>
Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: [patch 1/2] atomic_add_unless sadness
Date: Wed, 18 Jan 2006 07:36:37 +0100 [thread overview]
Message-ID: <20060118063636.GA14608@wotan.suse.de> (raw)
For some reason gcc 4 on at least i386 and ppc64 (that I have tested with)
emit two cmpxchges for atomic_add_unless unless we put branch hints in.
(it is unlikely for the "unless" case to trigger, and it is unlikely for
cmpxchg to fail).
So put these hints in for architectures which have a native cas, and
make the loop a bit more readable in the process.
So this patch isn't for inclusion just yet (incomplete, not widely tested),
however hopefully we can get some discussion about the best way to implement
this.
Index: linux-2.6/include/asm-i386/atomic.h
===================================================================
--- linux-2.6.orig/include/asm-i386/atomic.h
+++ linux-2.6/include/asm-i386/atomic.h
@@ -231,9 +231,15 @@ static __inline__ int atomic_sub_return(
({ \
int c, old; \
c = atomic_read(v); \
- while (c != (u) && (old = atomic_cmpxchg((v), c, c + (a))) != c) \
+ for (;;) { \
+ if (unlikely(c == (u))) \
+ break; \
+ old = atomic_cmpxchg((v), c, c + (a)); \
+ if (likely(old == c)) \
+ break; \
c = old; \
- c != (u); \
+ } \
+ likely(c != (u)); \
})
#define atomic_inc_not_zero(v) atomic_add_unless((v), 1, 0)
Index: linux-2.6/include/asm-ia64/atomic.h
===================================================================
--- linux-2.6.orig/include/asm-ia64/atomic.h
+++ linux-2.6/include/asm-ia64/atomic.h
@@ -95,9 +95,15 @@ ia64_atomic64_sub (__s64 i, atomic64_t *
({ \
int c, old; \
c = atomic_read(v); \
- while (c != (u) && (old = atomic_cmpxchg((v), c, c + (a))) != c) \
+ for (;;) { \
+ if (unlikely(c == (u))) \
+ break; \
+ old = atomic_cmpxchg((v), c, c + (a)); \
+ if (likely(old == c)) \
+ break; \
c = old; \
- c != (u); \
+ } \
+ likely(c != (u)); \
})
#define atomic_inc_not_zero(v) atomic_add_unless((v), 1, 0)
Index: linux-2.6/include/asm-x86_64/atomic.h
===================================================================
--- linux-2.6.orig/include/asm-x86_64/atomic.h
+++ linux-2.6/include/asm-x86_64/atomic.h
@@ -405,9 +405,15 @@ static __inline__ long atomic64_sub_retu
({ \
int c, old; \
c = atomic_read(v); \
- while (c != (u) && (old = atomic_cmpxchg((v), c, c + (a))) != c) \
+ for (;;) { \
+ if (unlikely(c == (u))) \
+ break; \
+ old = atomic_cmpxchg((v), c, c + (a)); \
+ if (likely(old == c)) \
+ break; \
c = old; \
- c != (u); \
+ } \
+ likely(c != (u)); \
})
#define atomic_inc_not_zero(v) atomic_add_unless((v), 1, 0)
Index: linux-2.6/include/asm-s390/atomic.h
===================================================================
--- linux-2.6.orig/include/asm-s390/atomic.h
+++ linux-2.6/include/asm-s390/atomic.h
@@ -89,11 +89,16 @@ static __inline__ int atomic_cmpxchg(ato
static __inline__ int atomic_add_unless(atomic_t *v, int a, int u)
{
int c, old;
-
c = atomic_read(v);
- while (c != u && (old = atomic_cmpxchg(v, c, c + a)) != c)
+ for (;;) {
+ if (unlikely(c == u))
+ break;
+ old = atomic_cmpxchg(v, c, c + a);
+ if (likely(old == c))
+ break;
c = old;
- return c != u;
+ }
+ return likely(c != u);
}
#define atomic_inc_not_zero(v) atomic_add_unless((v), 1, 0)
Index: linux-2.6/include/asm-sparc64/atomic.h
===================================================================
--- linux-2.6.orig/include/asm-sparc64/atomic.h
+++ linux-2.6/include/asm-sparc64/atomic.h
@@ -78,9 +78,15 @@ extern int atomic64_sub_ret(int, atomic6
({ \
int c, old; \
c = atomic_read(v); \
- while (c != (u) && (old = atomic_cmpxchg((v), c, c + (a))) != c) \
+ for (;;) { \
+ if (unlikely(c == (u))) \
+ break; \
+ old = atomic_cmpxchg((v), c, c + (a)); \
+ if (likely(old == c)) \
+ break; \
c = old; \
- c != (u); \
+ } \
+ likely(c != (u)); \
})
#define atomic_inc_not_zero(v) atomic_add_unless((v), 1, 0)
Index: linux-2.6/include/asm-m68k/atomic.h
===================================================================
--- linux-2.6.orig/include/asm-m68k/atomic.h
+++ linux-2.6/include/asm-m68k/atomic.h
@@ -146,9 +146,15 @@ static inline void atomic_set_mask(unsig
({ \
int c, old; \
c = atomic_read(v); \
- while (c != (u) && (old = atomic_cmpxchg((v), c, c + (a))) != c) \
+ for (;;) { \
+ if (unlikely(c == (u))) \
+ break; \
+ old = atomic_cmpxchg((v), c, c + (a)); \
+ if (likely(old == c)) \
+ break; \
c = old; \
- c != (u); \
+ } \
+ likely(c != (u)); \
})
#define atomic_inc_not_zero(v) atomic_add_unless((v), 1, 0)
next reply other threads:[~2006-01-18 6:36 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2006-01-18 6:36 Nick Piggin [this message]
2006-01-18 6:39 ` [patch 2/2] powerpc: native atomic_add_unless Nick Piggin
2006-01-18 17:11 ` Joel Schopp
2006-01-18 17:28 ` Nick Piggin
2006-01-18 21:05 ` Joel Schopp
2006-01-19 14:04 ` Nick Piggin
2006-01-18 16:48 ` [patch 1/2] atomic_add_unless sadness Linus Torvalds
2006-01-18 17:10 ` Nick Piggin
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