linux-kernel - Re: [cpuops cmpxchg V2 5/5] cpuops: Use cmpxchg for xchg to avoid lock semantics

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Date:	Tue, 14 Dec 2010 10:55:56 -0600 (CST)
From:	Christoph Lameter <cl@...ux.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
cc:	Tejun Heo <tj@...nel.org>, akpm@...ux-foundation.org,
	Pekka Enberg <penberg@...helsinki.fi>,
	linux-kernel@...r.kernel.org, "H. Peter Anvin" <hpa@...or.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: Re: [cpuops cmpxchg V2 5/5] cpuops: Use cmpxchg for xchg to avoid
 lock semantics

On Tue, 14 Dec 2010, Eric Dumazet wrote:

> You should use the fact that the failed cmpxchg loads in al/ax/eax/rax
> the current value, so :
>
> 	"\n\tmov "__percpu_arg(1)",%%al"
> 	"\n1:\tcmpxchgb %2, "__percpu_arg(1)
> 	"\n\tjnz 1b"
>
> (No need to reload the value again)

Subject: cpuops xchg: Exploit the fact that failed cmpxchges return old value

We do not need to reload the value as pointed out by Eric. It is already in
the correct register so just rerun the cmpxchg without the load.

Signed-off-by: Christoph Lameter <cl@...ux.com>

---
 arch/x86/include/asm/percpu.h |   16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

Index: linux-2.6/arch/x86/include/asm/percpu.h
===================================================================
--- linux-2.6.orig/arch/x86/include/asm/percpu.h	2010-12-14 10:51:57.000000000 -0600
+++ linux-2.6/arch/x86/include/asm/percpu.h	2010-12-14 10:54:46.000000000 -0600
@@ -223,32 +223,32 @@ do {									\
 	typeof(var) __new = (nval);					\
 	switch (sizeof(var)) {						\
 	case 1:								\
-		asm("\n1:mov "__percpu_arg(1)",%%al"			\
-		    "\n\tcmpxchgb %2, "__percpu_arg(1)			\
+		asm("\n\tmov "__percpu_arg(1)",%%al"			\
+		    "\n1:cmpxchgb %2, "__percpu_arg(1)		\
 		    "\n\tjnz 1b"					\
 			    : "=a" (__ret), "+m" (var)			\
 			    : "q" (__new)				\
 			    : "memory");				\
 		break;							\
 	case 2:								\
-		asm("\n1:mov "__percpu_arg(1)",%%ax"			\
-		    "\n\tcmpxchgw %2, "__percpu_arg(1)			\
+		asm("\n\tmov "__percpu_arg(1)",%%ax"			\
+		    "\n1:cmpxchgw %2, "__percpu_arg(1)		\
 		    "\n\tjnz 1b"					\
 			    : "=a" (__ret), "+m" (var)			\
 			    : "r" (__new)				\
 			    : "memory");				\
 		break;							\
 	case 4:								\
-		asm("\n1:mov "__percpu_arg(1)",%%eax"			\
-		    "\n\tcmpxchgl %2, "__percpu_arg(1)			\
+		asm("\n\tmov "__percpu_arg(1)",%%eax"			\
+		    "\n1:cmpxchgl %2, "__percpu_arg(1)		\
 		    "\n\tjnz 1b"					\
 			    : "=a" (__ret), "+m" (var)			\
 			    : "r" (__new)				\
 			    : "memory");				\
 		break;							\
 	case 8:								\
-		asm("\n1:mov "__percpu_arg(1)",%%rax"			\
-		    "\n\tcmpxchgq %2, "__percpu_arg(1)			\
+		asm("\n\tmov "__percpu_arg(1)",%%rax"			\
+		    "\n1:cmpxchgq %2, "__percpu_arg(1)		\
 		    "\n\tjnz 1b"					\
 			    : "=a" (__ret), "+m" (var)			\
 			    : "r" (__new)				\
--
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