Subject: [PATCH] ARM support single byte cmpxchg and cmpxchg_local on ARMv6

From: [email-masked] (Mathieu Desnoyers)
Date: Wed, 10 Mar 2010 15:02:36 -0500

* Russell King - ARM Linux (linux at arm.linux.org.uk) wrote:
> On Wed, Mar 10, 2010 at 06:22:21PM +0200, Imre Deak wrote:
> > recently you've added support for __cmpxchg for ARMv6+. There the assumption
> > is that ldrex[bh]/strex[bh] is only supported on platforms with the 32v6K
> > extension.
> > 
> > Currently the __xchg code uses these even without the extension. Should this
> > be fixed?
> 
> From what I remember, the half-word versions definitely aren't supported
> on anything without V6K extensions.  I think that the byte and word
> versions are supported on V6 and up though.
> 
> That'd make both __cmpxchg and __xchg slightly buggy, in different ways.
> 
> What it does mean is that atomic operations on unsigned shorts using
> ldrex/strex will only be possible on V6K and up.

Does the following help (addressing the single byte cmpxchg part of the
problem) ?

ARM support single byte cmpxchg and cmpxchg_local on ARMv6

Signed-off-by: Mathieu Desnoyers <[email-masked]>
CC: Russell King - ARM Linux <[email-masked]>
CC: Imre Deak <[email-masked]>
---
 arch/arm/include/asm/system.h |    3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

Index: linux-2.6-lttng.git/arch/arm/include/asm/system.h
===================================================================
--- linux-2.6-lttng.git.orig/arch/arm/include/asm/system.h	2010-03-10 14:49:59.000000000 -0500
+++ linux-2.6-lttng.git/arch/arm/include/asm/system.h	2010-03-10 14:57:41.000000000 -0500
@@ -352,7 +352,6 @@
 	unsigned long oldval, res;
 
 	switch (size) {
-#ifdef CONFIG_CPU_32v6K
 	case 1:
 		do {
 			asm volatile("@ __cmpxchg1\n"
@@ -365,6 +364,7 @@
 				: "memory", "cc");
 		} while (res);
 		break;
+#ifdef CONFIG_CPU_32v6K
 	case 2:
 		do {
 			asm volatile("@ __cmpxchg1\n"
@@ -424,7 +424,6 @@
 
 	switch (size) {
 #ifndef CONFIG_CPU_32v6K
-	case 1:
 	case 2:
 		ret = __cmpxchg_local_generic(ptr, old, new, size);
 		break;

-- 
Mathieu Desnoyers
Operating System Efficiency Consultant
EfficiOS Inc.
http://www.efficios.com