On Fri, Jul 30, 2010 at 05:04:46PM +1000, Benjamin Herrenschmidt wrote:

On Thu, 2010-07-08 at 16:08 +1000, Benjamin Herrenschmidt wrote:

quoted

On Fri, 2010-06-25 at 11:56 +0200, Jakub Jelinek wrote:

quoted

 static inline void sync(void)

diff --git a/arch/powerpc/include/asm/atomic.h b/arch/powerpc/include/asm/atomic.h
index b8f152e..288d8b2 100644
--- a/arch/powerpc/include/asm/atomic.h
+++ b/arch/powerpc/include/asm/atomic.h

@@ -19,14 +19,14 @@ static __inline__ int atomic_read(const atomic_t *v)
 {
 	int t;
 
-	__asm__ __volatile__("lwz%U1%X1 %0,%1" : "=r"(t) : "m"(v->counter));
+	__asm__ __volatile__("lwz%U1%X1 %0,%1" : "=r"(t) : "m<>"(v->counter));
 
 	return t;
 }
 

This gives me:

/home/benh/linux-powerpc-test/arch/powerpc/kernel/time.c: In function ‘timer_interrupt’:
/home/benh/linux-powerpc-test/arch/powerpc/include/asm/atomic.h:22: error: ‘asm’ operand has impossible constraints
make[2]: *** [arch/powerpc/kernel/time.o] Error 1

$ gcc --version
gcc (Debian 4.4.4-1) 4.4.4

Ping :-)

Do that mean that 4.4.4 doesn't understand your new constraints or are
we doing something incorrect ?

The constraints weren't new, so in theory everything would work fine.
Except because < and > were so rarely used on many targets before,
there were backend bugs on PowerPC and SPARC at least related to that.
See
http://gcc.gnu.org/PR44707
http://gcc.gnu.org/PR44701
http://gcc.gnu.org/PR44492

So, in short, I'm afraid "m<>" needs to be used only for GCC 4.6+
(and, vendors which backported the inline-asm handling changes
to their older gcc would need to adjust for their gccs too).
When "m<>" isn't used, it just leads to potential code pessimization
in inline-asms that are prepared for handling side-effects.

	Jakub