[PATCH -mm] ACPI lock: cpu_relax() (was: [RFC -mm] more cpu_relax() places?)

[Andreas Mohr]

Hi all,

On Mon, Jun 12, 2006 at 08:37:43PM +0200, Andreas Mohr wrote:
> Hi all,
> 
> while reviewing 2.6.17-rc6-mm1, I found some places that might
> want to make use of cpu_relax() in order to not block secondary
> pipelines while busy-polling (probably especially useful on SMT CPUs):

Patch no. 2 of 3.

This could be considered overkill given the previous unlikely(),
but it is busy-looping on failure after all...

Signed-off-by: Andreas Mohr <andi@xxxxxxxx>


diff -urN linux-2.6.17-rc6-mm2.orig/include/asm-i386/acpi.h linux-2.6.17-rc6-mm2.my/include/asm-i386/acpi.h
--- linux-2.6.17-rc6-mm2.orig/include/asm-i386/acpi.h	2006-06-08 10:38:10.000000000 +0200
+++ linux-2.6.17-rc6-mm2.my/include/asm-i386/acpi.h	2006-06-13 19:35:41.000000000 +0200
@@ -61,11 +61,14 @@
 __acpi_acquire_global_lock (unsigned int *lock)
 {
 	unsigned int old, new, val;
-	do {
+	while (1) {
 		old = *lock;
 		new = (((old & ~0x3) + 2) + ((old >> 1) & 0x1));
 		val = cmpxchg(lock, old, new);
-	} while (unlikely (val != old));
+		if (likely(val == old))
+			break;
+		cpu_relax();
+	}
 	return (new < 3) ? -1 : 0;
 }
 
@@ -73,11 +76,14 @@
 __acpi_release_global_lock (unsigned int *lock)
 {
 	unsigned int old, new, val;
-	do {
+	while (1) {
 		old = *lock;
 		new = old & ~0x3;
 		val = cmpxchg(lock, old, new);
-	} while (unlikely (val != old));
+		if (likely(val == old))
+			break;
+		cpu_relax();
+	}
 	return old & 0x1;
 }
 
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