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From: Konstantin Belousov <kib@FreeBSD.org>
Date: Fri, 5 Dec 2008 21:17:54 +0000 (UTC)
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Subject: svn commit: r185651 - head/sys/i386/include
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Author: kib
Date: Fri Dec  5 21:17:54 2008
New Revision: 185651
URL: http://svn.freebsd.org/changeset/base/185651

Log:
  Unconditionally use locked addition of zero to tip of the stack for
  memory barriers on i386. It works as a serialization instruction on
  all IA32 CPUs.
  
  Alternative solution of using {s,l,}fence requires run-time checking
  of the presense of the corresponding SSE or SSE2 extensions, and
  possible boot-time patching of the kernel text.
  
  Suggested by:	many

Modified:
  head/sys/i386/include/atomic.h

Modified: head/sys/i386/include/atomic.h
==============================================================================
--- head/sys/i386/include/atomic.h	Fri Dec  5 21:17:19 2008	(r185650)
+++ head/sys/i386/include/atomic.h	Fri Dec  5 21:17:54 2008	(r185651)
@@ -32,20 +32,9 @@
 #error this file needs sys/cdefs.h as a prerequisite
 #endif
 
-
-#if defined(I686_CPU)
-#define mb()	__asm__ __volatile__ ("mfence;": : :"memory")
-#define wmb()	__asm__ __volatile__ ("sfence;": : :"memory")
-#define rmb()	__asm__ __volatile__ ("lfence;": : :"memory")
-#else
-/*
- * do we need a serializing instruction?
- */
-#define mb()
-#define wmb()
-#define rmb()
-#endif
-
+#define	mb()	__asm __volatile("lock;addl	$0,(%esp)")
+#define	wmb()	__asm __volatile("lock;addl	$0,(%esp)")
+#define	rmb()	__asm __volatile("lock;addl	$0,(%esp)")
 
 /*
  * Various simple operations on memory, each of which is atomic in the