[ported from trunk r15949, adjusted to apply to linux-atomic.c
 in config/m68k/ <mikpe@it.uu.se>]

Subject: [PATCH] ARM: fix __sync_lock_release for linux
From: Richard Earnshaw <rearnsha at arm dot com>
Date: Thu, 03 Dec 2009 14:28:43 +0000
List-Archive: <http://gcc.gnu.org/ml/gcc-patches/>
List-Post: <mailto:gcc-patches at gcc dot gnu dot org>

When releasing a lock the key memory barrier is there to protect the
writes that occur earlier from being seen after the release of the lock
itself.  In order to do that the dmb instruction needs to appear before
the unlock write not after it.

gcc/

2009-12-03  Richard Earnshaw  <rearnsha@arm.com>

	* arm/linux-atomic.c (SYNC_LOCK_RELEASE): Place memory barrier
	before the lock release.

--- gcc-4.6.3/gcc/config/m68k/linux-atomic.c.~1~	2012-06-18 22:05:40.000000000 +0200
+++ gcc-4.6.3/gcc/config/m68k/linux-atomic.c	2012-06-18 22:08:45.000000000 +0200
@@ -271,8 +271,10 @@ SUBWORD_TEST_AND_SET (char,  1)
   void HIDDEN								\
   __sync_lock_release_##WIDTH (TYPE *ptr)				\
   {									\
-    *ptr = 0;								\
+    /* All writes before this point must be seen before we release	\
+       the lock itself.  */						\
     __kernel_dmb ();							\
+    *ptr = 0;								\
   }
 
 SYNC_LOCK_RELEASE (int,   4)