[PATCH] cmpxchg8b needs lock prefix

From 2.4

diff --git a/include/asm-i386/system.h b/include/asm-i386/system.h
index 5d5588788fe5f1810bc7b64396d68a09e75cf867..fc8f116430bc6376abc373b0d84ed0af9f975682 100644 (file)
--- a/include/asm-i386/system.h
+++ b/include/asm-i386/system.h
@@ -150,6 +150,11 @@ struct __xchg_dummy { unsigned long a[100]; };
  * to do an SIMD/3DNOW!/MMX/FPU 64-bit store here, but that
  * might have an implicit FPU-save as a cost, so it's not
  * clear which path to go.)
+ *
+ * cmpxchg8b must be used with the lock prefix here to allow
+ * the instruction to be executed atomically, see page 3-102
+ * of the instruction set reference 24319102.pdf. We need
+ * the reader side to see the coherent 64bit value.
  */
 static inline void __set_64bit (unsigned long long * ptr,
                unsigned int low, unsigned int high)
@@ -158,7 +163,7 @@ static inline void __set_64bit (unsigned long long * ptr,
                "\n1:\t"
                "movl (%0), %%eax\n\t"
                "movl 4(%0), %%edx\n\t"
-               "cmpxchg8b (%0)\n\t"
+               "lock cmpxchg8b (%0)\n\t"
                "jnz 1b"
                : /* no outputs */
                :       "D"(ptr),