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Diffstat (limited to 'arch/x86/include/asm/percpu.h')
-rw-r--r--arch/x86/include/asm/percpu.h21
1 files changed, 15 insertions, 6 deletions
diff --git a/arch/x86/include/asm/percpu.h b/arch/x86/include/asm/percpu.h
index b85ade511a53..8ee45167e817 100644
--- a/arch/x86/include/asm/percpu.h
+++ b/arch/x86/include/asm/percpu.h
@@ -263,8 +263,9 @@ do { \
})
/*
- * Beware: xchg on x86 has an implied lock prefix. There will be the cost of
- * full lock semantics even though they are not needed.
+ * xchg is implemented using cmpxchg without a lock prefix. xchg is
+ * expensive due to the implied lock prefix. The processor cannot prefetch
+ * cachelines if xchg is used.
*/
#define percpu_xchg_op(var, nval) \
({ \
@@ -272,25 +273,33 @@ do { \
typeof(var) pxo_new__ = (nval); \
switch (sizeof(var)) { \
case 1: \
- asm("xchgb %2, "__percpu_arg(1) \
+ asm("\n1:mov "__percpu_arg(1)",%%al" \
+ "\n\tcmpxchgb %2, "__percpu_arg(1) \
+ "\n\tjnz 1b" \
: "=a" (pxo_ret__), "+m" (var) \
: "q" (pxo_new__) \
: "memory"); \
break; \
case 2: \
- asm("xchgw %2, "__percpu_arg(1) \
+ asm("\n1:mov "__percpu_arg(1)",%%ax" \
+ "\n\tcmpxchgw %2, "__percpu_arg(1) \
+ "\n\tjnz 1b" \
: "=a" (pxo_ret__), "+m" (var) \
: "r" (pxo_new__) \
: "memory"); \
break; \
case 4: \
- asm("xchgl %2, "__percpu_arg(1) \
+ asm("\n1:mov "__percpu_arg(1)",%%eax" \
+ "\n\tcmpxchgl %2, "__percpu_arg(1) \
+ "\n\tjnz 1b" \
: "=a" (pxo_ret__), "+m" (var) \
: "r" (pxo_new__) \
: "memory"); \
break; \
case 8: \
- asm("xchgq %2, "__percpu_arg(1) \
+ asm("\n1:mov "__percpu_arg(1)",%%rax" \
+ "\n\tcmpxchgq %2, "__percpu_arg(1) \
+ "\n\tjnz 1b" \
: "=a" (pxo_ret__), "+m" (var) \
: "r" (pxo_new__) \
: "memory"); \