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Subject[PATCH 04/15] x86/split_lock: Use non locked bit set instruction in set_cpu_cap
set_bit() called by set_cpu_cap() is a locked bit set instruction for
atomic operation.

Since the c->x86_capability is not aligned to cache line depending on
compiler, the locked bit set instruction falls into split lock
situation which causes #AC exception when #AC exception is enabled by
split locked accesses.

But set_cpu_cap() is only called in early init phase on a CPU and
therefore there is no contention for set_cpu_cap(). So it's unnecessary
to be atomic operation.

Using __set_bit(), which is not a locked instruction, can fix the
kernel split lock issue and is faster than locked instruction.

Signed-off-by: Fenghua Yu <>
Acked-by: Dave Hansen <>
arch/x86/include/asm/cpufeature.h | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/cpufeature.h b/arch/x86/include/asm/cpufeature.h
index b27da9602a6d..604f4c514efa 100644
--- a/arch/x86/include/asm/cpufeature.h
+++ b/arch/x86/include/asm/cpufeature.h
@@ -128,7 +128,8 @@ extern const char * const x86_bug_flags[NBUGINTS*32];

#define boot_cpu_has(bit) cpu_has(&boot_cpu_data, bit)

-#define set_cpu_cap(c, bit) set_bit(bit, (unsigned long *)((c)->x86_capability))
+#define set_cpu_cap(c, bit) \
+ __set_bit(bit, (unsigned long *)((c)->x86_capability))

extern void setup_clear_cpu_cap(unsigned int bit);
extern void clear_cpu_cap(struct cpuinfo_x86 *c, unsigned int bit);
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