x86_capability can span two cache lines depending on kernel configuration and building environment. When #AC exception is enabled for split locked accesses, clear_cpufeature() may generate #AC exception because of atomic setting or clearing bits in x86_capability.
But kernel clears cpufeature only during a CPU is booting up. Therefore, there is no racing condition when clear_cpufeature() is called and no need to atomically clear or set bits in x86_capability. To avoid #AC exception caused by split lock, call non atomic __set_bit() and __clear_bit(). They are faster than atomic set_bit() and clear_bit() as well. Signed-off-by: Fenghua Yu <[email protected]> --- arch/x86/kernel/cpu/cpuid-deps.c | 10 +++++----- 1 file changed, 5 insertions(+), 5 deletions(-) diff --git a/arch/x86/kernel/cpu/cpuid-deps.c b/arch/x86/kernel/cpu/cpuid-deps.c index 2c0bd38a44ab..b2c2a004c769 100644 --- a/arch/x86/kernel/cpu/cpuid-deps.c +++ b/arch/x86/kernel/cpu/cpuid-deps.c @@ -65,15 +65,15 @@ static const struct cpuid_dep cpuid_deps[] = { static inline void clear_feature(struct cpuinfo_x86 *c, unsigned int feature) { /* - * Note: This could use the non atomic __*_bit() variants, but the - * rest of the cpufeature code uses atomics as well, so keep it for - * consistency. Cleanup all of it separately. + * Because this code is only called during boot time and there + * is no need to be atomic, use non atomic __*_bit() for better + * performance and to avoid #AC exception for split locked access. */ if (!c) { clear_cpu_cap(&boot_cpu_data, feature); - set_bit(feature, (unsigned long *)cpu_caps_cleared); + __set_bit(feature, (unsigned long *)cpu_caps_cleared); } else { - clear_bit(feature, (unsigned long *)c->x86_capability); + __clear_bit(feature, (unsigned long *)c->x86_capability); } } -- 2.5.0
