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    SubjectRe: [tip:x86/mm] x86/pat: Avoid contention on cpa_lock if possible
    On 06/06/2012 10:24 AM, Peter Zijlstra wrote:
    > On Wed, 2012-06-06 at 09:18 -0700, tip-bot for Shai Fultheim wrote:
    >
    >> [ I absolutely hate these locking patterns ... yet I have no better idea. Maybe the gents on Cc: ... ]
    >> Signed-off-by: Ingo Molnar <mingo@kernel.org>
    >
    > Oh yuck, this is vile..
    >
    > static struct static_key scale_mp_trainwreck = STATIC_KEY_INIT_FALSE;
    >
    > static DEFINE_SPINLOCK(_cpa_lock);
    >
    > static inline void cpa_lock(void)
    > {
    > if (static_key_false(&scale_mp_trainwreck))
    > return;
    >
    > spin_lock(&_cpa_lock);
    > }
    >
    > static inline void cpa_unlock(void)
    > {
    > if (static_key_false(&scale_mp_trainwreck))
    > return;
    >
    > spin_lock(&_cpa_lock);
    > }
    >
    > And then use cpa_{,un}lock(), and the scale-mp guys can
    > static_key_slow_inc(&scale_mp_trainwreck).
    >

    Actually, for this particular subcase I would use a synthetic CPUID bit
    and use static_cpu_has().

    -hpa

    --
    H. Peter Anvin, Intel Open Source Technology Center
    I work for Intel. I don't speak on their behalf.



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