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    SubjectRe: [PATCH 1/2] mm/slub: optimize alloc/free fastpath by removing preemption on/off
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    On Mon, 2015-01-05 at 10:36 +0900, Joonsoo Kim wrote:
    > - preempt_disable();
    > - c = this_cpu_ptr(s->cpu_slab);
    > + do {
    > + tid = this_cpu_read(s->cpu_slab->tid);
    > + c = this_cpu_ptr(s->cpu_slab);
    > + } while (IS_ENABLED(CONFIG_PREEMPT) && unlikely(tid != c->tid));
    > + barrier();

    I don't see the compiler reodering the object/page stores below, since c
    is updated in the loop anyway. Is this really necessary (same goes for
    slab_free)? The generated code by gcc 4.8 looks correct without it.
    Additionally, the implied barriers for preemption control aren't really
    the same semantics used here (if that is actually the reason why you are
    using them).

    Thanks,
    Davidlohr



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