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    SubjectRe: [PATCH v2] netlink: Replace rhash_portid with bound
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    On Tue, Sep 22, 2015 at 9:10 AM, Tejun Heo <tj@kernel.org> wrote:
    >
    > That's a pentium pro era errata. Virtually no working machine is
    > affected by that anymore and nobody builds kernel with that option.
    > In most cases, store_release and load_acquire are cheaper as they're
    > more specific. On x86, store_release and load_acquire boil down to
    > compiler reordering barriers. You're running in the opposite
    > direction.

    Well, to be fair, there are lots of machines where acquire/release is
    actually quite expensive.

    In general, the cheapest barrier there is (apart from the "no barrier
    at all" or just "compiler barrier") is "smp_wmb()". If an
    architecture gets that one wrong, the architects were f*cking morons.
    It should be a fundamentally cheap operation, since writes are
    buffered and it should simply be a buffer barrier.

    The acquire/release things are generally fairly cheap on modern
    architectures. Not free (except on x86), but fairly low-cost. HOWEVER,
    they are not at all free on some older architectures, including 32-bit
    ARM.

    smp_rmb() should generally be about the same cost as an acquire. It
    can go either way.

    So *if* the algorithm is amenable to smp_wmb()/smp_rmb() kind of
    barriers, that's actually quite possibly better than acquire/release.

    smp_mb() is expensive pretty much everywhere.

    Looking forward, I suspect long-term acquire/release is what hardware
    is going to be "reasonably good at", but as things are right now, you
    can't necessarily rely on them being fast.

    Linus


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