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    SubjectRe: [patch 09/10] SLUB: Do our own locking via slab_lock and slab_unlock.
    On Tue, 30 Oct 2007, Nick Piggin wrote:

    > Is this actually a speedup on any architecture to roll your own locking
    > rather than using bit spinlock?

    It avoids one load from memory when allocating and the release is simply
    writing the page->flags back. Less instructions.

    > I am not exactly convinced that smp_wmb() is a good idea to have in your
    > unlock, rather than the normally required smp_mb() that every other open
    > coded lock in the kernel is using today. If you comment every code path
    > where a load leaking out of the critical section would not be a problem,
    > then OK it may be correct, but I still don't think it is worth the
    > maintenance overhead.

    I thought you agreed that release semantics only require a write barrier?
    The issue here is that other processors see the updates before the
    updates to page-flags.

    A load leaking out of a critical section would require that the result of
    the load is not used to update other information before the slab_unlock
    and that the source of the load is not overwritten in the critical
    section. That does not happen in sluib.

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