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    SubjectRe: Async resume patch (was: Re: [GIT PULL] PM updates for 2.6.33)
    On Tue, 8 Dec 2009, Linus Torvalds wrote:

    > Side note: if this was a real lock, you'd also needed an smp_wmb() in the
    > 'wait_lock()' path after the atomic_inc(), to make sure that others see
    > the atomic lock was seen by other people before the suspend started.
    >
    > In your usage scenario, I don't think it would ever be noticeable, since
    > the other users are always going to start running from the same thread
    > that did the wait_lock(), so even if they run on other CPU's, we'll have
    > scheduled _to_ those other CPU's and done enough memory ordering to
    > guarantee that they will see the thing.
    >
    > So it would be ok in this situation, simply because it acts as an
    > initializer and never sees any real SMP issues.

    Yes. I would have brought this up, but you made the point for me.

    > But it's an example of how you now don't just depend on the locking
    > primitives themselves doing the right thing, you end up depending very
    > subtly on exactly how the lock is used. The standard locks do have the
    > same kind of issue for initializers, but we avoid it elsewhere because
    > it's so risky.

    No doubt there are other reasons why the "wait-lock" pattern doesn't
    get used enough to be noticed.

    Alan Stern



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