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    SubjectRe: [RFC 09/15] PM / Hibernate: user, implement user_ops writer
    On 03/25/2010 11:14 PM, Rafael J. Wysocki wrote:
    > On Wednesday 24 March 2010, Jiri Slaby wrote:
    >> On 03/24/2010 09:42 PM, Pavel Machek wrote:
    >>>> + if (test_bit(TODO_CLOSED, to_do_flags))
    >>>> + return -EIO;
    >>>> +
    >>>> + to_do_buf = buf;
    >>>> + wmb();
    >>>> + set_bit(TODO_WORK, to_do_flags);
    >>>> + wake_up_interruptible(&to_do_wait);
    >>> Uhuh, open-coded barriers... these need to be commented, and I guess
    >>> you just should not play this kind of trickery.
    >> It's just to ensure the to_do_buf store is not reordered with the
    >> set_bit. I wanted to avoid locks as too heavy tools here.
    > No, please use them, at least in a prototype version.
    > We can always optimize things out later, but doing optimizations upfront
    > doesn't really work well from my experience.
    > So, if you'd use a lock somewhere, please use it, or maybe use a completion if
    > that fits the design better.

    That's it, I don't think a lock is appropriate here (I didn't even think
    of that) -- I don't know what to lock (OK, I see it, but it's not that
    clear). There is no potential for race per se, I only need to disable
    reordering (which locks do as a side-effect). I need the steps to be
    done in the A-B order where there is a barrier appropriate. Here, A is
    store to to_do_buf, B is set_bit. It's I set to_do_buf, flag that it may
    be used, the consumer will see the flag and use to_do_buf, in this order.

    Above that if I introduce locks the wait_event on the other side will
    grow into an unreadable mess. I would need to hold a lock when checking
    the condition and hold it until I reach to_do_buf use, but also unlock
    it on all paths that do not reach that point. Yeah, it's indeed doable,
    but I don't think, it will improve things.

    I also don't think completion is appropriate here, as I have a condition
    to check for and it differs over wake_up sites.


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