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    SubjectRe: [Linux-ia64] reader-writer livelock problem
    Date
    In article <1036777105.13021.13.camel@ixodes.goop.org>,
    Jeremy Fitzhardinge <jeremy@goop.org> wrote:
    >
    >Even without interrupts that would be a bug. It isn't ever safe to
    >attempt to retake a read lock if you already hold it, because you may
    >deadlock with a pending writer. Fair multi-reader locks aren't
    >recursive locks.

    .. but I don't think we have any real users who use them for recursion,
    so the only "recursion" right now is through interrupts that use this
    feature.

    (At least that was true a long time time ago, maybe we've added truly
    recursive users since)

    >> Actually, giving this som emore thought, I really suspect that the
    >> simplest solution is to alloc a separate "fair_read_lock()", and paths
    >> that need to care about fairness (and know they don't have the irq
    >> issue)
    >> can use that, slowly porting users over one by one...
    >
    >Do you mean have a separate lock type, or have two different read_lock
    >operations on the current type?

    That depends on whether it is even sanely implementable to share the
    same lock. It may not be.

    From a migration standpoint it would be easiest (by far) to be able to
    share the lock type and to mix operations (ie an interrupt - or
    recursive user - could just use the non-fair version, while others could
    use the fair version on the same lock). However, I have this nagging
    suspicion that it might be a total nightmare to implement efficiently in
    practice.

    I've not looked at it. Any ideas?

    Linus
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