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    SubjectRe: MM deadlock [was: Re: arca-vm-8...]


    On Mon, 11 Jan 1999, Savochkin Andrey Vladimirovich wrote:
    > On Sun, Jan 10, 1999 at 10:35:10AM -0800, Linus Torvalds wrote:
    > > The thing I want to make re-entrant is just semaphore accesses: at the
    > > point where we would otherwise deadlock on the writer semaphore it's much
    > > better to just allow nested writes. I suspect all filesystems can already
    > > handle nested writes - they are a lot easier to handle than truly
    > > concurrent ones.
    >
    > You're an optimist, aren't you? :-)

    No, drugged to my eye-brows.

    > In any case I've checked your recursive semaphore code on a news server
    > which reliably deadlocked with the previous kernels.
    > The code seems to work well.

    I found a rather nasty race in my implementation - it's basically
    impossible to triggerin real life, but quite frankly I don't want to have
    semaphores that have a really subtle bug in them.

    However much I tried, I couldn't make the race go away without using a
    spinlock in the critical path of the semaphore, something which I very
    much want to avoid.

    Unless I find a good recursive semaphore implementation (and I'm starting
    to despair about finding one that is lock-free for the non-contention
    case), I'll have to come up with something else (like letting only kswapd
    swap out pages as has been discussed here).

    Linus


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