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


    On Sat, 9 Jan 1999, Savochkin Andrey Vladimirovich wrote:
    >
    > I've found an another deadlock.

    Yes. This is a case I knew about, and that Alan already mentioned. Trying
    to write from a shared mapping has a path that can take the write
    semaphore twice.

    This one is a whole lot harder to fix - the previous one needed only a
    simple extra flag, this one is truly nasty.

    The cleanest solution I can think of is actually to allow semaphores to be
    recursive. I can do that with minimal overhead (just one extra instruction
    in the non-contention case), so it's not too bad, and I've wanted to do it
    for certain other things, but it's still a nasty piece of code to mess
    around with.

    Oh, well. I don't think I have much choice. Making the swap-out routines
    refuse to touch an inode that is busy is a sure way to allow people to
    let bad users lock down infinite amounts of memory.

    Linus


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