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    SubjectRe: kernel bug: futex_wait hang
    Jamie Lokier <jamie@shareable.org> wrote:
    >
    > Andrew Morton wrote:
    > > iirc we ended up deciding that the futex problems around that time were due
    > > to userspace problems (a version of libc). But then, there's no discussion
    > > around Seto's patch and it didn't get applied. So I don't know what
    > > happened to that work - it's all a bit mysterious.
    >
    > It can be fixed _either_ in Glibc, or by changing the kernel.
    >
    > That problem is caused by differing assumptions between Glibc and the
    > kernel about subtle futex semantics. Which goes to show they are
    > really clever, or something.
    >
    > I provided pseudo-code for the Glibc fix, but not an actual patch
    > because NPTL is quite complicated and I wanted to know the Glibc
    > people were interested, but apparently they were too busy at the time
    > - benchmarks would have made sense for such a patch.
    >
    > Scott Snyder started fixing part of Glibc, and that did fix his
    > instance of this problem so we know the approach works. But a full
    > patch for Glibc was not prepared.
    >
    > The most recent messages under "Futex queue_me/get_user ordering",
    > with a patch from Jakub Jelinek will fix this problem by changing the
    > kernel. Yes, you should apply Jakub's most recent patch, message-ID
    > "<20050318165326.GB32746@devserv.devel.redhat.com>".
    >
    > I have not tested the patch, but it looks convincing.

    OK, thanks. Lee && Paul, that's at
    ftp://ftp.kernel.org/pub/linux/kernel/people/akpm/patches/2.6/2.6.12-rc1/2.6.12-rc1-mm1/broken-out/futex-queue_me-get_user-ordering-fix.patch


    > I argued for fixing Glibc on the grounds that the changed kernel
    > behaviour, or more exactly having Glibc depend on it, loses a certain
    > semantic property useful for unusual operations on multiple futexes at
    > the same time. But I appear to have lost the argument, and Jakub's
    > latest patch does clean up some cruft quite nicely, with no expected
    > performance hit.

    Futexes were initially so simple.
    -
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