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    SubjectRe: [PATCH] seqlock: serialize against writers
    Andi Kleen wrote:
    > On Fri, Aug 29, 2008 at 12:29:42PM -0400, Gregory Haskins wrote:
    >
    >> Andi Kleen wrote:
    >>
    >>>> Im running it on a x86_64 box as we speak. How can I tell if there is a
    >>>> certain mode that is permitting this?
    >>>>
    >>>>
    >>> If the boot up says you're running with PMtimer then it uses the fallback
    >>> (usually happens on pre Fam10h AMD boxes). A typical Intel box
    >>> would use the faster ring 3 only TSC path and then explode with your
    >>> change I bet.
    >>>
    >>>
    >> Thinking about this some more, perhaps the issue is I am not hitting the
    >> contended path in vsyscall?
    >>
    >
    > Yes it will be only contended when gettimeofday() races with the timer
    > interrupt. You could try to run gettimeofday() in a loop and see how
    > long it holds up.
    >
    > But anyways from the theory you should crash when it happens.
    > Writes to kernel data are not allowed in vsyscalls and your read_lock clearly
    > does a write.
    >

    Oh I don't deny that it does. The compiler neatly reminded me that it
    could no longer be "const". I just was ignorant of the userspace
    requirement ;)

    But we *do* have a serious problem here. A non-preemptible seqlock_t
    will do bad things if it preempts the writer, so we need some kind of
    solution here, one way or the other. So suggestions welcome :) I
    realize this is only an issue currently in PREEMPT_RT so perhaps most
    will not care... but I do need to solve this at least for this branch.

    I currently do not see a way to solve this problem that doesn't involve
    some heavier involvement in the read path (read: const seqlock_t need
    not apply). Given that, we either need to address the const requirement
    for userspace, alter userspace's usage, or find another way to address
    the deadlock. Any ideas?

    -Greg


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