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    SubjectRe: [RFC/PATCH] mm/futex: Fix futex writes on archs with SW tracking of dirty & young
    On Fri, 22 Jul 2011 08:52:06 +1000
    Benjamin Herrenschmidt <benh@kernel.crashing.org> wrote:

    > On Thu, 2011-07-21 at 15:36 -0700, Andrew Morton wrote:
    > > On Tue, 19 Jul 2011 14:29:22 +1000
    > > Benjamin Herrenschmidt <benh@kernel.crashing.org> wrote:
    > >
    > > > The futex code currently attempts to write to user memory within
    > > > a pagefault disabled section, and if that fails, tries to fix it
    > > > up using get_user_pages().
    > > >
    > > > This doesn't work on archs where the dirty and young bits are
    > > > maintained by software, since they will gate access permission
    > > > in the TLB, and will not be updated by gup().
    > > >
    > > > In addition, there's an expectation on some archs that a
    > > > spurious write fault triggers a local TLB flush, and that is
    > > > missing from the picture as well.
    > > >
    > > > I decided that adding those "features" to gup() would be too much
    > > > for this already too complex function, and instead added a new
    > > > simpler fixup_user_fault() which is essentially a wrapper around
    > > > handle_mm_fault() which the futex code can call.
    > > >
    > > > Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
    > > > ---
    > > >
    > > > Shan, can you test this ? It might not fix the problem
    > >
    > > um, what problem. There's no description here of the user-visible
    > > effects of the bug hence it's hard to work out what kernel version(s)
    > > should receive this patch.
    >
    > Shan could give you an actual example (it was in the previous thread),
    > but basically, livelock as the kernel keeps trying and trying the
    > in_atomic op and never resolves it.
    >
    > > What kernel version(s) should receive this patch?
    >
    > I haven't dug. Probably anything it applies on as far as we did that
    > trick of atomic + gup() for futex.

    You're not understanding me.

    I need a good reason to merge this into 3.0.

    The -stable maintainers need even better reasons to merge this into
    earlier kernels.

    Please provide those reasons!

    (Documentation/stable_kernel_rules.txt, 4th bullet)

    (And it's not just me and -stable maintainers. Distro maintainers will
    also look at this patch and wonder whether they should merge it)


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