Messages in this thread | | | Date | Thu, 21 Jul 2011 15:59:38 -0700 | From | Andrew Morton <> | Subject | Re: [RFC/PATCH] mm/futex: Fix futex writes on archs with SW tracking of dirty & young |
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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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