Re: [RFC/PATCH] mm/futex: Fix futex writes on archs with SWtracking of dirty & young

From: Benjamin Herrenschmidt
Date: Thu Jul 21 2011 - 18:53:04 EST

On Thu, 2011-07-21 at 15:36 -0700, Andrew Morton wrote:
> On Tue, 19 Jul 2011 14:29:22 +1000
> Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxx>
> > ---
> >
> > 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.

> > since I'm
> > starting to have the nasty feeling that you are hitting what is
> > somewhat a subtly different issue or my previous patch should
> > have worked (but then I might have done a stupid mistake as well)
> > but let us know anyway.
> I assume that Shan reported the secret problem so I added the
> reported-by to the changelog.

He did :-) Shan, care to provide a rough explanation of what you
observed ?

Also Russell confirmed that ARM should be affected as well.


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