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

From: Peter Zijlstra
Date: Tue Jul 19 2011 - 07:11:01 EST

On Tue, 2011-07-19 at 14:29 +1000, Benjamin Herrenschmidt 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>

Acked-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
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