Re: [PATCH] futex: Set USER_DS for the futex_detect_cmpxchg() test

Thomas Gleixner
2018-09-28T14:40:58Z

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On Fri, 28 Sep 2018, Andy Lutomirski wrote:
> > On Sep 28, 2018, at 1:42 AM, Thomas Gleixner <tglx@linutronix.de> wrote:
> >
> >> On Fri, 28 Sep 2018, Martin Schwidefsky wrote:
> >> On Fri, 28 Sep 2018 09:12:10 +0200
> >> Geert Uytterhoeven <geert@linux-m68k.org> wrote:
> >>> I don't know if that has happened, and whether it would work on s390 now.
> >>
> >> commit 03b8c7b623c80af264c4c8d6111e5c6289933666
> >> Author: Heiko Carstens <heiko.carstens@de.ibm.com>
> >> Date: Sun Mar 2 13:09:47 2014 +0100
> >>
> >> futex: Allow architectures to skip futex_atomic_cmpxchg_inatomic() test
> >>
> >> If an architecture has futex_atomic_cmpxchg_inatomic() implemented and there
> >> is no runtime check necessary, allow to skip the test within futex_init().
> >>
> >> This allows to get rid of some code which would always give the same result,
> >> and also allows the compiler to optimize a couple of if statements away.
> >>
> >> Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
> >> Cc: Finn Thain <fthain@telegraphics.com.au>
> >> Cc: Geert Uytterhoeven <geert@linux-m68k.org>
> >> Link: http://lkml.kernel.org/r/20140302120947.GA3641@osiris
> >> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> >>
> >>
> >> Heiko created the CONFIG_HAVE_FUTEX_CMPXCHG to get around this issue.
> >> We just skip the runtime check as well as arc, m68k and sh. Not sure
> >> about xtensa, the set it config option only for !MMU.
> >
> > Duh. grep would have told me. -ENOTENOUGHCOFFEE
> >
> >
> There’s another way to skin this cat: keep KERNEL_DS but pass a valid
> pointer. I don’t suppose you remember why you didn’t do that?

IIRC, there was an issue with extra checks in some architectures when you
handed in a kernel address spitting warnings or such. That's probably gone
by now, but I can't tell for sure. At least the requirement to do runtime
detection for x86 is gone. Don't know if any other architecture still has
it.

Thanks,

tglx