linux-kernel - Re: [PATCH] futex: Set USER_DS for the futex_detect

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Date:   Fri, 28 Sep 2018 21:38:37 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andy Lutomirski <luto@...capital.net>
cc:     Martin Schwidefsky <schwidefsky@...ibm.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Andrew Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Darren Hart <dvhart@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>, linux-s390@...r.kernel.org,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        fthain@...egraphics.com.au
Subject: Re: [PATCH] futex: Set USER_DS for the futex_detect_cmpxchg() test

On Fri, 28 Sep 2018, Andy Lutomirski wrote:
> On Fri, Sep 28, 2018 at 7:53 AM Martin Schwidefsky <schwidefsky@...ibm.com> wrote:
> > On Fri, 28 Sep 2018 07:11:44 -0700 Andy Lutomirski <luto@...capital.net> wrote:
> >
> > > 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?
> >
> > No, I don't remember. To use a valid kernel pointer with KERNEL_DS and
> > then test for == 0 (vs -ENOSYS) imho should work.
> >
> 
> There may be a much nicer solution.  Unless I missed something, only
> mips and xtensa even have the possibility of cmpxchg being missing.
> We could just make those arches supply a futex-detecting helper.

That makes sense.

Thanks,

	tglx