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

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Message-ID: <alpine.DEB.2.21.1809290832360.1432@nanos.tec.linutronix.de>
Date:   Sat, 29 Sep 2018 08:32:55 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Max Filippov <jcmvbkbc@...il.com>
cc:     Andy Lutomirski <luto@...capital.net>,
        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 <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, Max Filippov wrote:
> On Fri, Sep 28, 2018 at 2:08 PM, Andy Lutomirski <luto@...capital.net> wrote:
> >> On Sep 28, 2018, at 1:26 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> >>
> >>> On Fri, 28 Sep 2018, Max Filippov wrote:
> >>>
> >>>> On Fri, Sep 28, 2018 at 11:02 AM, Andy Lutomirski <luto@...capital.net> wrote:
> >>>> 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.
> >>>
> >>> In case of xtensa availability of cmpxchg is known at build time.
> >>
> >> That makes it even simpler. Could you provide a patch which selects
> >> CONFIG_HAVE_FUTEX_CMPXCHG for the right set of CPUs please?
> >>
> >>
> >
> > I think that’s the wrong approach, since it won’t cover mips. How about adding this to mips and xtensa only:
> >
> > static inline void arch_have_futex_cmpxchg(void) {...};
> > #define arch_have_futex_cmpxchg arch_have_futex_cmpxchg
> >
> > And getting rid of the config option.
> 
> I'd rather do that, given that defining Kconfig entries that describe
> parts of xtensa configuration is somewhat awkward and redundant.

Fair enough.

Thanks,

	tglx