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

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Message-Id: <A3EEA386-DC02-432B-8B53-F1ECD035F431@amacapital.net>
Date:   Fri, 28 Sep 2018 14:08:10 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Max Filippov <jcmvbkbc@...il.com>,
        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 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.