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

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Message-Id: <0B38810F-3E22-45A8-9D5D-EDEABD214148@amacapital.net>
Date:   Fri, 28 Sep 2018 07:11:44 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Martin Schwidefsky <schwidefsky@...ibm.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Andy Lutomirski <luto@...nel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Darren Hart <dvhart@...radead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-s390 <linux-s390@...r.kernel.org>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Finn Thain <fthain@...egraphics.com.au>
Subject: Re: [PATCH] futex: Set USER_DS for the futex_detect_cmpxchg() test



> On Sep 28, 2018, at 1:42 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
> 
>> On Fri, 28 Sep 2018, Martin Schwidefsky wrote:
>> On Fri, 28 Sep 2018 09:12:10 +0200
>> Geert Uytterhoeven <geert@...ux-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@...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@...ibm.com>
>>    Cc: Finn Thain <fthain@...egraphics.com.au>
>>    Cc: Geert Uytterhoeven <geert@...ux-m68k.org>
>>    Link: http://lkml.kernel.org/r/20140302120947.GA3641@osiris
>>    Signed-off-by: Thomas Gleixner <tglx@...utronix.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?