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

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Message-Id: <20180928165321.0ddaf948@mschwideX1>
Date:   Fri, 28 Sep 2018 16:53:21 +0200
From:   Martin Schwidefsky <schwidefsky@...ibm.com>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        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 Fri, 28 Sep 2018 07:11:44 -0700
Andy Lutomirski <luto@...capital.net> wrote:

> > 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?

No, I don't remember. To use a valid kernel pointer with KERNEL_DS and
then test for == 0 (vs -ENOSYS) imho should work.

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.