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

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Message-ID: <alpine.DEB.2.21.1809280823220.8118@nanos.tec.linutronix.de>
Date:   Fri, 28 Sep 2018 08:24:44 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andy Lutomirski <luto@...nel.org>
cc:     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,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Finn Thain <fthain@...egraphics.com.au>,
        Geert Uytterhoeven <geert@...ux-m68k.org>
Subject: Re: [PATCH] futex: Set USER_DS for the futex_detect_cmpxchg() test

On Fri, 28 Sep 2018, Thomas Gleixner wrote:

> On Thu, 27 Sep 2018, Andy Lutomirski wrote:
> > I have a couple questions here:
> > 
> >  - Is this actually okay on all architectures?  That is, are there
> >    cases where we'll screw up if we fail a USER_DS access this early?
> >    s390 stands out as the obvious special case (where USER_DS is not
> >    than just a subset of KERNEL_DS), but s390 opts out.
> >
> >  - Why doesn't x86 set HAVE_FUTEX_CMPXCHG?  Or do we still support
> >    some 32-bit configurations that don't have cmpxchg and don't know
> >    about it at compile time?
> 
> I'm not entirely sure. Have to dig into the details. I assume S390 just can
> set it though.

x86 as well. It's supported from 486 onwards and we ripped out 386 years ago.

Thanks,

	tglx