linux-kernel - Re: [PATCH 2/2] ARM: futex: make futex_detect

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Date:   Thu, 7 Mar 2019 16:04:23 -0800
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Russell King - ARM Linux admin <linux@...linux.org.uk>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Darren Hart <dvhart@...radead.org>,
        Mikael Pettersson <mikpe@...uu.se>,
        Mikael Pettersson <mikpelinux@...il.com>,
        Dave Martin <Dave.Martin@....com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] ARM: futex: make futex_detect_cmpxchg more reliable

On Thu, Mar 7, 2019 at 3:49 PM Russell King - ARM Linux admin
<linux@...linux.org.uk> wrote:
>
> On Thu, Mar 07, 2019 at 11:39:08AM -0800, Nick Desaulniers wrote:
> > Underspecification of constraints to extended inline assembly is a
> > common issue exposed by other compilers (and possibly but in-effect
> > infrequently compiler upgrades).
>
> I don't see what is "underspecified" in the original constraints.
> Please explain.

>From the link:

The problem is that in the T(streq) insn, %3 and %4 MUST be different registers,
but nothing in the asm() constrains them to be different.

> > >  "One way of fixing this is to make uaddr an input/output register, since
> > >  "that prevents it from overlapping any other input or output."

-- 
Thanks,
~Nick Desaulniers