linux-kernel - Re: [tip:core/rseq] rseq/selftests/x86: Work around bogus gcc-8 optimisation

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Message-ID: <CAHk-=wieuxm+B-Bmhcy+Sszqs6h=iTNMEP=TeSBFKxtnEsKHUg@mail.gmail.com>
Date:   Fri, 19 Apr 2019 11:53:52 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Peter Anvin <hpa@...or.com>, Ingo Molnar <mingo@...nel.org>,
        Will Deacon <will.deacon@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Christoph Lameter <cl@...ux.com>,
        Shuah Khan <shuah@...nel.org>, josh@...htriplett.org,
        Dave Watson <davejwatson@...com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Boqun Feng <boqun.feng@...il.com>, linux@....linux.org.uk,
        Paul Turner <pjt@...gle.com>, bmaurer@...com,
        Joel Fernandes <joelaf@...gle.com>,
        Michael Kerrisk-manpages <mtk.manpages@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...capital.net>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     linux-tip-commits@...r.kernel.org
Subject: Re: [tip:core/rseq] rseq/selftests/x86: Work around bogus gcc-8 optimisation

On Fri, Apr 19, 2019 at 11:46 AM tip-bot for Mathieu Desnoyers
<tipbot@...or.com> wrote:
>
> rseq/selftests/x86: Work around bogus gcc-8 optimisation
>
> At least the following versions of gcc-8:
>
>  - gcc version 8.0.1 20180414 (experimental) [trunk revision 259383] (Ubuntu 8-20180414-1ubuntu2)
>  - gcc 8.2.0-7ubuntu1 (Ubuntu 18.10 (Cosmic)),
>
> generate broken assembler with asm goto that have a thread-local storage
> "m" input operand on both x86-32 and x86-64. For instance:

Is there a gcc bugzilla for this? Shouldn't that be mentioned here?

Also, we use "asm goto" together with "m" all the time in the kernel.
In fact, it's the most common case, with the RMWcc ops being generated
with that. I realize that we don't use the gcc thread-local storage
for them (we often do use our *own* thread-local storage), but it
would be good to have that gcc bugzilla to see why it can only affect
those user level "__thread" cases..

                    Linus