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From:   Andy Lutomirski <luto@kernel.org>
Date:   Fri, 26 Jan 2018 08:22:48 -0800
Message-ID: <CALCETrXVFC=6wPNpL0Dc7pkSv4JSaoEZZcu3Rtw0sg7KJsE5Pg@mail.gmail.com>
Subject: Re: selftests/x86/fsgsbase_64 test problem
To:     Dan Rue <dan.rue@linaro.org>
Cc:     Shuah Khan <shuah@kernel.org>, Ingo Molnar <mingo@kernel.org>,
        Andy Lutomirski <luto@kernel.org>,
        Dmitry Safonov <dsafonov@virtuozzo.com>,
        Borislav Petkov <bp@alien8.de>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
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On Fri, Jan 26, 2018 at 7:36 AM, Dan Rue <dan.rue@linaro.org> wrote:
>
> We've noticed that fsgsbase_64 can fail intermittently with the
> following error:
>
>         [RUN]   ARCH_SET_GS(0x0) and clear gs, then schedule to 0x1
>                 Before schedule, set selector to 0x1
>                 other thread: ARCH_SET_GS(0x1) -- sel is 0x0
>         [FAIL]  GS/BASE changed from 0x1/0x0 to 0x0/0x0
>
> This can be reliably reproduced by running fsgsbase_64 in a loop. i.e.
>
>     for i in $(seq 1 10000); do ./fsgsbase_64 || break; done
>
> This problem isn't new - I've reproduced it on latest mainline and every
> release going back to v4.12 (I did not try earlier). This was tested on
> a Supermicro board with a Xeon E3-1220 as well as an Intel Nuc with an
> i3-5010U.
>

Hmm, I can reproduce it, too.  I'll look in a bit.