linux-kernel - selftests/x86/fsgsbase

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Date:   Fri, 26 Jan 2018 09:36:31 -0600
From:   Dan Rue <dan.rue@...aro.org>
To:     Shuah Khan <shuah@...nel.org>, Ingo Molnar <mingo@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Dmitry Safonov <dsafonov@...tuozzo.com>,
        Borislav Petkov <bp@...en8.de>,
        linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: selftests/x86/fsgsbase_64 test problem

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.

Thanks,
Dan