Re: [Draft] selftests/x86/fsgsbase_64 test problem

On 24 January 2018 at 20:08, Dan Rue dan.rue@linaro.org wrote:

I don't know if this is worth reporting - please let me know what your thoughts are on the following email. Recipients are from 'get_maintainer.pl -f tools/testing/selftests/x86/fsgsbase.c'.

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To: Shuah Khan shuah@kernel.org, Ingo Molnar mingo@kernel.org, Andy Lutomirski luto@kernel.org, Dmitry Safonov dsafonov@virtuozzo.com, linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org Subject: selftests/x86/fsgsbase_64 test problem

I might also include Boris Petkov who reviewed at least the initial change adding the test.

We've noticed that fsgsbase_64 can fail periodically with the following error:

Periodically or intermittently?

    [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).

Possibly worth mentioning what sort of systems you're using in case they're unusual ones for that feature.

Thanks, Dan