----- On Jul 6, 2020, at 9:59 AM, Florian Weimer fweimer@xxxxxxxxxx wrote:
> * Mathieu Desnoyers:
>
>> When available, use the cpu_id field from __rseq_abi on Linux to
>> implement sched_getcpu(). Fall-back on the vgetcpu vDSO if
>> unavailable.
>
> I've pushed this to glibc master, but unfortunately it looks like this
> exposes a kernel bug related to affinity mask changes.
>
> After building and testing glibc, this
>
> for x in {1..2000} ; do posix/tst-affinity-static & done
>
> produces some “error:” lines for me:
>
> error: Unexpected CPU 2, expected 0
> error: Unexpected CPU 2, expected 0
> error: Unexpected CPU 2, expected 0
> error: Unexpected CPU 2, expected 0
> error: Unexpected CPU 138, expected 0
> error: Unexpected CPU 138, expected 0
> error: Unexpected CPU 138, expected 0
> error: Unexpected CPU 138, expected 0
>
> “expected 0” is a result of how the test has been written, it bails out
> on the first failure, which happens with CPU ID 0.
>
> Smaller systems can use a smaller count than 2000 to reproduce this. It
> also happens sporadically when running the glibc test suite itself
> (which is why it took further testing to reveal this issue).
>
> I can reproduce this with the Debian 4.19.118-2+deb10u1 kernel, the
> Fedora 5.6.19-300.fc32 kernel, and the Red Hat Enterprise Linux kernel
> 4.18.0-193.el8 (all x86_64).
>
> As to the cause, I'd guess that the exit path in the sched_setaffinity
> system call fails to update the rseq area, so that userspace can observe
> the outdated CPU ID there.
Hi Florian,
We have a similar test in Linux, see tools/testing/selftests/rseq/basic_test.c.
That test does not trigger this issue, even when executed repeatedly.
I'll investigate further what is happening within the glibc test.
Thanks,
Mathieu
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
