From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To: Florian Weimer <fweimer@redhat.com>
Cc: carlos <carlos@redhat.com>,
Joseph Myers <joseph@codesourcery.com>,
Szabolcs Nagy <szabolcs.nagy@arm.com>,
libc-alpha <libc-alpha@sourceware.org>,
Thomas Gleixner <tglx@linutronix.de>, Ben Maurer <bmaurer@fb.com>,
Peter Zijlstra <peterz@infradead.org>,
Paul <paulmck@linux.vnet.ibm.com>,
Boqun Feng <boqun.feng@gmail.com>,
Will Deacon <will.deacon@arm.com>, Paul Turner <pjt@google.com>,
linux-kernel <linux-kernel@vger.kernel.org>,
linux-api <linux-api@vger.kernel.org>
Subject: Re: [PATCH 2/3] Linux: Use rseq in sched_getcpu if available (v9)
Date: Mon, 6 Jul 2020 10:49:38 -0400 (EDT) [thread overview]
Message-ID: <942999672.22574.1594046978937.JavaMail.zimbra@efficios.com> (raw)
In-Reply-To: <877dvg4ud4.fsf@oldenburg2.str.redhat.com>
----- On Jul 6, 2020, at 9:59 AM, Florian Weimer fweimer@redhat.com 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 the outdated CPU ID there.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
next prev parent reply other threads:[~2020-07-06 14:49 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <20200629190036.26982-1-mathieu.desnoyers@efficios.com>
2020-06-29 19:00 ` [PATCH 1/3] glibc: Perform rseq registration at C startup and thread creation (v22) Mathieu Desnoyers
2020-07-02 14:46 ` Florian Weimer
2020-07-03 17:50 ` Carlos O'Donell
2020-06-29 19:00 ` [PATCH 2/3] Linux: Use rseq in sched_getcpu if available (v9) Mathieu Desnoyers
2020-07-06 13:59 ` Florian Weimer
2020-07-06 14:49 ` Mathieu Desnoyers [this message]
2020-07-06 17:33 ` Mathieu Desnoyers
2020-07-06 17:50 ` Florian Weimer
2020-07-06 18:02 ` Mathieu Desnoyers
2020-07-06 18:11 ` Florian Weimer
2020-07-06 21:08 ` Mathieu Desnoyers
[not found] <20200622180803.1449-1-mathieu.desnoyers@efficios.com>
2020-06-22 18:08 ` Mathieu Desnoyers
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