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From:   Florian Weimer <fweimer@redhat.com>
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>,
        Darren Hart <darren@dvhart.com>,
        Yi Wang <wang.yi59@zte.com.cn>,
        Yang Tao <yang.tao172@zte.com.cn>,
        Oleg Nesterov <oleg@redhat.com>,
        Carlos O'Donell <carlos@redhat.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>
Subject: Re: [patch 00/12] futex: Cure robust/PI futex exit races
References: <20191106215534.241796846@linutronix.de>
        <87zhh78gnf.fsf@oldenburg2.str.redhat.com>
        <87v9rv8g44.fsf@oldenburg2.str.redhat.com>
        <87o8xm95rt.fsf@oldenburg2.str.redhat.com>
        <alpine.DEB.2.21.1911080912520.27903@nanos.tec.linutronix.de>
Date:   Fri, 08 Nov 2019 11:17:16 +0100
In-Reply-To: <alpine.DEB.2.21.1911080912520.27903@nanos.tec.linutronix.de>
        (Thomas Gleixner's message of "Fri, 8 Nov 2019 10:18:08 +0100 (CET)")
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* Thomas Gleixner:

> On Fri, 8 Nov 2019, Florian Weimer wrote:
>> * Florian Weimer:
>> > * Florian Weimer:
>> >> I ran the glibc upstream test suite (which has some robust futex test=
s)
>> >> against b21be7e942b49168ee15a75cbc49fbfdeb1e6a97 on x86-64, both nati=
ve
>> >> and 32-bit/i386 compat mode.
>> >>
>> >> compat mode seems broken, nptl/tst-thread-affinity-pthread fails.  Th=
is
>> >> is probably *not* due to
>> >> <https://bugzilla.kernel.org/show_bug.cgi?id=3D154011> because the fa=
ilure
>> >> is non-sporadic, but reliable fails for thread 253:
>> >>
>> >> info: Detected CPU set size (in bits): 225
>> >> info: Maximum test CPU: 255
>> >> error: pthread_create for thread 253 failed: Resource temporarily una=
vailable
>> >>
>> >> I'm running this on a large box as root, so ulimits etc. do not apply=
.
>> >>
>> >> I did not see this failure with the x86-64 test.
>> >>
>> >> You should be able to reproduce with (assuming you've got a multilib =
gcc):
>> >>
>> >> git clone git://sourceware.org/git/glibc.git git
>> >> mkdir build
>> >> cd build
>> >> ../git/configure --prefix=3D/usr CC=3D"gcc -m32" CXX=3D"g++ -m32" --b=
uild=3Di686-linux
>> >> make -j`nproc`
>> >> make test t=3Dnptl/tst-thread-affinity-pthread
>> >
>> > Sorry, I realized that I didn't actually verify that this is a
>> > regression caused by your patches.  Maybe I can do that tomorrow.
>>=20
>> Confirmed as a regression caused by the patches.  Depending on the
>> nature of the bug, you need a machine which has or pretends to have many
>> CPUs (this one has 256 CPUs).
>
> Sure I can do that, but I completely fail to see how that's a
> regression.
>
> Unpatched 5.4-rc6:
>
> FAIL: nptl/tst-thread-affinity-pthread
> original exit status 1
> info: Detected CPU set size (in bits): 225
> info: Maximum test CPU: 255
> error: pthread_create for thread 253 failed: Resource temporarily unavail=
able

Huh.  Reverting your patches (at commit 26bc672134241a080a83b2ab9aa8abede8d=
30e1c)
fixes the test for me.

> TBH, the futex changes have absolutely nothing to do with that resource
> fail.

I suspect that there are some changes to task exit latency, which
triggers the latent resource management bug.

Thanks,
Florian