linux-kernel - Re: INFO: rcu detected stall in sys

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Date:   Sat, 4 Jan 2020 20:09:00 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     syzbot <syzbot+bcad772bbc241b4c6147@...kaller.appspotmail.com>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        James Morris <jmorris@...ei.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-security-module@...r.kernel.org,
        "Serge E. Hallyn" <serge@...lyn.com>
Subject: Re: INFO: rcu detected stall in sys_sendfile64

On 2018/12/20 3:42, Dmitry Vyukov wrote:
> On Wed, Dec 19, 2018 at 11:13 AM Tetsuo Handa
> <penguin-kernel@...ove.sakura.ne.jp> wrote:
>>
>> On 2018/12/19 18:27, syzbot wrote:
>>> HEAD commit:    ddfbab46539f Merge tag 'scsi-fixes' of git://git.kernel.or..
>>> git tree:       upstream
>>> console output: https://syzkaller.appspot.com/x/log.txt?x=15b87fa3400000
>>> kernel config:  https://syzkaller.appspot.com/x/.config?x=861a3573f4e78ba1
>>> dashboard link: https://syzkaller.appspot.com/bug?extid=bcad772bbc241b4c6147
>>> compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
>>> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=13912ccd400000
>>> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=145781db400000
>>
>> This is not a LSM problem, for the reproducer is calling
>> sched_setattr(SCHED_DEADLINE) with very large values.
>>
>>   sched_setattr(0, {size=0, sched_policy=0x6 /* SCHED_??? */, sched_flags=0, sched_nice=0, sched_priority=0, sched_runtime=2251799813724439, sched_deadline=4611686018427453437, sched_period=0}, 0) = 0
>>
>> I think that this problem is nothing but an insane sched_setattr() parameter.
>>
>> #syz invalid
> 
> Note there was another one with sched_setattr, which turned out to be
> some serious problem in kernel (sched_setattr should not cause CPU
> stall for 3 minutes):
> INFO: rcu detected stall in do_idle
> https://syzkaller.appspot.com/bug?extid=385468161961cee80c31
> https://groups.google.com/forum/#!msg/syzkaller-bugs/crrfvusGtwI/IoD_zus4BgAJ
> 
> Maybe it another incarnation of the same bug, that one is still not fixed.
> 

Can we let syzbot blacklist sched_setattr() for now? There are many stall reports
doing sched_setattr(SCHED_RR) which makes it difficult to find stall reports not
using sched_setattr().