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

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Date:   Fri, 20 Sep 2019 19:22:10 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Dmitry Vyukov <dvyukov@...gle.com>, paulmck@...nel.org,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:     syzbot <syzbot+18379f2a19bc62c12565@...kaller.appspotmail.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>, avagin@...il.com,
        Christian Brauner <christian@...uner.io>, dbueso@...e.de,
        LKML <linux-kernel@...r.kernel.org>,
        Oleg Nesterov <oleg@...hat.com>, prsood@...eaurora.org,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Thomas Gleixner <tglx@...utronix.de>, Tejun Heo <tj@...nel.org>
Subject: Re: INFO: rcu detected stall in sys_exit_group

Calling printk() people.

On 2019/09/20 16:50, Dmitry Vyukov wrote:
>>>                                   How it runs on top of an interrupt?
>>
>> It is not running on top of an interrupt.  Its stack was dumped
>> separately.
> 
> I see. Usually the first stack is the traceback of the current stack.
> So I was confused.
> 
>>> And why one cpu tracebacks another one?
>>
>> The usual reason is because neither CPU's quiescent state was reported
>> to the RCU core, so the stall-warning code dumped both stacks.
> 
> But should the other CPU traceback _itself_? Rather than being traced
> back by another CPU?
> E.g. see this report:
> https://github.com/google/syzkaller/blob/master/pkg/report/testdata/linux/report/350#L61-L83
> Here the overall problem was detected by C2, but then C1 traces back itself.
> 
> ... however even in that case C0 and C3 are traced by C2:
> https://github.com/google/syzkaller/blob/master/pkg/report/testdata/linux/report/350#L84-L149
> I can't understand this...
> This makes understanding what happened harder because it's not easy to
> exclude things on other CPUs.

I think this should be
https://github.com/google/syzkaller/blob/master/pkg/report/testdata/linux/report/350#L84-L172
than #L84-L149 .

Is the reason these lines have "[    C2]" is that these lines were flushed (printk_caller_id()
was called) from log_output() from vprintk_store() from vprintk_emit() from vprintk_deferred()
 from printk_deferred() from printk_safe_flush_line() from __printk_safe_flush() from
printk_safe_flush() from printk_safe_flush_on_panic() from panic() ?