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Date:   Thu, 21 Dec 2017 11:22:45 +0100
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:     syzbot 
        <bot+e38be687a2450270a3b593bacb6b5795a7a74edb@...kaller.appspotmail.com>,
        syzkaller-bugs@...glegroups.com,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Kate Stewart <kstewart@...uxfoundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Philippe Ombredanne <pombredanne@...b.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: BUG: workqueue lockup (2)

On Wed, Dec 20, 2017 at 11:55 AM, Tetsuo Handa
<penguin-kernel@...ove.sakura.ne.jp> wrote:
> Dmitry Vyukov wrote:
>> On Tue, Dec 19, 2017 at 3:27 PM, Tetsuo Handa
>> <penguin-kernel@...ove.sakura.ne.jp> wrote:
>> > syzbot wrote:
>> >>
>> >> syzkaller has found reproducer for the following crash on
>> >> f3b5ad89de16f5d42e8ad36fbdf85f705c1ae051
>> >
>> > "BUG: workqueue lockup" is not a crash.
>>
>> Hi Tetsuo,
>>
>> What is the proper name for all of these collectively?
>
> I think that things which lead to kernel panic when /proc/sys/kernel/panic_on_oops
> was set to 1 are called an "oops" (or a "kerneloops").
>
> Speak of "BUG: workqueue lockup", this is not an "oops". This message was
> added by 82607adcf9cdf40f ("workqueue: implement lockup detector"), and
> this message does not always indicate a fatal problem. This message can be
> printed when the system is really out of CPU and memory. As far as I tested,
> I think that workqueue was not able to run on specific CPU due to a soft
> lockup bug.
>
>>
>>
>> >> git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/master
>> >> compiler: gcc (GCC) 7.1.1 20170620
>> >> .config is attached
>> >> Raw console output is attached.
>> >> C reproducer is attached
>> >> syzkaller reproducer is attached. See https://goo.gl/kgGztJ
>> >> for information about syzkaller reproducers
>> >>
>> >>
>> >> BUG: workqueue lockup - pool cpus=1 node=0 flags=0x0 nice=0 stuck for 37s!
>> >> BUG: workqueue lockup - pool cpus=1 node=0 flags=0x0 nice=-20 stuck for 32s!
>> >> Showing busy workqueues and worker pools:
>> >> workqueue events: flags=0x0
>> >>    pwq 2: cpus=1 node=0 flags=0x0 nice=0 active=1/256
>> >>      pending: cache_reap
>> >> workqueue events_power_efficient: flags=0x80
>> >>    pwq 2: cpus=1 node=0 flags=0x0 nice=0 active=2/256
>> >>      pending: neigh_periodic_work, do_cache_clean
>> >> workqueue mm_percpu_wq: flags=0x8
>> >>    pwq 2: cpus=1 node=0 flags=0x0 nice=0 active=1/256
>> >>      pending: vmstat_update
>> >> workqueue kblockd: flags=0x18
>> >>    pwq 3: cpus=1 node=0 flags=0x0 nice=-20 active=1/256
>> >>      pending: blk_timeout_work
>> >
>> > You gave up too early. There is no hint for understanding what was going on.
>> > While we can observe "BUG: workqueue lockup" under memory pressure, there is
>> > no hint like SysRq-t and SysRq-m. Thus, I can't tell something is wrong.
>>
>> Do you know how to send them programmatically? I tried to find a way
>> several times, but failed. Articles that I've found talk about
>> pressing some keys that don't translate directly to us-ascii.
>
> # echo t > /proc/sysrq-trigger
> # echo m > /proc/sysrq-trigger


This requires working ssh connection, but we routinely deal with
half-dead kernels. I think that sysrq over console is as reliable as
we can get in this context. But I don't know how to send them.

But thinking more about this, I am leaning towards the direction that
kernel just need to do the right thing and print that info.
In lots of cases we get a panic and as far as I understand kernel
won't react on sysrq in that state. Console is still unreliable too.
If a message is not useful, the right direction is to make it useful.

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