linux-kernel - Re: BUG: workqueue lockup (2)

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Message-Id: <201712192327.FIJ64026.tMQFOOVFFLHOSJ@I-love.SAKURA.ne.jp>
Date:   Tue, 19 Dec 2017 23:27:24 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     bot+e38be687a2450270a3b593bacb6b5795a7a74edb@...kaller.appspotmail.com,
        syzkaller-bugs@...glegroups.com
Cc:     dvyukov@...gle.com, gregkh@...uxfoundation.org,
        kstewart@...uxfoundation.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, pombredanne@...b.com, tglx@...utronix.de
Subject: Re: BUG: workqueue lockup (2)

syzbot wrote:
> 
> syzkaller has found reproducer for the following crash on  
> f3b5ad89de16f5d42e8ad36fbdf85f705c1ae051

"BUG: workqueue lockup" is not a crash.

> 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.

At least you need to confirm that lockup lasts for a few minutes. Otherwise,
this might be just overstressing. (According to repro.c , 12 threads are
created and soon SEGV follows? According to above message, only 2 CPUs?
Triggering SEGV suggests memory was low due to saving coredump?)