linux-kernel - Re: possible deadlock in lru_add_drain

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Date:   Fri, 27 Oct 2017 11:44:58 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     syzbot 
        <bot+e7353c7141ff7cbb718e4c888a14fa92de41ebaa@...kaller.appspotmail.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Johannes Weiner <hannes@...xchg.org>, Jan Kara <jack@...e.cz>,
        jglisse@...hat.com, LKML <linux-kernel@...r.kernel.org>,
        linux-mm@...ck.org, shli@...com, syzkaller-bugs@...glegroups.com,
        Thomas Gleixner <tglx@...utronix.de>,
        Vlastimil Babka <vbabka@...e.cz>, ying.huang@...el.com
Subject: Re: possible deadlock in lru_add_drain_all

On Fri, Oct 27, 2017 at 11:34 AM, Michal Hocko <mhocko@...nel.org> wrote:
> On Fri 27-10-17 02:22:40, syzbot wrote:
>> Hello,
>>
>> syzkaller hit the following crash on
>> a31cc455c512f3f1dd5f79cac8e29a7c8a617af8
>> git://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/master
>> compiler: gcc (GCC) 7.1.1 20170620
>> .config is attached
>> Raw console output is attached.
>
> I do not see such a commit. My linux-next top is next-20171018
>
> [...]
>> Chain exists of:
>>   cpu_hotplug_lock.rw_sem --> &pipe->mutex/1 --> &sb->s_type->i_mutex_key#9
>>
>>  Possible unsafe locking scenario:
>>
>>        CPU0                    CPU1
>>        ----                    ----
>>   lock(&sb->s_type->i_mutex_key#9);
>>                                lock(&pipe->mutex/1);
>>                                lock(&sb->s_type->i_mutex_key#9);
>>   lock(cpu_hotplug_lock.rw_sem);
>
> I am quite confused about this report. Where exactly is the deadlock?
> I do not see where we would get pipe mutex from inside of the hotplug
> lock. Is it possible this is just a false possitive due to cross release
> feature?


As far as I understand this CPU0/CPU1 scheme works only for simple
cases with 2 mutexes. This seem to have larger cycle as denoted by
"the existing dependency chain (in reverse order) is:" section.