linux-kernel - Re: possible deadlock in lru_add_drain

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Message-ID: <20171027093418.om5e566srz2ztsrk@dhcp22.suse.cz>
Date:   Fri, 27 Oct 2017 11:34:18 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     syzbot 
        <bot+e7353c7141ff7cbb718e4c888a14fa92de41ebaa@...kaller.appspotmail.com>
Cc:     akpm@...ux-foundation.org, dan.j.williams@...el.com,
        hannes@...xchg.org, jack@...e.cz, jglisse@...hat.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org, shli@...com,
        syzkaller-bugs@...glegroups.com, tglx@...utronix.de,
        vbabka@...e.cz, ying.huang@...el.com
Subject: Re: possible deadlock in lru_add_drain_all

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?
-- 
Michal Hocko
SUSE Labs