Re: possible deadlock in lru_add_drain_all

[Michal Hocko]

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