Re: possible deadlock in lru_add_drain_all

[Dmitry Vyukov]

On Fri, Oct 27, 2017 at 11:34 AM, Michal Hocko <mhocko@xxxxxxxxxx> 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.