Re: possible deadlock in lru_add_drain_all

[Michal Hocko]

On Fri 27-10-17 11:44:58, Dmitry Vyukov wrote:
> 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.

My point was that lru_add_drain_all doesn't take any external locks
other than lru_lock and that one is not anywhere in the chain AFAICS.

-- 
Michal Hocko
SUSE Labs