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SubjectRe: possible deadlock in lru_add_drain_all
[Cc Byungchul. The original full report is
http://lkml.kernel.org/r/089e0825eec8955c1f055c83d476@google.com]

Could you have a look please? This smells like a false positive to me.

On Fri 27-10-17 15:42:34, Michal Hocko wrote:
> On Fri 27-10-17 11:44:58, Dmitry Vyukov wrote:
> > On Fri, Oct 27, 2017 at 11:34 AM, Michal Hocko <mhocko@kernel.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.
>
> 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

--
Michal Hocko
SUSE Labs

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