On Mon 30-10-17 19:26:19, Byungchul Park wrote: > On Mon, Oct 30, 2017 at 09:22:03AM +0100, Michal Hocko wrote: > > [Cc Byungchul. The original full report is > > http://lkml.kernel.org/r/089e0825eec8955c1f055c83d476@xxxxxxxxxx] > > > > 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@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. > > I think lru_add_drain_all() takes cpu_hotplug_lock.rw_sem implicitly in > get_online_cpus(), which appears in the chain. Yes, but it doesn't take any _other_ locks which are externally visible except for lru_lock and that itself doesn't provide any further dependency AFAICS. So what exactly is the deadlock scenario? -- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
