Re: [PATCH] mm, hotplug: protect nr_zones with pgdat_resize_lock()

[Michal Hocko <mhocko@suse.com>]

On Wed 21-11-18 02:44:35, Wei Yang wrote:
> On Tue, Nov 20, 2018 at 08:31:41AM +0100, Michal Hocko wrote:
> >On Tue 20-11-18 09:48:22, Wei Yang wrote:
> >> After memory hot-added, users could online pages through sysfs, and this
> >> could be done in parallel.
> >> 
> >> In case two threads online pages in two different empty zones at the
> >> same time, there would be a contention to update the nr_zones.
> >
> >No, this shouldn't be the case as I've explained in the original thread.
> >We use memory hotplug lock over the online phase. So there shouldn't be
> >any race possible.
> 
> Sorry for misunderstanding your point.
> 
> >
> >On the other hand I would like to see the global lock to go away because
> >it causes scalability issues and I would like to change it to a range
> >lock. This would make this race possible.
> 
> The global lock you want to remove is mem_hotplug_begin() ?

Yes

> 
> Hmm... my understanding may not correct. While mem_hotplug_begin() use
> percpu lock, which means if there are two threads running on two
> different cpus to online pages at the same time, they could get their
> own lock?

No. The per-cpu is a mere implementation detail on how the
synchronization is done. Only one path might aquire the exclusive lock.

-- 
Michal Hocko
SUSE Labs