Re: mm: deadlock between get_online_cpus/pcpu_alloc

[Michal Hocko]

On Tue 07-02-17 23:25:17, Thomas Gleixner wrote:
> On Tue, 7 Feb 2017, Christoph Lameter wrote:
> > On Tue, 7 Feb 2017, Michal Hocko wrote:
> > 
> > > I am always nervous when seeing hotplug locks being used in low level
> > > code. It has bitten us several times already and those deadlocks are
> > > quite hard to spot when reviewing the code and very rare to hit so they
> > > tend to live for a long time.
> > 
> > Yep. Hotplug events are pretty significant. Using stop_machine_XXXX() etc
> > would be advisable and that would avoid the taking of locks and get rid of all the
> > ocmplexity, reduce the code size and make the overall system much more
> > reliable.
> 
> Huch? stop_machine() is horrible and heavy weight. Don't go there, there
> must be simpler solutions than that.

Absolutely agreed. We are in the page allocator path so using the
stop_machine* is just ridiculous. And, in fact, there is a much simpler
solution [1]

[1] http://lkml.kernel.org/r/20170207201950.20482-1-mhocko@xxxxxxxxxx
-- 
Michal Hocko
SUSE Labs