Re: Crashes with 874bbfe600a6 in 3.18.25

[Tejun Heo]

On Wed, Feb 03, 2016 at 01:28:56PM +0100, Michal Hocko wrote:
> > The CPU was 168, and that one was offlined in the meantime. So
> > __queue_work fails at:
> >   if (!(wq->flags & WQ_UNBOUND))
> >     pwq = per_cpu_ptr(wq->cpu_pwqs, cpu);
> >   else
> >     pwq = unbound_pwq_by_node(wq, cpu_to_node(cpu));
> >     ^^^                           ^^^^ NODE is -1
> >       \ pwq is NULL
> > 
> >   if (last_pool && last_pool != pwq->pool) { <--- BOOM

So, the proper fix here is keeping cpu <-> node mapping stable across
cpu on/offlining which has been being worked on for a long time now.
The patchst is pending and it fixes other issues too.

> So I think 874bbfe600a6 is really bogus. It should be reverted. We
> already have a proper fix for vmstat 176bed1de5bf ("vmstat: explicitly
> schedule per-cpu work on the CPU we need it to run on"). This which
> should be used for the stable trees as a replacement.

It's not bogus.  We can't flip a property that has been guaranteed
without any provision for verification.  Why do you think vmstat blow
up in the first place?  vmstat would be the canary case as it runs
frequently on all systems.  It's exactly the sign that we can't break
this guarantee willy-nilly.

Thanks.

-- 
tejun