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SubjectRe: possible circular locking dependency mmap_sem/cpu_hotplug_lock.rw_sem
On Mon, 4 Sep 2017, Michal Hocko wrote:

> Thomas, Johannes,
> could you double check my thinking here? I will repost the patch to
> Andrew if you are OK with this.
> > + /*
> > + * The only protection from memory hotplug vs. drain_stock races is
> > + * that we always operate on local CPU stock here with IRQ disabled
> > + */
> > local_irq_save(flags);
> >
> > stock = this_cpu_ptr(&memcg_stock);
> > @@ -1807,26 +1811,27 @@ static void drain_all_stock(struct mem_cgroup *root_memcg)
> > if (!mutex_trylock(&percpu_charge_mutex))
> > return;
> > /* Notify other cpus that system-wide "drain" is running */
> > - get_online_cpus();
> > curcpu = get_cpu();

The problem here is that this does only protect you against a CPU being
unplugged, but not against a CPU coming online concurrently. I have no idea
whether that might be a problem, but at least you should put a comment in
which explains why it is not.

Thanks,

tglx

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