linux-kernel - Re: possible circular locking dependency mmap_sem/cpu_hotplug_lock.rw

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Date:   Tue, 5 Sep 2017 10:19:13 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Michal Hocko <mhocko@...nel.org>
cc:     Johannes Weiner <hannes@...xchg.org>,
        Artem Savkov <asavkov@...hat.com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org
Subject: Re: 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