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

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Date:   Tue, 5 Sep 2017 13:04:05 +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 Tue, 5 Sep 2017, Michal Hocko wrote:
> On Tue 05-09-17 10:19:13, Thomas Gleixner wrote:
> > 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.
> 
> Yes but same as the drain_all_pages we do not have any cpu up specific
> intialization so there is no specific action to race against AFAICS.
> 
> > 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.
> 
> What about this?

Looks good.

> ---
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 5c70f47abb3d..ff9b0979ccc3 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1810,7 +1810,12 @@ static void drain_all_stock(struct mem_cgroup *root_memcg)
>  	/* If someone's already draining, avoid adding running more workers. */
>  	if (!mutex_trylock(&percpu_charge_mutex))
>  		return;
> -	/* Notify other cpus that system-wide "drain" is running */
> +	/*
> +	 * Notify other cpus that system-wide "drain" is running
> +	 * We do not care about races with the cpu hotplug because cpu down
> +	 * as well as workers from this path always operate on the local
> +	 * per-cpu data. CPU up doesn't touch memcg_stock at all.
> +	 */
>  	curcpu = get_cpu();
>  	for_each_online_cpu(cpu) {
>  		struct memcg_stock_pcp *stock = &per_cpu(memcg_stock, cpu);
> 
> -- 
> Michal Hocko
> SUSE Labs
>