linux-kernel - Re: [PATCH-next v2] mm/memcg: Properly handle memcg_stock access for PREEMPT

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Message-ID: <YbOBmYrInQVI+VOR@linutronix.de>
Date:   Fri, 10 Dec 2021 17:34:33 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Waiman Long <longman@...hat.com>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
        linux-mm@...ck.org, Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH-next v2] mm/memcg: Properly handle memcg_stock access for
 PREEMPT_RT

On 2021-12-10 11:29:31 [-0500], Waiman Long wrote:
> 
> On 12/10/21 08:01, Sebastian Andrzej Siewior wrote:
> > On 2021-12-09 21:52:28 [-0500], Waiman Long wrote:
> > …
> > > --- a/mm/memcontrol.c
> > > +++ b/mm/memcontrol.c
> > …
> > > @@ -2210,7 +2211,7 @@ static void refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
> > >   	struct memcg_stock_pcp *stock;
> > >   	unsigned long flags;
> > > -	local_irq_save(flags);
> > > +	local_lock_irqsave(&memcg_stock.lock, flags);
> > Why is this one using the lock? It isn't accessing irq_obj, right?
> Well, the lock isn't just for irq_obj. It protects the whole memcg_stock
> structure which include irq_obj. Sometimes, data in irq_obj (or task_obj)
> will get transfer to nr_pages and vice versa. So it is easier to use one
> single lock for the whole thing.

This needs way better documentation what protects what any why.
I don't like the quick slapping for RT only usage without any kind of
explanation. Once you think you know it is irq_obj only you end up here
where you have the lock again for no obvious reason.

> 
> Cheers,
> Longman

Sebastian