linux-kernel - Re: [PATCH-next v3] mm/memcg: Properly handle memcg_stock access for PREEMPT

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Message-ID: <20211215043657.ngmxlk6rgc2ysbmz@offworld>
Date:   Tue, 14 Dec 2021 20:36:57 -0800
From:   Davidlohr Bueso <dave@...olabs.net>
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,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH-next v3] mm/memcg: Properly handle memcg_stock access for
 PREEMPT_RT

On Tue, 14 Dec 2021, Waiman Long wrote:

>@@ -2189,7 +2194,7 @@ static void drain_local_stock(struct work_struct *dummy)
>	 * drain_stock races is that we always operate on local CPU stock
>	 * here with IRQ disabled
>	 */
>-	local_irq_save(flags);
>+	local_lock_irqsave(&memcg_stock.lock, flags);
>
>	stock = this_cpu_ptr(&memcg_stock);
>	drain_obj_stock(&stock->irq_obj);

So here there is still the problem that you can end up taking sleeping locks
with irqs disabled via obj_cgroup_put() >> obj_cgroup_release() - ie: the
percpu_ref_switch_lock and css_set_lock. It had occurred to me to promote
the former to a raw spinlock, but doubt we can get away with the latter.

Thanks,
Davidlohr