linux-kernel - Re: [RFC][4.1.15-rt17 PATCH] mm: swap: lru drain don't use workqueue with PREEMPT_RT

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Date:	Tue, 12 Jan 2016 13:01:09 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	l@...ileo.org
cc:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>, rostedt@...dmis.org,
	John Kacur <jkacur@...hat.com>, linux-mm@...ck.org,
	Leandro Dorileo <leandro.maciel.dorileo@...el.com>
Subject: Re: [RFC][4.1.15-rt17 PATCH] mm: swap: lru drain don't use workqueue
 with PREEMPT_RT_FULL

On Sun, 10 Jan 2016, l@...ileo.org wrote:
> +#ifdef CONFIG_PREEMPT_RT_FULL
> +void lru_add_drain_all(void)
> +{
> +	static DEFINE_MUTEX(lock);
> +	int cpu;
> +
> +	mutex_lock(&lock);
> +	get_online_cpus();
> +
> +	for_each_online_cpu(cpu) {
> +		smp_call_function_single(cpu, lru_add_drain, NULL, 1);

How is that supposed to work on RT? Not at all, because lru_add_drain() takes
'sleeping' spinlocks and you cannot do that from hard interrupt context.

Enable lockdep (what you should have done before posting) and watch the
fireworks.

Thanks,

	tglx