linux-kernel - Re: [patch 2/2] mm: lru_cache_disable: replace work queue synchronization with synchronize

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Date:   Tue, 22 Feb 2022 16:53:30 +0100
From:   Nicolas Saenz Julienne <nsaenzju@...hat.com>
To:     Marcelo Tosatti <mtosatti@...hat.com>, linux-kernel@...r.kernel.org
Cc:     linux-mm@...ck.org, Minchan Kim <minchan@...nel.org>,
        Matthew Wilcox <willy@...radead.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Juri Lelli <juril@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        "Paul E. McKenney" <paulmck@...nel.org>
Subject: Re: [patch 2/2] mm: lru_cache_disable: replace work queue
 synchronization with synchronize_rcu

On Tue, 2022-02-22 at 11:47 -0300, Marcelo Tosatti wrote:
> @@ -918,14 +917,23 @@ atomic_t lru_disable_count = ATOMIC_INIT
>  void lru_cache_disable(void)
>  {
>  	atomic_inc(&lru_disable_count);
> +	synchronize_rcu();
>  #ifdef CONFIG_SMP
>  	/*
> -	 * lru_add_drain_all in the force mode will schedule draining on
> -	 * all online CPUs so any calls of lru_cache_disabled wrapped by
> -	 * local_lock or preemption disabled would be ordered by that.
> -	 * The atomic operation doesn't need to have stronger ordering
> -	 * requirements because that is enforced by the scheduling
> -	 * guarantees.
> +	 * synchronize_rcu() waits for preemption disabled
> +	 * and RCU read side critical sections
> +	 * For the users of lru_disable_count:
> +	 *
> +	 * preempt_disable, local_irq_disable() [bh_lru_lock()]
> +	 * rcu_read_lock			[lru_pvecs CONFIG_PREEMPT_RT]
> +	 * preempt_disable			[lru_pvecs !CONFIG_PREEMPT_RT]
> +	 *
> +	 *
> +	 * so any calls of lru_cache_disabled wrapped by
> +	 * local_lock+rcu_read_lock or preemption disabled would be
> +	 * ordered by that. The atomic operation doesn't need to have
> +	 * stronger ordering requirements because that is enforced
> +	 * by the scheduling guarantees.

"The atomic operation doesn't need to have stronger ordering requirements
because that is enforced by the scheduling guarantees."

This is no longer needed.

Regards,

-- 
Nicolás Sáenz