Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Message-ID: <b3d35a3e605880034a619e9a3113980107b6d18d.camel@redhat.com>
Subject: Re: [patch 2/2] mm: lru_cache_disable: replace work queue
 synchronization with synchronize_rcu
From:   Nicolas Saenz Julienne <nsaenzju@redhat.com>
To:     Marcelo Tosatti <mtosatti@redhat.com>, linux-kernel@vger.kernel.org
Cc:     linux-mm@kvack.org, Minchan Kim <minchan@kernel.org>,
        Matthew Wilcox <willy@infradead.org>,
        Mel Gorman <mgorman@techsingularity.net>,
        Juri Lelli <juril@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        "Paul E. McKenney" <paulmck@kernel.org>
Date:   Tue, 22 Feb 2022 16:53:30 +0100
In-Reply-To: <20220222144907.056089321@redhat.com>
References: <20220222144706.937848439@redhat.com>
         <20220222144907.056089321@redhat.com>
Content-Type: text/plain; charset="UTF-8"
User-Agent: Evolution 3.42.4 (3.42.4-1.fc35) 
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

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