linux-kernel - Re: [PATCH] hotplug: Optimize {get,put}_online

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Date:	Mon, 23 Sep 2013 19:50:52 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...nel.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH] hotplug: Optimize {get,put}_online_cpus()

And somehow I didn't notice that cpuhp_set_state() doesn't look right,

On 09/19, Peter Zijlstra wrote:
>  void cpu_hotplug_begin(void)
>  {
> -	cpu_hotplug.active_writer = current;
> +	lockdep_assert_held(&cpu_add_remove_lock);
>  
> -	for (;;) {
> -		mutex_lock(&cpu_hotplug.lock);
> -		if (likely(!cpu_hotplug.refcount))
> -			break;
> -		__set_current_state(TASK_UNINTERRUPTIBLE);
> -		mutex_unlock(&cpu_hotplug.lock);
> -		schedule();
> -	}
> +	__cpuhp_writer = current;
> +
> +	/* After this everybody will observe _writer and take the slow path. */
> +	synchronize_sched();
> +
> +	/* Wait for no readers -- reader preference */
> +	cpuhp_wait_refcount();
> +
> +	/* Stop new readers. */
> +	cpuhp_set_state(1);

But this stops all readers, not only new. Even if cpuhp_wait_refcount()
was correct, a new reader can come right before cpuhp_set_state(1) and
then it can call another recursive get_online_cpus() right after.

Oleg.

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