linux-kernel - Re: [PATCH 01/51] CPU hotplug: Provide lockless versions of callback registration functions

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Date:	Tue, 11 Feb 2014 18:15:53 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Cc:	Toshi Kani <toshi.kani@...com>, paulus@...ba.org,
	rusty@...tcorp.com.au, peterz@...radead.org, tglx@...utronix.de,
	akpm@...ux-foundation.org, mingo@...nel.org,
	paulmck@...ux.vnet.ibm.com, tj@...nel.org, walken@...gle.com,
	ego@...ux.vnet.ibm.com, linux@....linux.org.uk,
	linux-kernel@...r.kernel.org,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>
Subject: Re: [PATCH 01/51] CPU hotplug: Provide lockless versions of
	callback registration functions

On 02/11, Srivatsa S. Bhat wrote:
>
> +static DECLARE_RWSEM(cpu_hotplug_rwsem);
> +
> +void cpu_notifier_register_begin(void)
> +{
> +	down_read(&cpu_hotplug_rwsem);
> +}
> +
> +void cpu_notifier_register_end(void)
> +{
> +	up_read(&cpu_hotplug_rwsem);
> +}
> +
>  /* Serializes the updates to cpu_online_mask, cpu_present_mask */
>  static DEFINE_MUTEX(cpu_add_remove_lock);
>  
> @@ -32,12 +45,14 @@ static DEFINE_MUTEX(cpu_add_remove_lock);
>   */
>  void cpu_maps_update_begin(void)
>  {
> +	down_write(&cpu_hotplug_rwsem);
>  	mutex_lock(&cpu_add_remove_lock);
>  }
>  
>  void cpu_maps_update_done(void)
>  {
>  	mutex_unlock(&cpu_add_remove_lock);
> +	up_write(&cpu_hotplug_rwsem);
>  }

I am a bit confused... If we do this, why we can't simply turn
cpu_add_remove_lock into rw_semaphore?

Oleg.

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