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

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Message-ID: <20140213174452.GB14455@redhat.com>
Date:	Thu, 13 Feb 2014 18:44:52 +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 02/12, Srivatsa S. Bhat wrote:
>
> On 02/11/2014 10:45 PM, Oleg Nesterov wrote:
> >
> > I am a bit confused... If we do this, why we can't simply turn
> > cpu_add_remove_lock into rw_semaphore?

[...snip...]

> 	cpu_notifier_register_begin();                |  Run in parallel
>                                                       |  with similar phases
> 	for_each_online_cpu(cpu)                      |  from other subsystems.
> 		init_cpu(cpu);                        |
>
> 	/* Updates the cpu notifier chain. */
> 	register_cpu_notifier(&foobar_cpu_notifier); ||| -- Must run serially

Ah indeed, we can't use a single lock, thanks. Perhaps we can simply
add a spinlock_t which only protects cpu_chain though, but I am not
sure and currently this is off-topic anyway.

Thanks,

Oleg.

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