linux-kernel - Re: [RFC PATCH v2 01/10] CPU hotplug: Provide APIs for "light" atomic readers to prevent CPU offline

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Date:	Thu, 6 Dec 2012 17:18:50 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Cc:	tj@...nel.org, tglx@...utronix.de, peterz@...radead.org,
	paulmck@...ux.vnet.ibm.com, rusty@...tcorp.com.au,
	mingo@...nel.org, akpm@...ux-foundation.org, namhyung@...nel.org,
	vincent.guittot@...aro.org, sbw@....edu, amit.kucheria@...aro.org,
	rostedt@...dmis.org, rjw@...k.pl, wangyun@...ux.vnet.ibm.com,
	xiaoguangrong@...ux.vnet.ibm.com, nikunj@...ux.vnet.ibm.com,
	linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v2 01/10] CPU hotplug: Provide APIs for "light"
	atomic readers to prevent CPU offline

On 12/06, Srivatsa S. Bhat wrote:
>
> +void get_online_cpus_atomic(void)
> +{
> +	int c, old;
> +
> +	preempt_disable();
> +	read_lock(&hotplug_rwlock);

Confused... Why it also takes hotplug_rwlock?

> +
> +	for (;;) {
> +		c = atomic_read(&__get_cpu_var(atomic_reader_refcount));
> +		if (unlikely(writer_active(c))) {
> +			cpu_relax();
> +			continue;
> +		}
> +
> +		old = atomic_cmpxchg(&__get_cpu_var(atomic_reader_refcount),
> +				     c, c + 1);
> +
> +		if (likely(old == c))
> +			break;
> +
> +		c = old;
> +	}
> +}

	while (!atomic_inc_unless_negative(...))
		cpu_relax();
	
and atomic_dec_unless_positive() in disable_atomic_reader().


Obviously you can't use get_online_cpus_atomic() under rq->lock or
task->pi_lock or any other lock CPU_DYING can take. Probably this is
fine, but perhaps it makes sense to add the lockdep annotations.

Oleg.

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