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:	Fri, 7 Dec 2012 20:56:29 +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 12/07, Srivatsa S. Bhat wrote:
>
> On 12/06/2012 09:48 PM, Oleg Nesterov wrote:
> > 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?
>
> To avoid ABBA deadlocks.
>
> hotplug_rwlock was meant for the "light" readers.
> The atomic counters were meant for the "heavy/full" readers.

OK, I got lost a bit. I'll try to read v3 tomorrow.

> > 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.
>
> Hmm, you are right. We can't use _atomic() in the CPU_DYING path.

Not sure I undestand... I simply meant that, say,
get_online_cpus_atomic() under task->pi_lock can obviously deadlock
with take_cpu_down() which can want the same task->pi_lock after
disable_atomic_reader().

Oleg.

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