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

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Message-ID: <20121207205935.GA17608@redhat.com>
Date:	Fri, 7 Dec 2012 21:59:35 +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 12/08, Srivatsa S. Bhat wrote:
>
> On 12/08/2012 01:26 AM, Oleg Nesterov wrote:
> > 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().
>
> Right, I mistook your point for something else (i.e., ability for
> the writer to do get_online_cpus_atomic() safely, which I fixed in
> v3).
>
> So, your point above is very valid. And yes, we can't do much
> about it, we'll just have to teach lockdep to catch such usages.

Afaics, this is simple. Just add the "fake" lockdep_map as, say,
lglock does. Except, you need rwlock_acquire_read(map, 0, 1, IP)
because this lock is recursive.

But. There is another email from you about the possible deadlock.
I'll write the reply in a minute...

Oleg.

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