linux-kernel - Re: [RFC PATCH v3 1/9] CPU hotplug: Provide APIs to prevent CPU offline from atomic context

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Message-ID: <50C72F4B.9090805@linux.vnet.ibm.com>
Date:	Tue, 11 Dec 2012 18:34:11 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	Oleg Nesterov <oleg@...hat.com>
CC:	rostedt@...dmis.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, tj@...nel.org, sbw@....edu,
	amit.kucheria@...aro.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 v3 1/9] CPU hotplug: Provide APIs to prevent CPU offline
 from atomic context

On 12/10/2012 11:45 PM, Oleg Nesterov wrote:
> On 12/10, Srivatsa S. Bhat wrote:
>>
>> On 12/10/2012 02:27 AM, Oleg Nesterov wrote:
>>> However. If this is true, then compared to preempt_disable/stop_machine
>>> livelock is possible. Probably this is fine, we have the same problem with
>>> get_online_cpus(). But if we can accept this fact I feel we can simmplify
>>> this somehow... Can't prove, only feel ;)
>>
>> Not sure I follow..
> 
> I meant that write_lock_irqsave(&hotplug_rwlock) in take_cpu_down()
> can spin "forever".
> 
> Suppose that reader_acked() == T on every CPU, so that
> get_online_cpus_atomic() always takes read_lock(&hotplug_rwlock).
> 
> It is possible that this lock will be never released by readers,
> 
> 	CPU_0				CPU_1
> 
> 	get_online_cpus_atomic()
> 					get_online_cpus_atomic()
> 	put_online_cpus_atomic()
> 
> 	get_online_cpus_atomic()
> 					put_online_cpus_atomic()
> 
> 					get_online_cpus_atomic()
> 	put_online_cpus_atomic()
> 
> and so on.
> 

Right, and we can't do anything about it :(
Regards,
Srivatsa S. Bhat

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