Re: [RFC PATCH v3 1/9] CPU hotplug: Provide APIs to prevent CPU offlinefrom atomic context

[Srivatsa S. Bhat]

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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