linux-kernel - Re: [RFC PATCH v4 1/9] CPU hotplug: Provide APIs to prevent CPU offline from atomic context

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Date:	Thu, 13 Dec 2012 00:42:57 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	Oleg Nesterov <oleg@...hat.com>
CC:	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, 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 v4 1/9] CPU hotplug: Provide APIs to prevent CPU offline
 from atomic context

On 12/13/2012 12:18 AM, Oleg Nesterov wrote:
> On 12/13, Srivatsa S. Bhat wrote:
>>
>> On 12/12/2012 11:32 PM, Oleg Nesterov wrote:
>>> And _perhaps_ get_ can avoid it too?
>>>
>>> I didn't really try to think, probably this is not right, but can't
>>> something like this work?
>>>
>>> 	#define XXXX	(1 << 16)
>>> 	#define MASK	(XXXX -1)
>>>
>>> 	void get_online_cpus_atomic(void)
>>> 	{
>>> 		preempt_disable();
>>>
>>> 		// only for writer
>>> 		__this_cpu_add(reader_percpu_refcnt, XXXX);
>>>
>>> 		if (__this_cpu_read(reader_percpu_refcnt) & MASK) {
>>> 			__this_cpu_inc(reader_percpu_refcnt);
>>> 		} else {
>>> 			smp_wmb();
>>> 			if (writer_active()) {
>>> 				...
>>> 			}
>>> 		}
>>>
>>> 		__this_cpu_dec(reader_percpu_refcnt, XXXX);
>>> 	}
>>>
>>
>> Sorry, may be I'm too blind to see, but I didn't understand the logic
>> of how the mask helps us avoid disabling interrupts..
> 
> Why do we need cli/sti at all? We should prevent the following race:
> 
> 	- the writer already holds hotplug_rwlock, so get_ must not
> 	  succeed.
> 
> 	- the new reader comes, it increments reader_percpu_refcnt,
> 	  but before it checks writer_active() ...
> 
> 	- irq handler does get_online_cpus_atomic() and sees
> 	  reader_nested_percpu() == T, so it simply increments
> 	  reader_percpu_refcnt and succeeds.
> 
> OTOH, why do we need to increment reader_percpu_refcnt the counter
> in advance? To ensure that either we see writer_active() or the
> writer should see reader_percpu_refcnt != 0 (and that is why they
> should write/read in reverse order).
> 
> The code above tries to avoid this race using the lower 16 bits
> as a "nested-counter", and the upper bits to avoid the race with
> the writer.
> 
> 	// only for writer
> 	__this_cpu_add(reader_percpu_refcnt, XXXX);
> 
> If irq comes and does get_online_cpus_atomic(), it won't be confused
> by __this_cpu_add(XXXX), it will check the lower bits and switch to
> the "slow path".
> 

This is a very clever scheme indeed! :-) Thanks a lot for explaining
it in detail.

> 
> But once again, so far I didn't really try to think. It is quite
> possible I missed something.
> 

Even I don't spot anything wrong with it. But I'll give it some more
thought..

Regards,
Srivatsa S. Bhat

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