linux-kernel - Re: Deadlock between cpu_hotplug_begin and cpu_add_remove

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Message-ID: <52E7C072.80704@linux.vnet.ibm.com>
Date:	Tue, 28 Jan 2014 20:06:34 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	Rusty Russell <rusty@...tcorp.com.au>
CC:	Paul Mackerras <paulus@...ba.org>, linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...nel.org>,
	Oleg Nesterov <oleg@...hat.com>, Tejun Heo <tj@...nel.org>,
	Michel Lespinasse <walken@...gle.com>, ego@...ux.vnet.ibm.com,
	Thomas Gleixner <tglx@...utronix.de>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
Subject: Re: Deadlock between cpu_hotplug_begin and cpu_add_remove_lock

On 01/24/2014 04:31 AM, Rusty Russell wrote:
> "Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com> writes:
>> On 01/23/2014 07:59 AM, Rusty Russell wrote:
>>> "Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com> writes:
>>>> On 01/22/2014 02:00 PM, Srivatsa S. Bhat wrote:
>>>>> Hi Paul,
>>>
>>> I find an old patch for register_allcpu_notifier(), but the "bool
>>> replay_history" should be eliminated (always true): it's too weird.
>>>
>>
>> Sorry, I didn't get this part. Why do you say that replay_history
>> will always be true?
> 
> OK, let me start again and try to explain myself properly:
> 
> register_cpu_notifier is a bad API.  It's hard to get right because:
> 1) You need to loop over online (or present) cpus once before you call
>    it.
> 2) You have to beware the race between the loop and registration, but
>    much example code happens at boot time where it doesn't matter,
>    so random author is likely to copy that and have a race.
> 3) You have two paths doing the same thing: the loop which is run on
>    every machine (cpu hotplug or not), and the notifier callback which
>    is run far less rarely.
> 
> What we actually *want* is a routine which will reliably call for every
> current and future CPU, and then there are very few places which should
> use the current register_cpu_notifier().
> 
> ie. halfway between register_cpu_notifier() (too racy) and
> register_allcpu_notifier() (too simplified).
> 
> Let's call it register_cpu_callback / unregister_cpu_callback?
> 

Thanks a lot for the detailed and profound explanation! It makes perfect
sense to me now.

>> By the way, I'm still tempted to try out the simpler-looking alternative
>> idea of exporting cpu_maps_update_begin() and cpu_maps_update_done()
>> and then mandating that the callers do:
>>
>> cpu_maps_update_begin();
>> for_each_online_cpu(cpu) {
>> 	...
>> }
>>
>> __register_cpu_notifier(); // this doesn't take the add_remove_lock
>> cpu_maps_update_done();
> 
> Sure, fix this one for -stable.  But let's create an idiom we can be
> proud of for the longer term.
> 

Ok, that sounds good, will work on that.

Thank you very much!
 
Regards,
Srivatsa S. Bhat

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