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Date:	Thu, 06 Feb 2014 21:39:58 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	ego@...ux.vnet.ibm.com
CC:	paulus@...ba.org, oleg@...hat.com, rusty@...tcorp.com.au,
	peterz@...radead.org, tglx@...utronix.de,
	akpm@...ux-foundation.org, mingo@...nel.org,
	paulmck@...ux.vnet.ibm.com, tj@...nel.org, walken@...gle.com,
	linux@....linux.org.uk, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 00/51] CPU hotplug: Fix issues with callback registration

On 02/06/2014 05:44 PM, Gautham R Shenoy wrote:
> On Thu, Feb 06, 2014 at 04:34:33PM +0530, Srivatsa S. Bhat wrote:
>>>
>>> CPU_POST_DEAD notification, is invoked with the cpu_hotplug.lock
>>> dropped. This was necessary for subsystems which would be waiting for
>>> some other thread to finish some work, and that other thread could
>>> invoke get_online_cpus(). If CPU_POST_DEAD notification were issued
>>> without dropping the cpu_hotplug.lock, this would lead to a deadlock
>>> as the notifier would be left stuck waiting for the thread which is
>>> blocked in get_online_cpus().
>>>
>>> It was introduced to ensure that multithreaded workqueues can safely
>>> use get_online_cpus() [https://lkml.org/lkml/2008/6/29/121].
>>>
>>> As of now, only two subsystems use this notification and workqueues is
>>> _not_ one of them!
>>>   * arch/x86/kernel/cpu/mcheck/mce.c:mce_cpu_callback()
>>>   * drivers/cpufreq/cpufreq.c:cpufreq_cpu_callback()
>>> I haven't yet audited these two cases to see if they really need this
>>> to be handled in CPU_POST_DEAD or if they can be handled in CPU_DEAD.
>>>
>>
>> Well, cpufreq had a legitimate need to use POST_DEAD to avoid the
>> deadlock described in commit 1aee40ac9c. However, there had been some
>> discussion some time ago about reorganizing the cpufreq's hotplug callback
>> so as to move most (but not all) of its work outside of POST_DEAD [1].
>> But as it stands, I don't think it would be easy to totally get rid of
>> cpufreq's dependence on the POST_DEAD notifier.
>>
> 
> Right, I see the reason why cpufreq needs POST_DEAD.
> 
>> Besides, I think its good to retain the POST_DEAD notifier option in
>> the CPU hotplug core code. It has come handy several times to fix hard
>> deadlock issues.
>>
> 
> I know. I am not denying the usefulness of POST_DEAD. But the fact
> that some of the CPU_* notifiers are invoked with the cpu_hotplug.lock
> held while CPU_POST_DEAD is invoked with the lock dropped looks a bit
> asymmetrical. At the moment I cannot think of a simpler alternative.
> 

Hmmm...

> 
>>> Also can we have an alternate API, something like
>>> cpu_hotplug_register_begin/end() instead of reusing
>>> cpu_maps_update_begin/end() for this usage, since in most of the
>>> patches that follow, we're not touching the any of the cpu_*_maps!
>>>
>>
>> Yes, the function names cpu_maps_update_begin/end() don't really suit
>> the kind of usage I'm proposing in this patchset, and hence is kind of
>> a misnomer. For better readability, I'm thinking of defining a macro
>> such as say, cpu_hotplug_notifier_lock()/unlock() that redirects to
>> cpu_maps_update_begin/end() respectively. That way, we can export just
>> those former symbols for use by modules, and thereby the code would look
>> more intuitive, like this:
>>
>> 	cpu_hotplug_notifier_lock();
>>
>> 	for_each_online_cpu(cpu)
>> 		init_cpu(cpu);
>>
>> 	/* This doesn't take the cpu_add_remove_lock */
>> 	__register_cpu_notifier(&foobar_cpu_notifier);
>>
>> 	cpu_hotplug_notifier_unlock();
>>
>> What do you think?
> 
> Sounds good.

Cool! If there are no objections, I'll use this naming for the APIs
and spin a v2 of the patchset soon.

Thank you!
 
Regards,
Srivatsa S. Bhat

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