Re: [RFC PATCH v2 01/10] CPU hotplug: Provide APIs for "light" atomic readers to prevent CPU offline

["Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>]

On 12/06/2012 12:37 AM, Oleg Nesterov wrote:
> I'll try to read this series later,
> 
> one minor and almost offtopic nit.
> 
> On 12/06, Srivatsa S. Bhat wrote:
>>
>>  static int __ref take_cpu_down(void *_param)
>>  {
>>  	struct take_cpu_down_param *param = _param;
>> +	unsigned long flags;
>>  	int err;
>>
>> +	/*
>> +	 *  __cpu_disable() is the step where the CPU is removed from the
>> +	 *  cpu_online_mask. Protect it with the light-lock held for write.
>> +	 */
>> +	write_lock_irqsave(&light_hotplug_rwlock, flags);
>> +
>>  	/* Ensure this CPU doesn't handle any more interrupts. */
>>  	err = __cpu_disable();
>> -	if (err < 0)
>> +	if (err < 0) {
>> +		write_unlock_irqrestore(&light_hotplug_rwlock, flags);
>>  		return err;
>> +	}
>> +
>> +	/*
>> +	 * We have successfully removed the CPU from the cpu_online_mask.
>> +	 * So release the light-lock, so that the light-weight atomic readers
>> +	 * (who care only about the cpu_online_mask updates, and not really
>> +	 * about the actual cpu-take-down operation) can continue.
>> +	 *
>> +	 * But don't enable interrupts yet, because we still have work left to
>> +	 * do, to actually bring the CPU down.
>> +	 */
>> +	write_unlock(&light_hotplug_rwlock);
>>
>>  	cpu_notify(CPU_DYING | param->mod, param->hcpu);
>> +
>> +	local_irq_restore(flags);
>>  	return 0;
> 
> This is subjective, but imho _irqsave and the fat comment look confusing.
> 
> Currently take_cpu_down() is always called with irqs disabled, so you
> do not need to play with interrupts.
> 
> 10/10 does s/__stop_machine/stop_cpus/ and that patch could simply add
> local_irq_disable/enable into take_cpu_down().
> 

Hmm, we could certainly do that, but somehow I felt it would be easier to
read if we tinker and fix up the take_cpu_down() logic at one place, as a
whole, instead of breaking up into pieces in different patches. And that
also makes the last patch look really cute: it just replaces stop_machine()
with stop_cpus(), as the changelog intended.

I'll see if doing like what you suggested improves the readability, and
if yes, I'll change it. Thank you!

Regards,
Srivatsa S. Bhat