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

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

On 12/12/2012 10:47 PM, Oleg Nesterov wrote:
> On 12/11, Srivatsa S. Bhat wrote:
>>
>> IOW, the hotplug readers just increment/decrement their per-cpu refcounts
>> when no writer is active.
> 
> plus cli/sti ;)

Of course, forgot to mention it, again! :)

> and increment/decrement are atomic.
> 
> At first glance looks correct to me, but I'll try to read it carefully
> later.
> 
> A couple of minor nits,
> 
>> +static DEFINE_PER_CPU(bool, writer_signal);
> 
> Why it needs to be per-cpu? It can be global and __read_mostly to avoid
> the false-sharing. OK, perhaps to put reader_percpu_refcnt/writer_signal
> into a single cacheline...
> 

Even I realized this (that we could use a global) after posting out the
series.. But do you think that it would be better to retain the per-cpu
variant itself, due to the cache effects?

>> +void get_online_cpus_atomic(void)
>> +{
>> +	unsigned long flags;
>> +
>> +	preempt_disable();
>> +
>> +	if (cpu_hotplug.active_writer == current)
>> +		return;
>> +
>> +	local_irq_save(flags);
> 
> Yes... this is still needed, we are going to increment reader_percpu_refcnt
> unconditionally and this makes reader_nested_percpu() == T.
> 
> But,
> 
>> +void put_online_cpus_atomic(void)
>> +{
>> +	unsigned long flags;
>> +
>> +	if (cpu_hotplug.active_writer == current)
>> +		goto out;
>> +
>> +	local_irq_save(flags);
>> +
>> +	/*
>> +	 * We never allow heterogeneous nesting of readers. So it is trivial
>> +	 * to find out the kind of reader we are, and undo the operation
>> +	 * done by our corresponding get_online_cpus_atomic().
>> +	 */
>> +	if (__this_cpu_read(reader_percpu_refcnt))
>> +		__this_cpu_dec(reader_percpu_refcnt);
>> +	else
>> +		read_unlock(&hotplug_rwlock);
>> +
>> +	local_irq_restore(flags);
>> +out:
>> +	preempt_enable();
>> +}
> 
> Do we really need local_irq_save/restore in put_ ?
>

Hmm.. good point! I don't think we need it.
 

Regards,
Srivatsa S. Bhat