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 09:48 PM, Oleg Nesterov wrote:
> On 12/06, Srivatsa S. Bhat wrote:
>>
>> +void get_online_cpus_atomic(void)
>> +{
>> +	int c, old;
>> +
>> +	preempt_disable();
>> +	read_lock(&hotplug_rwlock);
> 
> Confused... Why it also takes hotplug_rwlock?

To avoid ABBA deadlocks.

hotplug_rwlock was meant for the "light" readers.
The atomic counters were meant for the "heavy/full" readers.
I wanted them to be able to nest in any manner they wanted,
such as:

Full inside light:

get_online_cpus_atomic_light()
	...
	get_online_cpus_atomic_full()
	...
	put_online_cpus_atomic_full()
	...
put_online_cpus_atomic_light()

Or, light inside full:

get_online_cpus_atomic_full()
	...
	get_online_cpus_atomic_light()
	...
	put_online_cpus_atomic_light()
	...
put_online_cpus_atomic_full()

To allow this, I made the two sets of APIs take the locks
in the same order internally.

(I had some more description of this logic in the changelog
of 2/10; the only difference there is that instead of atomic
counters, I used rwlocks for the full-readers as well.
https://lkml.org/lkml/2012/12/5/320)

> 
>> +
>> +	for (;;) {
>> +		c = atomic_read(&__get_cpu_var(atomic_reader_refcount));
>> +		if (unlikely(writer_active(c))) {
>> +			cpu_relax();
>> +			continue;
>> +		}
>> +
>> +		old = atomic_cmpxchg(&__get_cpu_var(atomic_reader_refcount),
>> +				     c, c + 1);
>> +
>> +		if (likely(old == c))
>> +			break;
>> +
>> +		c = old;
>> +	}
>> +}
> 
> 	while (!atomic_inc_unless_negative(...))
> 		cpu_relax();
> 	
> and atomic_dec_unless_positive() in disable_atomic_reader().
> 

Ah, great! I was searching for them while writing the code, but somehow
overlooked them and rolled out my own. ;-)

> 
> Obviously you can't use get_online_cpus_atomic() under rq->lock or
> task->pi_lock or any other lock CPU_DYING can take. Probably this is
> fine, but perhaps it makes sense to add the lockdep annotations.
> 

Hmm, you are right. We can't use _atomic() in the CPU_DYING path.
So how about altering it to _allow_ that, instead of teaching lockdep
that we don't allow it? I mean, just like how the existing
get_online_cpus() allows such calls in the writer?

(I haven't thought it through; just thinking aloud...)

Regards,
Srivatsa S. Bhat