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
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