On 12/19/2012 01:13 AM, Oleg Nesterov wrote: > On 12/18, Srivatsa S. Bhat wrote: >> >> So now that we can't avoid disabling and enabling interrupts, > > Still I think it would be better to not use local_irq_save/restore > directly.
Sure, we can use this_cpu_add() itself. I explicitly used local_irq_save/restore here just to explain my question. > And, > >> I was >> wondering if we could exploit this to avoid the smp_mb().. >> >> Maybe this is a stupid question, but I'll shoot it anyway... >> Does local_irq_disable()/enable provide any ordering guarantees by any >> chance? > > Oh, I do not know. > > But please look at the comment above prepare_to_wait(). It assumes > that even spin_unlock_irqrestore() is not the full barrier. > Semi-permeable barrier.. Hmm.. > In any case. get_online_cpus_atomic() has to use irq_restore, not > irq_enable. And _restore does nothing "special" if irqs were already > disabled, so I think we can't rely on sti. > Right, I forgot about the _restore part. >> I tried thinking about other ways to avoid that smp_mb() in the reader, > > Just in case, I think there is no way to avoid mb() in _get (although > perhaps it can be "implicit"). > Actually, I was trying to avoid mb() in the _fastpath_, when there is no active writer. I missed stating that clearly, sorry. > The writer changes cpu_online_mask and drops the lock. We need to ensure > that the reader sees the change in cpu_online_mask after _get returns. > The write_unlock() will ensure the completion of the update to cpu_online_mask, using the same semi-permeable logic that you pointed above. So readers will see the update as soon as the writer releases the lock, right? >> but was unsuccessful. So if the above assumption is wrong, I guess we'll >> just have to go with the version that uses synchronize_sched() at the >> writer-side. > > In this case we can also convert get_online_cpus() to use percpu_rwsem > and avoid mutex_lock(&cpu_hotplug.lock), but this is minor I guess. > I do not think get_online_cpus() is called too often. > Yes, we could do that as well. I remember you saying that you had some patches for percpu_rwsem to help use it in cpu hotplug code (to make it recursive, IIRC). So, I guess we'll go with the synchronize_sched() approach for percpu rwlocks then. Tejun, it is still worthwhile to expose this as a generic percpu rwlock and then use it inside cpu hotplug code, right? Regards, Srivatsa S. Bhat -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
