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. 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. 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. > 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"). 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. > 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. Oleg. -- 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/