On Sun, Mar 02, 2014 at 10:42:05PM +0800, Ming Lei wrote:
> >> @@ -237,8 +242,11 @@ void percpu_ida_free(struct percpu_ida *pool,
> >> unsigned tag)
> >> spin_unlock(&tags->lock);
> >>
> >> if (nr_free == 1) {
> >> - cpumask_set_cpu(smp_processor_id(),
> >> - &pool->cpus_have_tags);
> >> + cpumask_set_cpu(smp_processor_id(), &pool->cpus_have_tags);
> >> + /*
> >> + * Pairs with smp_rmb() in steal_tags()
> >> + */
> >> + smp_wmb();
> >> wake_up(&pool->wait);
> >
> > I think I'm nacking this - there's a lot of code in the kernel that relies
> > on
> > the fact that prepare_to_wait)/wake_up() do the appropriate fences, we
> > really
> > shouldn't be adding to the barriers those do.
>
> In theory, it still might cause percpu_ida_alloc(TASK_RUNNING) failed,
> looks it isn't a big deal for the case.
>
> But I am wondering why cpumask_set_cpu() isn't called with
> holding lock inside percpu_ida_free()? Looks 'nr_free == 1'
> shouldn't have happened frequently.
Because bouncing on the lock is more expensive than occasionally putting
a thread into sleep.
>
>
> Thanks,
> --
> Ming Lei
--
Regards,
Alexander Gordeev
agord...@redhat.com
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