On 02/11/2013 01:43 AM, Oleg Nesterov wrote: > On 02/11, Srivatsa S. Bhat wrote: >> >> On 02/09/2013 04:40 AM, Paul E. McKenney wrote: >>>> +static void announce_writer_inactive(struct percpu_rwlock *pcpu_rwlock) >>>> +{ >>>> + unsigned int cpu; >>>> + >>>> + drop_writer_signal(pcpu_rwlock, smp_processor_id()); >>> >>> Why do we drop ourselves twice? More to the point, why is it important to >>> drop ourselves first? >>> >> >> I don't see where we are dropping ourselves twice. Note that we are no longer >> in the cpu_online_mask, so the 'for' loop below won't include us. So we need >> to manually drop ourselves. It doesn't matter whether we drop ourselves first >> or later. > > Yes, but this just reflects its usage in cpu-hotplug. cpu goes away under > _write_lock. > Ah, right. I guess the code still has remnants from the older version in which this locking scheme wasn't generic and was tied to cpu-hotplug alone.. > Perhaps _write_lock/unlock shoud use for_each_possible_cpu() instead? > Hmm, that wouldn't be too bad. > Hmm... I think this makes sense anyway. Otherwise, in theory, > percpu_write_lock(random_non_hotplug_lock) can race with cpu_up? > Yeah, makes sense. Will change it to for_each_possible_cpu(). And I had previously fixed such races with lglocks with a complicated scheme (to avoid the costly for_each_possible loop), which was finally rewritten to use for_each_possible_cpu() for the sake of simplicity.. Regards, Srivatsa S. Bhat -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html