On Tue, Feb 19, 2013 at 2:50 AM, Srivatsa S. Bhat <srivatsa.bhat@xxxxxxxxxxxxxxxxxx> wrote: > But, the whole intention behind removing the parts depending on the > recursive property of rwlocks would be to make it easier to make rwlocks > fair (going forward) right? Then, that won't work for CPU hotplug, because, > just like we have a legitimate reason to have recursive > get_online_cpus_atomic(), we also have a legitimate reason to have > unfairness in locking (i.e., for deadlock-safety). So we simply can't > afford to make the locking fair - we'll end up in too many deadlock > possibilities, as hinted in the changelog of patch 1. Grumpf - I hadn't realized that making the underlying rwlock fair would break your hotplug use case. But you are right, it would. Oh well :/ > So the only long-term solution I can think of is to decouple > percpu-rwlocks and rwlock_t (like what Tejun suggested) by implementing > our own unfair locking scheme inside. What do you think? I have no idea how hard would it be to change get_online_cpus_atomic() call sites so that the hotplug rwlock read side has a defined order vs other locks (thus making sure the situation you describe in patch 1 doesn't happen). I agree we shouldn't base our short term plans around that, but maybe that's doable in the long term ??? Otherwise, I think we should add some big-fat-warning that percpu rwlocks don't have reader/writer fairness, that the hotplug use case actually depends on the unfairness / would break if the rwlock was made fair, and that any new uses of percpu rwlocks should be very carefully considered because of the reader/writer fairness issues. Maybe even give percpu rwlocks a less generic sounding name, given how constrained they are by the hotplug use case. -- Michel "Walken" Lespinasse A program is never fully debugged until the last user dies. -- 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
