On 02/19/2013 03:10 PM, Michel Lespinasse wrote: > 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 :/ > Yeah :-/ >> 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 ??? > I think it should be possible in the longer term. I'm expecting it to be *much much* harder to audit and convert (requiring a lot of subsystem knowledge of each subsystem that we are touching), than the simpler tree-wide conversion that I did in this patchset... but I don't think it is impossible. > 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. In fact, when I started out, I actually contained all the new locking code inside CPU hotplug itself, and didn't even expose it as a generic percpu rwlock in some of the previous versions of this patchset... :-) But now that we already have a generic locking scheme exposed, we could add a warning against using it without due consideration. > Maybe even give percpu rwlocks a less generic sounding name, given how > constrained they are by the hotplug use case. I wouldn't go that far... ;-) Unfairness is not a show-stopper right? IMHO, the warning/documentation should suffice for anybody wanting to try out this locking scheme for other use-cases. 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
