Hi, Mike thanks for the reviewing, really appreciate it!
Mike Pattrick <[email protected]> 于2022年2月4日周五 02:08写道： > On Tue, Jan 18, 2022 at 10:01 AM Peng He <[email protected]> wrote: > > > > From hepeng: > > > https://patchwork.ozlabs.org/project/openvswitch/patch/[email protected]/#2487473 > > > > also from guohongzhi <[email protected]>: > > > http://patchwork.ozlabs.org/project/openvswitch/patch/[email protected]/ > > > > also from a discussion about the mixing use of RCU and refcount in the > mail > > list with Ilya Maximets, William Tu, Ben Pfaf, and Gaëtan Rivet. > > > > A summary, as quoted from Ilya: > > > > " > > RCU for ofproto was introduced for one > > and only one reason - to avoid freeing ofproto while rules are still > > alive. This was done in commit f416c8d61601 ("ofproto: RCU postpone > > rule destruction."). The goal was to allow using rules without > > refcounting them within a single grace period. And that forced us > > to postpone destruction of the ofproto for a single grace period. > > Later commit 39c9459355b6 ("Use classifier versioning.") made it > > possible for rules to be alive for more than one grace period, so > > the commit made ofproto wait for 2 grace periods by double postponing. > > As we can see now, that wasn't enough and we have to wait for more > > than 2 grace periods in certain cases. > > " > > > > In a short, the ofproto should have a longer life time than rule, if > > the rule lasts for more than 2 grace periods, the ofproto should live > > longer to ensure rule->ofproto is valid. It's hard to predict how long > > a ofproto should live, thus we need to use refcount on ofproto to make > > things easy. The controversial part is that we have already used RCU > postpone > > to delay ofproto destrution, if we have to add refcount, is it simpler to > > use just refcount without RCU postpone? > > > > IMO, I think going back to the pure refcount solution is more > > complicated than mixing using both. > > > > Gaëtan Rive asks some questions on guohongzhi's v2 patch: > > > > during ofproto_rule_create, should we use ofproto_ref > > or ofproto_try_ref? how can we make sure the ofproto is alive? > > > > By using RCU, ofproto has three states: > > > > state 1: alive, with refcount >= 1 > > state 2: dying, with refcount == 0, however pointer is valid > > state 3: died, memory freed, pointer might be dangling. > > > > Without using RCU, there is no state 2, thus, we have to be very careful > > every time we see a ofproto pointer. In contrast, with RCU, we can be > sure > > that it's alive at least in this grace peroid, so we can just check if > > it is dying by ofproto_try_ref. > > > > This shows that by mixing use of RCU and refcount we can save a lot of > work > > worrying if ofproto is dangling. > > > > In short, the RCU part makes sure the ofproto is alive when we use it, > > and the refcount part makes sure it lives longer enough. > > > > Also regarding a new patch filed recently, people are now making use > > of RCU to protect ofproto: > > > > > https://patchwork.ozlabs.org/project/openvswitch/patch/[email protected]/ > > > > In this patch, I have merged guohongzhi's patch and mine, and fixes > > accoring to the previous comments. > > > > Signed-off-by: Peng He <[email protected]> > > Signed-off-by: guohongzhi <[email protected]> > > --- > > Hello Peng, > > Excellent patch, I've read through the previous patch series and > associated comments, I think you've addressed all the issues. > > Acked-by: Mike Pattrick <[email protected]> > > -- hepeng _______________________________________________ dev mailing list [email protected] https://mail.openvswitch.org/mailman/listinfo/ovs-dev
