On Thu, Nov 17, 2016 at 10:28:00AM +0100, Peter Zijlstra wrote: > On Tue, Nov 15, 2016 at 10:19:09PM +0800, Boqun Feng wrote: > > But as I said, we actually only need the pairing of orderings: > > > > 1) load part of cmpxchg -> free() > > 2) object accesses -> store part of cmpxchg > > > > Ordering #1 can be achieved via control dependency as you pointed out > > that free()s very much includes stores. And ordering #2 can be achieved > > with RELEASE. > > > > So the code is right, I just thought the comment may be misleading. The > > reason we use cmpxchg_release() is just for achieving ordering #2, and > > not to order "prior loads and stores" with "a subsequent free". > > > > Am I missing some subtle orderings here? > > I would want to further quality 1), it must be no earlier than the load > of the last / successful ll/sc round. > > At that point we're guaranteed a reference count of 1 that _will_ drop > to 0, and thus nobody else (should) reference that memory anymore. > > If we agree on this, I'll update the comment :-) Will, do you too agree?
All sounds reasonable to me. It's worth pointing out that you can't create order using a control dependency hanging off the status flag of a store-conditional, but the code in question here has the dependency from the loaded value, which is sufficient. Will
