________________________________________ De: boost-bounces@lists.boost.org [boost-bounces@lists.boost.org] En nombre de Peter Dimov [pdimov@pdimov.com] Enviado el: sábado, 11 de abril de 2009 2:15 Para: boost@lists.boost.org Asunto: Re: [boost] atomic_count::operator++ return type
JOAQUIN M. LOPEZ MUÑOZ:
I'm afraid I don't quite get what you mean exactly by memory visibility in this context: I guess I need the same behavior (mutatis mutandis) as given by op--. In case it sheds some light, the usage case I talk about can be taken a a look at
https://svn.boost.org/trac/boost/browser/trunk/boost/flyweight/refcounted.hp...
The protocol is a classical refcounting mechanism with a twist due to the possiblity that values are retrieved even if their recount drops to zero.
I'm not sure how this works... what is TrackingEntry?
It is a façade to the functionality provided by the rest of Boost.Flyweight for the use of the refcounting mechanism: http://www.boost.org/libs/flyweight/doc/reference/tracking.html#tracking * TrackingHelper::entry(h) returns the value to which the refcounted_handle h refers (which is implicitly convertible to a refcounted_value). Think of it as a dereference operator. * TrackingHelper::erase(h,check_erase) deletes the value refered to by h iff check_erase(h) returns true. The execution of this routine happens in a locked context (including the call to check_erase and erasure itself). * Though it may not be apparent, the execution of explicit refcounted_handle(const Handle&) also happens in a locked context (this is important to the implementation of the refcounting protocol).
How do you handle the possibility of
- thread A detaches the last reference, calls erase, erase calls check_erase, gets true, and is preempted;
This cannot happen as check_erase is invoked in a locked context. The critical point where preemption can occur is between detaching of the last reference and execution of erase, which is precisely what the del_ref member variable keeps tracks of; if A resumes after preemption and the value has acquired a new reference the count of deleters will be >0 and check_delete will return false. Joaquín M López Muñoz Telefónica, Investigación y Desarrollo