[cpp-threads] Failed compare-and-swap

[Paul E. McKenney]

On Tue, Jul 31, 2007 at 10:14:17PM +0300, Peter Dimov wrote:
> Paul E. McKenney wrote:
> 
> >Sounds like we need to make sure that we should either define CAS to
> >have two modes of "success", or to get CAS to explicitly state
> >otherwise should it ever be added to ISO/IEC 9945.  Might be nice to
> >get the pthread_mutex_trylock() primitive to be added to the list of
> >"explicitly stated otherwise" primitives, but that doesn't sound like
> >a short-term project.  ;-)
> 
> I don't really understand the motivation to make code slower without 
> solving a practical problem. I could see some rationale for giving the 
> acquire CAS acquire semantics in the failure case (even though it still may 
> penalize code that doesn't need the synchronization), but for a trylock? A 
> failed trylock did not acquire the lock - pun intended and significant - so 
> you can't access the shared state without a data race; why do you need the 
> acquire semantics in this case? 

But given C++0x atomics, it -is- possible to access the shared state
without a data race if the shared state contains atomics.  And, again,
if trylock failure is frequent enough to cause performance problems,
you likely have other problems.

						Thanx, Paul