Matt Hurd wrote:
Have a load_load, load_store, store_store, store_load primitive set of functions for memory barriers [...]
The "unidirectional label" model proposed by Alexander Terekhov is better.
I think we need to assume that lock/unlock synchronisation of a non-null mutex is a full barrier. Is that correct?
No, lock+unlock is not a full barrier, although it imposes ordering with respect to another lock+unlock on the same mutex.
[...] This will become increasingly important in developing, so called, lock free methods. I call them "so called" as they usually require memory model guarantees through barriers or atomic ops, such as CAS, and these can be very expensive on some architectures. Lock free algorithms are often more complicated and if the non-locking concurrency costs are sufficiently high the benefits can quickly dissipate. For example, locking operations on an intel P4 are expensive. On an Athlon and ia64 they are considerably cheaper by more than a factor of two.
The canonical definition of "lock free" is that the system as a whole always makes progress. An ordinary lock-based algorithm does not have this property; if the thread that obtained the lock suddenly dies, no other thread can move past that lock.