On Tue, Jun 13, 2023 at 08:15:28PM -0400, nick black wrote: > enjoying the updated edition! > > "The lock operation is more expensive than CAS because it > requires two atomic operations on the lock data structure, one > for acquisition and the other for release." > > this might be worth expanding upon. it's not immediately obvious > to me why the unlock would need to be atomic. the rough 2x > certainly jives with this explanation, and i intend to go look > at some source, but i'd think it would be more: > > atomic update > fence > non-atomic update > > you've presumably got threads attempting to lock with a CAS > looking for a sentinel representing unlocked, right? so if i > update back to that sentinel in multiple steps, they're going to > miss in media res, no? > > feel no obligation to respond, of course. In some cases, you can release a lock with a simple store instruction. But much depends on the specifics of the implementation of the lock. Thanx, Paul
