On Sep 19, 2006, at 11:03 AM, Anthony Williams wrote:
You've already seen my comments on "convertible shared ownership" (from N2094) over on the committee reflector, but for everyone else: I think "convertible shared ownership" is a bad idea, as it raises the potential for deadlocks, and given exclusive ownership, shared ownership, and upgradable ownership, "convertible shared ownership" is not required.
One of the points I keep forgetting to raise on this subject: convertible_shared_mutex and upgradable_mutex represent two engineering tradeoffs. One can not have a "perfect upgradable mutex" because there is inherently no perfect design. It is a compromise. upgradable_mutex represents the case where one *must* be able to convert from shared locking to exclusive locking. But it gives up concurrency to do so: though upgradable ownership can share with sharable ownership, it can't share with upgradable ownership. convertible_shared_mutex represents the opposite compromise: You loose nothing in concurrency, but you do loose the ability of knowing that you can definitely atomically upgrade to an exclusive lock. You might be able to, and you might have to settle for a non-atomic upgrade to exclusive. I believe that applications exist which favor both tradeoffs. Thus neither is a complete substitute for the other. -Howard