[dmd-concurrency] CSP: Communicating sequential processes

[Michel Fortin]

Le 2010-01-20 à 10:57, Robert Jacques a écrit :

>>> I think read/write lock usage should only be used explicitly.
>>> 
>>> That's because you can never upgrade "safely" from a read lock to a write lock, or to be precise there is always a risk that you must release your read lock before upgrading. Two threads could acquire a read lock and then both might want to upgrade to a write lock, which can't be satisfied without one of them dropping its read lock first.
>>> 
>>> If the read lock drops because you call another function that wants a write lock, this could be trouble. Upgrades work well when there is only one thread requesting upgrades. Otherwise the upgrade must be seen as a drop-read-lock then acquire-write-lock non-atomic operation.
>>> 
>> 
>> Ah, the composability problem. Thanks for explaining.
> 
> But, since the reader lock would only be called from const functions, there would never be a situation where you'd want to upgrade a reader lock to a writer lock. The only issue would be writer locks trying to take a reader or a second writer lock. Or am I missing something?

A const function only makes the "this" reference const. Accessing the same objects through other references (such as a global or an argument) could cause non-const functions to be called on that same object, which could require a write lock even when calling a const function.

Pure functions are safe of course, but that's too restricting to be really useful. 

-- 
Michel Fortin
michel.fortin at michelf.com
http://michelf.com/