On Wednesday, Jun 18, 2003, at 01:17 Europe/Amsterdam, Greg Ewing wrote:
Note that this same problem turns up with Greg's "synchronised class" idea: if the language makes locking easy people will overuse it and it will come back to bite you
I don't think that should be used as a reason not to provide higher-level facilities such as synchronised classes. It just means we need to keep clearly in mind that they're not a magical solution that enables people to write threaded code without understanding the issues.
I had a half-baked idea yesterday that I'd like to dump here. I haven't thought it over, so it's probably bogus anyway, but still here goes:-)
The main deadlock problem is acquiring a second lock while you already hold a lock. But completely disallowing this in the runtime system is overly restrictive. So, what we need is knowledge about which locks we can acquire and which ones we can't. In other words, if there is a statement synchronize lock: code then while inside "code" you will not be allowed to do any more synchronize statements. (I'm re-using the synchronize statement here, but the object is really a lock, not a general object, and I'm not interested in syntax right now. Also, this idea would work as well, or maybe even better, with Greg's synchronized class scheme). Whether an attempt to do so results in an error or warning I'm not sure.
If the author of the code has investigated interaction between "lock", "otherlock" and "thirdlock" and believes the interaction is safe then you use the form synchronize lock allowing otherlock, thirdlock: code and "code" will be allowed to synchronize on the other two locks.
This scheme puts the annotation at the outer lock, an alternative would be to put it at the inner lock. That has its advantages too, because if you know your code does not block nor acquire locks you can say something like "synchronize lock allow_holding *". A third alternative would be to completely decouple the annotation from the locking code, with a statement that says "Locks lock, otherlock and thirdlock are mutually safe". -- Jack Jansen, Jack.Jansen@cwi.nl, http://www.cwi.nl/%7Ejack If I can't dance I don't want to be part of your revolution -- Emma Goldman