Re: [LAD] [somewhat OT] semaphores in python

On Tue, Aug 26, 2008 at 11:02:37PM +0200, Dominic Sacr� wrote: At first it would seem that solves the problem. But AFAICS it doesn't. A condition variable consists of - one or more state variables, on wich a condition is defined, - a mutex M to protect these variables from concurrent acceess, - a binary sema S (or equivalent) on which to wait until the condition is satisfied. To make all of this work correctly in all conditions, the essential part is that one can do [wait (S), release (M)] atomically. A counting sema is just one special case of this, the condition being _count > 0, where _count is an integer and the only state variable. The lock provided by the built-in thread module can play the role of both M and S, and that is indeed how Condition in threading is implemented (it's a bit more complicated, supporting multiple waiters, but that doesn't change the basic way it works). BUT: the python interpreter will release the GIL every so many bytecode instructions. There is AFAICS nothing that would prevent it from doing this in between the two operations that have to be atomic - it doesn't know they have to be.