[Python-Dev] Threading idea -- exposing a global thread lock (original) (raw)

Josiah Carlson jcarlson at uci.edu
Tue Mar 14 17:58:10 CET 2006

Samuele Pedroni <pedronis at strakt.com> wrote:

Raymond Hettinger wrote: > [Samuele Pedroni] > >> there's no sys.checkinterval in Jython. Implementing this would need the >> introduction of some kind of GIL implementation in Jython, the JVM >> has no primitive for global critical sections. > > > Wouldn't Java implement this directly by suspending and resuming the > other threads (being careful to avoid access to monitored resources > and to pair the suspend/resume operations in a try/finally or > with-statement to prevent deadlocks)? suspending a thread is a deprecated operation because it can cause deadlocks.

There are two assumptions that one can make about code using the "gil", or the equivalent of suspending all threads but the current one, or in Python, just disabling thread switching; I'll call it a (global) 'critical section'.

Either the user is going to rely on just the critical section for locking, or the user is going to mix locks too. If the user doesn't mix (even implicitly with Queue, etc.), then there can be no deadlocks caused by the critical section. If the user is mixing standard locks with critical sections, the only new potential cause of deadlocks is if the user attempts to acquire locks within the critical section which have already been acquired by another thread. Deadlocks of this particular type, however, can be generally prevented by making locks aware of critical sections and raising an exception whenever a lock acquisition is taking place within a critical section. You wouldn't want the exception to only be raised if the acquisition would block, as this would result in intermittant errors; just make it an error.

It would be nice if Jython or IronPython could (and would) implement these 'critical sections'. Whether they can or not, I think that it would be a useful feature in the CPython runtime. It could be considered a platform-specific feature, similar to how you can use select on any file handle on *nix, but you need to jump through hoops to get a similar thing on Windows.

I'm +1, but only because I've spent more than my share of time digging around with threads, locks, Rlocks, conditions, events, etc.

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