python threads and the linuxthread pthread library

[anton wilson]

Re: python threads and the linuxthread pthread library
From: anton wilson <anton.wilson at camotion.com>
 To: Tim Peters <tim.one at comcast.net>
 Date: Mon, 1 Jul 2002 16:52:12 -0400
 
> > How do threads aquire the global interpreter lock once they are awoken?
>
> I'm not sure what you mean by "awoken".  Python doesn't normally put
> threads to sleep <wink>.  Threads that don't hold the GIL are normally
> blocked trying to acquire the GIL.  If by "awoken" you mean "become
> unblocked", then awakening and acquiring the GIL are exactly the same thing
> -- one doesn't precede the other.  The details of how the GIL gets acquired
> live inside your platform thread implementation.  On Linux, Python may use
> either pthread condition variables, or POSIX semaphores, to implement the
> GIL.


I think the reason why I'm confused here is that when the 
PyThread_release_lock call ias executed, it simply locks the global int lock, 
changes the locked variable to 0, unlocks the global interpreter lock, and 
then signals a thread waiting on the unlocked mutex condition to wake up. I 
can't find where a thread that gets swapped out by the interpreter blocks 
when it tries to require the lock again. 

Where does a swapped out thread block?

>From thread_pthread.h:

       status = pthread_mutex_lock( &thelock->mut );
        CHECK_STATUS("pthread_mutex_lock[3]");

        thelock->locked = 0;

        status = pthread_mutex_unlock( &thelock->mut );
        CHECK_STATUS("pthread_mutex_unlock[3]");

        /* wake up someone (anyone, if any) waiting on the lock */
        status = pthread_cond_signal( &thelock->lock_released );
        CHECK_STATUS("pthread_cond_signal");