Python threading (was: Re: global interpreter lock not working as it should)

Tim Peters tim.one at comcast.net
Tue Aug 6 00:04:38 CEST 2002


[Bengt Richter]
> ...
> My impression was that it requires the mutex to be locked when called,
> and unlocks it itself so as to wait unlocked, and then re-locks it for
> the waiter. I.e., inside pthread_cond_wait:
>   ...
>   pthread_mutex_unlock(mutex);
>   suspend_with_cancellation(self);
>   pthread_mutex_lock(mutex);
>   ...
>
> I guess the re-lock involves trying, though -- is that a tiny crack
> for Murphy to sneak through?

The implementation of pthread_cond_wait can't be that simple; it's required
to act "as if atomic":  a thread returning from that call *must* hold the
mutex, and it's pthread_cond_wait's job to guarantee that.

> Ah, I guess that's why there's that 'while' in PyThread_acquire_lock:
> 		...
> 		while ( thelock->locked ) {
> 			status = pthread_cond_wait(&thelock->lock_released,
> 						   &thelock->mut);
> 			CHECK_STATUS("pthread_cond_wait");
> 		}
> 		thelock->locked = 1;
> 		...

No, this isn't to guard against broken pthread_cond_wait implementations.
The POSIX spec for pthread_cond_signal allows that any number of threads may
be signaled; specific implementations may guarantee to wake up exactly one,
but that goes beyond what the phtread spec promises, so can't be relied on.
If the signal does wake more than one thread, all but one of them will
typically loop back to the pthread_cond_wait here.

This is all (including the loop) a perfectly vanilla use of the condvar
protocol, BTW.

> ...
> Thanks for your pointers. BTW, the source for the linuxthreads
> (or whatever pthread package is actually used) is (AFAICS) not
> included in the win32 python distribution.

No C source code is included in the Windows distro, beyond the .h files
extension modules may need.

> Might this be a good idea for cross-platform documentation purposes?

That's what the source distributino is for.  The typical Windows user has no
use for the C source code, and a few Windows users have kindly <wink> let it
be known that they think the installer is already quite large enough.





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