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

Duncan Booth duncan at NOSPAMrcp.co.uk
Mon Aug 5 12:09:28 CEST 2002


a-steinhoff at web.de (Armin Steinhoff) wrote in
news:ddc19db7.0208050112.56a26827 at posting.google.com: 
>>      CHECK_STATUS("pthread_mutex_lock[1]");
>>      success = thelock->locked == 0;
>>      if (success) thelock->locked = 1;
> 
> 'success' will always be 0 ... so why 'if(success)' ??????
> We are in a critical section !!!

success will be 1 if another thread had previously acquired this lock and 
not yet released it.


> The while loop is plain nonsens and dangerous because 'thelock->mut'
> isn't acquired again!!

Have you read the documentation for pthread_cond_wait? It acquires the 
lock.

> 
>>           while ( thelock->locked ) {
>>                status = pthread_cond_wait(&thelock->lock_released,
>>                                  &thelock->mut);
>>                CHECK_STATUS("pthread_cond_wait");
> 
> After calling pthread_cond_wait() the mutex 'thelock->mut' will be
> released!

Just as well, since it has just been acquired.

> It should  be possible to implement the GIL by a single condition
> variable!

How do you expect a thread to block on a simple variable? How do you expect 
to ensure that a variable is tested and updated atomically?


-- 
Duncan Booth                                             duncan at rcp.co.uk
int month(char *p){return(124864/((p[0]+p[1]-p[2]&0x1f)+1)%12)["\5\x8\3"
"\6\7\xb\1\x9\xa\2\0\4"];} // Who said my code was obscure?



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