Re: For review: pthread_cleanup_push.3

[Loic Domaigne <tech-Z4JMKDdsf89Wk0Htik3J/w@public.gmane.org>]

Gidday Michael,

IIRC, I had an issue left:

>>> I see a major deficiency in your code. Unless I am mistaken, the global
>>> variable <done> and <cleanup_pop_arg> are accessed from two different
>>> threads.
>>
>> True.
>>
>>> Following the POSIX memory model, you need mutex to synchronize the
>>> visibility.
>>
>> Please educate me about the POSIX memory model ;-).  Say some more
>> here please.  Are you meaning that the change made in main are not
>> guaranteed to visible in the other thread, unless I use a
>> synchronization mechanism?  (or, perhaps, a barrier?)
> 
> Oh, that's a long story. Tomorrow perhaps (I just came home, and I guess 
> Antje would like to spend some time with me...)

Please refer to St Butenhof's bible "Programming with POSIX Threads", 
§3.4, pp88-95.

An excerpt can be found at:
http://markmail.org/message/24zdwjjkbe4dqz5o

Basically:

1) the values you write prior to pthread_create(3) can be seen in the 
new thread.

2) the values you write prior to mutex_unlock(3) can be seen in any 
thread that locks later the same mutex.

3) the values you write prior to terminating a thread can be seen in the 
thread that joins.

4) the values you write prior to pthread_cond_{signal,broadcast}(3) can 
be seen by any thread that is awakened.

Failing to comply these rules may cause your program to fail in very 
subtle ways on multi-processors machine. Even if I may save here and 
there a mutex; I apply those rules religiously (but, yeah, I am a 
Pthreads fanatic ;-)

HTH,
Loïc.
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