Optimization of conditional access to globals: thread-unsafe?

[Ross Smith]

Erik Trulsson wrote:
> On Sun, Oct 28, 2007 at 06:06:17PM -0000, Dave Korn wrote:
>>
>>   As far as I know, there is no separate 'pthreads' spec apart from what is
>> defined in the Threads section (2.9) of the SUS (http://tinyurl.com/2wdq2u)
>> and what it says about the various pthread_ functions in the system interfaces
>> (http://tinyurl.com/2r7c5k) chapter.  None of that, as far as I have been able
>> to determine, makes any kind of claims about access to shared state or the use
>> of volatile.
> 
> Having just been pointed to that copy of the SUS, I must agree.  I can't
> find anything in there saying anything at all about what is required to
> safely share data between threads.  If that is really so it seems 'pthreads'
> are even more under-specified than I thought (and I had fairly low
> expectations in that regard.)
> I really hope there is something I have missed.

I think the relevant part is here:
http://www.opengroup.org/onlinepubs/009695399/basedefs/xbd_chap04.html#tag_04_10

[begin quote]

4.10 Memory Synchronization

Applications shall ensure that access to any memory location by more 
than one thread of control (threads or processes) is restricted such 
that no thread of control can read or modify a memory location while 
another thread of control may be modifying it. Such access is restricted 
using functions that synchronize thread execution and also synchronize 
memory with respect to other threads. The following functions 
synchronize memory with respect to other threads:

fork()
pthread_barrier_wait()
pthread_cond_broadcast()
pthread_cond_signal()
pthread_cond_timedwait()
pthread_cond_wait()
pthread_create()
pthread_join()
pthread_mutex_lock()
pthread_mutex_timedlock()
pthread_mutex_trylock()
pthread_mutex_unlock()
pthread_spin_lock()
pthread_spin_trylock()
pthread_spin_unlock()
pthread_rwlock_rdlock()
pthread_rwlock_timedrdlock()
pthread_rwlock_timedwrlock()
pthread_rwlock_tryrdlock()
pthread_rwlock_trywrlock()
pthread_rwlock_unlock()
pthread_rwlock_wrlock()
sem_post()
sem_trywait()
sem_wait()
wait()
waitpid()

The pthread_once() function shall synchronize memory for the first call 
in each thread for a given pthread_once_t object.

Unless explicitly stated otherwise, if one of the above functions 
returns an error, it is unspecified whether the invocation causes memory 
to be synchronized.

Applications may allow more than one thread of control to read a memory 
location simultaneously.

[end quote]


-- Ross Smith