Subj : Re: Memory visibility with Pthreads
To   : comp.programming.threads
From : Maciej Sobczak
Date : Thu Aug 11 2005 10:02 am

David Schwartz wrote:

>>int A = 0;
>>int B = 0;
>>
>>Thread 1 executes the following code:
>>
>>A = 10;
>>pthread_mutex_lock(mutex);
>>B = 10;
>>pthread_mutex_unlock(mutex);
>>
>>Thread 2 executes another codesequence:
>>
>>pthread_mutex_lock(mutex);
>>if (B == 10) {
>> printf("The value of A is %d\n", A);
>>} else {
>> printf("B is not 10\n");
>>}
>>pthread_mutex_unlock(mutex);
>>
>>One question is, whether it is legal to use the variable A in thread 2? 
>>The
>>other question is, whether one can be sure that the value is always 10 
>>when
>>the if condition is true?
> 
>     Yes, the mutex functions are defined by POSIX to fully synchronize 
> memory.

Yes, but isn't it OK for the platform (compiler+CPU+memory+...) to 
reorder actions performed by thread 1 so that A = 10; is done after 
unlock? Something like this:

lock
B = 10;
unlock
A = 10;

In other discussions about this issue it was usually stressed that 
writes cannot be moved *from inside to outside* of the critical section 
embraced by lock+unlock. I got an impression that it is always OK for 
the platform to move "outer" writes *into* the section, which here would 
mean this:

lock
A = 10;
B = 10;
unlock

Is it allowed?

But is it allowed also to reorder the critical section with regard to 
other outer writes? If yes, then the answer to the OP's question is no - 
there is nothing that guarantees anything about the value of A when it's 
read in thread 2. But if it is not allowed to move A=10; from before 
lock to after unlock, then it means that lock and unlock are not really 
in pairs with regard to memory visibility and each of them, even in 
separation, provides some guarantees that can be relied upon (in the 
OP's case, the assumption is that if lock2 is after unlock1, then 
everything after lock2 sees the changes from before lock1). Is that true?


-- 
Maciej Sobczak : http://www.msobczak.com/
Programming    : http://www.msobczak.com/prog/

.