Subj : Re: double-checked locking in C
To   : comp.programming.threads
From : Laurent Deniau
Date : Wed Jul 06 2005 11:44 am

David Hopwood wrote:
> amorox@gmail.com wrote:
> 
>> Hi,
>>
>> I would like to use the DCL idiom (double-checked locking) to guarantee
>> the once and only once execution of an initialization code:
>>
>> bool initialized = false;
>> pthread_mutex_t mutex;
>>
>> void function(void)
>> {
>>    if (!initialized) {
>>       pthread_mutex_lock(&mutex);
>>       if (!initialized) {
>>          initialized = true;
>>          init_code();
>>       }
>>       pthread_mutex_unlock(&mutex);
>>    }
>> }
>>
>> I know that in Java there were problems with this idiom due to the
>> memory operation reorderings and these have been apparently solved in
>> Java 1.4.
> 
> 
> It has not been "solved"; it was not a goal of the new memory model to
> solve it. Just don't use this idiom in Java.
> 
>> Is there any problem using this idiom in a multithread C program? If
>> affirmative would you explained why?
> 
> 
> To a first approximation, anything that can go wrong with this idiom in
> Java can also go wrong in C. While it is possible to solve the problems
> by (nonportable, processor and compiler-specific) use of memory barriers,
> it really isn't worth the hassle to do this in an application.
> It might be worth it in a language implementation if you really know
> what you're doing.

There is a simple, portable, processor independent, compiler independent 
idiom which consists of delegating the work to an extern function, a 
function with an implementation in a different translation unit.

void function(void)
{
     if (!initialized) {
        pthread_mutex_lock(&mutex);
        if (!initialized)
           initialized = extern_init_code();
        pthread_mutex_unlock(&mutex);
     }
}

The compiler is not allowed to assign initialized *before* 
extern_init_code() returns. The processor will not be able to do the 
same since this is a true jump with a return value. The assignment could 
be delayed but not after the unlock, so it is not a problem.

a+, ld.

.