Re: [boost] [FSM] Review library version update

From: Steven Watanabe (steven_at_[hidden])
Date: 2007-02-05 17:04:48

AMDG

Andreas Huber <ahd6974-spamgroupstrap <at> yahoo.com> writes:

> 
> As Andrei Alexandrescu explains in the following article:
> 
> http://www.ddj.com/dept/cpp/184403766
> 
> a volatile-declared variable of primitive type only hinders the compiler 
> to cache its value in a processor register. It doesn't prevent any race 
> conditions in an MT application. In your code one thread can be in the 
> middle of executing init() when another thread evaluates the condition 
> in the if statement. Clearly, this will lead to two threads calling 
> init(), possibly even at the same time.

The volatile is necessary to prevent the
compiler from reordering the statements.
Because init just does assignment of PODs
it is ok if it is executed more than once.
It is only a problem if executing
    x = y;
in two threads at the same time does
not work.

> 
> Even if you rewrite the code as follows
> 
>        // above same as before
>        if(!initialized) {
>            initialized = true;
>            const_cast<volatile Dispatcher&>(dispatcher).init();
>        }
>        // below same as before
> 
> there's still a race condition. Two threads could evaluate initialized 
> at the same time... All this even ignores weaker memory models of SMP 
> machines where changes to initialized in one processor are not even 
> visible to threads running on another processor unless you use a memory 
> barrier.
> 

This definitely bad. One thread could set
the condition and another could then use
dispatcher before it is initialized.

In Christ,
Steven Watanabe