how to cast away 'volatile'?

[Ian Lance Taylor]

Sergei Organov <osv@javad.com> writes:

> Ian Lance Taylor <iant@google.com> writes:
> [...]
> > And once you are using a mutex, there is no reason to use volatile.
> > Using volatile will give you no extra protection, it will merely cost
> > you some efficiency.
> 
> Yes, that's how it is intended to work. However practice may vary. Some
> time ago there was an interesting example posted in a newsgroup[1].
> 
> The initial program was:
> 
> for (...) {
>         if (...) pthread_mutex_lock(...);
>         x = ... x ...
>         if (...) pthread_mutex_unlock(...);
> }
> 
> and compiler did the following transformation that breaks thread-safety
> ('r' is register):
> 
> r = x;
> for (...) {
>         if (...) { x = r; pthread_mutex_lock(...); r = x; }
>         r = ... r ...
>         if (...) { x = r; pthread_mutex_unlock(...); r = x; }
> }
> x = r;
> 
> This transformation seems to be OK both from the C language POV and from
> pthread standard POV (though the latter obviously was not intentional),
> but it creates accesses to 'x' outside of the mutex-protected section
> that in turn breaks thread-safety.
> 
> Making 'x' volatile would prevent such an optimization in practice,
> isn't it?

That is an interesting example.  The question is whether declaring x
volatile would prevent the compiler from moving a load or store to x
across the call to pthread_mutex_lock or pthread_mutex_unlock.  I
guess I agree that it would.  x must have the correct value when the
functions are called.  The use of volatile would ensure that x can
only be read and written once.  So there is no way to move the load or
store.

Interesting.  Thanks.

Ian