Re: Question about C language.

[Chris Snook <csnook@redhat.com>]

Francis Moreau wrote:
> Hello,
> 
> I know it's a bit out of topic but this is something I need to clarify for
> writing a Linux driver... hope you don't mind.
> 
> In my driver I have a global variable that controls a loop such as:
> 
> int my_condition;
> 
> void change_my_condition(int new)
> {
>     my_condition = new;
> }
> 
> int foo(void)
> {
>     /* irqs are disabled */
>     my_condition = 1;
>     do {
>         ....
>         local_irq_enable();
>         cpu_sleep();
>         local_irq_disable();
> 
>    } while (my_condition);
> 
> }
> 
> This variable is modified by an interrupt handler define in another file
> by using 'change_my_condition' function.
> 
> By reading the ISO C99 specification, I _think_ that I needn't any
> kind of barrier
> or even use the volatile type qualifier for my_condition variable to make a true
> access to 'my_condition' in the controlling expression of the while, but I'm not
> sure.
> 
> Coud anybody confirm ?
> 
> Thanks,

Even volatile may be insufficient with some architecture/compiler 
combinations.  You should use explicit barriers wherever you need them, 
or Bad Things will happen.

-- Chris