Re: Question about C language.

[Chris Snook]

Chris Snook wrote:
> 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

Oops, forgot to mention, you should use atomic_t, to avoid aliasing 
problems, and ALSO use explicit barriers wherever you need them.

-- Chris
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