Re: [PATCH 6/24] make atomic_read() behave consistently on frv

Paul E. McKenney
2007-08-13T06:03:24Z

On Mon, Aug 13, 2007 at 01:15:52PM +0800, Herbert Xu wrote:
> Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:
> > On Sat, Aug 11, 2007 at 08:54:46AM +0800, Herbert Xu wrote:
> >> Chris Snook <csnook@redhat.com> wrote:
> >> >
> >> > cpu_relax() contains a barrier, so it should do the right thing. For
> >> > non-smp architectures, I'm concerned about interacting with interrupt
> >> > handlers. Some drivers do use atomic_* operations.
> >>
> >> What problems with interrupt handlers? Access to int/long must
> >> be atomic or we're in big trouble anyway.
> >
> > Reordering due to compiler optimizations. CPU reordering does not
> > affect interactions with interrupt handlers on a given CPU, but
> > reordering due to compiler code-movement optimization does. Since
> > volatile can in some cases suppress code-movement optimizations,
> > it can affect interactions with interrupt handlers.
>
> If such reordering matters, then you should use one of the
> *mb macros or barrier() rather than relying on possibly
> hidden volatile cast.

If communicating among CPUs, sure. However, when communicating between
mainline and interrupt/NMI handlers on the same CPU, the barrier() and
most expecially the *mb() macros are gross overkill. So there really
truly is a place for volatile -- not a large place, to be sure, but a
place nonetheless.

Thanx, Paul