Re: [RFC patch 08/18] cnt32_to_63 should use smp_rmb()

Steven Rostedt
2008-11-08T00:55:39Z

On Fri, 7 Nov 2008, David Howells wrote:

> Steven Rostedt <rostedt@goodmis.org> wrote:
>
> > > I use smp_rmb() to do this on SMP systems (hrm, actually, a rmb() could
> > > be required so it works also on UP systems safely wrt interrupts).
> >
> > smp_rmb turns into a compiler barrier on UP and should prevent the below
> > description.
>
> Note that that does not guarantee that the two reads will be done in the order
> you want. The compiler barrier _only_ affects the compiler. It does not stop
> the CPU from doing the reads in any order it wants. You need something
> stronger than smp_rmb() if you need the reads to be so ordered.

For reading hardware devices that can indeed be correct. But for normal
memory access on a uniprocessor, if the CPU were to reorder the reads that
would effect the actual algorithm then that CPU is broken.

read a
<--- interrupt - should see read a here before read b is done.
read b

Now the fact that one of the reads is a hardware clock, then this
statement might not be too strong. But the fact that it is a clock, and
not some memory mapped device register, I still think smp_rmb is
sufficient.

-- Steve