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

Paul E. McKenney
2008-11-08T00:34:37Z

On Fri, Nov 07, 2008 at 04:04:48PM -0500, Steven Rostedt wrote:
>
> On Fri, 7 Nov 2008, Paul E. McKenney wrote:
> > > > Would that make more sense ?
> > > >
> > >
> > > Oh, actually, I got things reversed in this email : the readl(io_addr)
> > > must be done _after_ the __m_cnt_hi read.
> > >
> > > Therefore, two consecutive executions would look like :
> > >
> > > barrier(); /* Make sure the compiler does not reorder __m_cnt_hi and
> > > previous mmio read. */
> > > read __m_cnt_hi
> > > smp_rmb(); /* Waits for every cached memory reads to complete */
> >
> > If these are MMIO reads, then you need rmb() rather than smp_rmb(),
> > at least on architectures that can reorder writes (Power, Itanium,
> > and I believe also ARM, ...).
>
> The read is from a clock source. The only writes that are happening is
> by the clock itself.
>
> On a UP system, is a rmb still needed? That is, can you have two reads on
> the same CPU from the clock source that will produce a backwards clock?
> That to me sounds like the clock interface is broken.

I do not believe that all CPUs are guaranteed to execute a sequence
of MMIO reads in order.

Thanx, Paul