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

Steven Rostedt
2008-11-07T21:05:43Z

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.

-- Steve