[PATCH] Document Linux's memory barriers [try #2]

[Alan Cox]

On Wed, Mar 08, 2006 at 06:35:07PM +0000, David Howells wrote:
> Alan Cox <alan at redhat.com> wrote:
> 
> > spin_unlock ensures that local CPU writes before the lock are visible
> > to all processors before the lock is dropped but it has no effect on 
> > I/O ordering. Just a need for clarity.
> 
> So I can't use spinlocks in my driver to make sure two different CPUs don't
> interfere with each other when trying to communicate with a device because the
> spinlocks don't guarantee that I/O operations will stay in effect within the
> locking section?

If you have 

CPU #0

	spin_lock(&foo->lock)
	writel(0, &foo->regnum)
	writel(1, &foo->data);
	spin_unlock(&foo->lock);

					CPU #1
						spin_lock(&foo->lock);
						writel(4, &foo->regnum);
						writel(5, &foo->data);
						spin_unlock(&foo->lock);

then on some NUMA infrastructures the order may not be as you expect. The
CPU will execute writel 0, writel 1 and the second CPU later will execute
writel 4 writel 5, but the order they hit the PCI bridge may not be the
same order. Usually such things don't matter but in a register windowed
case getting 0/4/1/5 might be rather unfortunate.

See Documentation/DocBook/deviceiobook.tmpl (or its output)

The following case is safe

	spin_lock(&foo->lock);
	writel(0, &foo->regnum);
	reg = readl(&foo->data);
	spin_unlock(&foo->lock);

as the real must complete and it forces the write to complete. The pure write
case used above should be implemented as

	spin_lock(&foo->lock);
	writel(0, &foo->regnum);
	writel(1, &foo->data);
	mmiowb();
	spin_unlock(&foo->lock);

The mmiowb ensures that the writels will occur before the writel from another
CPU then taking the lock and issuing a writel.

Welcome to the wonderful world of NUMA

Alan