linux-kernel - Re: I/O memory barriers vs SMP memory barriers

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Date:	Mon, 26 Mar 2007 10:46:39 +0200
From:	Lennert Buytenhek <buytenh@...tstofly.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	David Howells <dhowells@...hat.com>,
	Catalin Marinas <catalin.marinas@....com>,
	ARM Linux Mailing List 
	<linux-arm-kernel@...ts.arm.linux.org.uk>,
	Dan Williams <dan.j.williams@...el.com>,
	linux-kernel@...r.kernel.org, torvalds@...l.org
Subject: Re: I/O memory barriers vs SMP memory barriers

On Sun, Mar 25, 2007 at 08:24:18PM -0700, Paul E. McKenney wrote:

> > > > > [ background: On ARM, SMP synchronisation does need barriers but device
> > > > >   synchronisation does not.  The question is that given this, whether
> > > > >   mb() and friends can be NOPs on ARM or not (i.e. whether mb() is
> > > > >   supposed to sync against other CPUs or not, or whether only smp_mb()
> > > > >   can be used for this.)  ]
> > > > 
> > > > Hmmmm...
> > > > 
> > > > [snip]
> > > 
> > > 3.	Orders memory accesses and device accesses, but not necessarily
> > > 	the union of the two -- mb(), rmb(), wmb().
> > 
> > If mb/rmb/wmb are required to order normal memory accesses, that means
> > that the change made in commit 9623b3732d11b0a18d9af3419f680d27ea24b014
> > to always define mb/rmb/wmb as barrier() on ARM systems was wrong.
> 
> This was on UP ARM systems, right?

No.

If you look at commit 9623b3732d11b0a18d9af3419f680d27ea24b014, you can
see that it defines mb/rmb/wmb as barrier() on both ARM UP and SMP systems.
The UP part is obviously fine, the SMP part is what is under debate here.

> Assuming that ARM CPUs respect the usual CPU-self-consistency
> semantics, and given the background that device accesses are ordered,
> then it might well be OK to have mb/rmb/wmb be barrier() on UP ARM
> systems.
> 
> Most likely not on SMP ARM systems, however.

Given the semantics above, mb/rmb/wmb can obviously be just barrier()s
on ARM UP systems.. I don't think anyone ever disagreed about that.

> > Does everybody agree on these semantics, though?  At least David
> > seems to think that mb/rmb/wmb aren't required to order normal
> > memory accesses against each other..
> 
> Not on UP.  On SMP, ordering is (almost certainly) required.

'almost certainly'?  That sounds like there is a possibility that it
wouldn't have to?  What does this depend on?

At least David and Catalin seem to disagree with the statement
that mb/rmb/wmb should order accesses from different CPUs.  And
memory-barriers.txt is pretty vague about this..
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