Re: [rfc][patch 3/3] x86: optimise barriers

[Nick Piggin <npiggin@suse.de>]

On Fri, Oct 12, 2007 at 11:12:13AM +0200, Jarek Poplawski wrote:
> On Fri, Oct 12, 2007 at 10:42:34AM +0200, Helge Hafting wrote:
> > Jarek Poplawski wrote:
> > >On 04-10-2007 07:23, Nick Piggin wrote:
> > >  
> > >>According to latest memory ordering specification documents from Intel and
> > >>AMD, both manufacturers are committed to in-order loads from cacheable 
> > >>memory
> > >>for the x86 architecture. Hence, smp_rmb() may be a simple barrier.
> > >>    
> > >...
> > >
> > >Great news!
> > >
> > >First it looks like a really great thing that it's revealed at last.
> > >But then... there is probably some confusion: did we have to use
> > >ineffective code for so long?
> > >  
> > You could have tried the optimization before, and
> > gotten better performance. But if without solid knowledge that
> > the optimization is _valid_, you risk having a kernel
> > that performs great but suffer the occational glitch and
> > therefore is unstable and crash the machine "now and then".
> > This sort of thing can't really be figured out by experimentation, because
> > the bad cases might happen only with some processors, some
> > combinations of memory/chipsets, or with some minimum
> > number of processors.  Such problems can be very hard
> > to find, especially considering that other plain bugs also
> > cause crashes.
> > 
> > Therefore, the "ineffective code" was used because it was
> > the only safe alternative. Now we know, so now we may optimize.
> 
> Sorry, I don't understand this logic at all. Since bad cases
> happen independently from any specifications and Intel doesn't
> take any legal responsibility for such information, it seems we
> should better still not optimize?

We already do in probably more critical and lible to be problematic
cases (notably, spin_unlock).

So unless there is reasonable information for us to believe this
will be a problem, IMO the best thing to do is stick with the
specs. Intel is pretty reasonable with documenting errata I think.

With memory barriers specifically, I'm sure we have many more bugs
in the kernel than AMD or Intel have in their chips ;)