Re: [PATCH 1/23] document preferred use of volatile with atomic_t

[Chris Snook <csnook@redhat.com>]

Christoph Lameter wrote:
> On Tue, 14 Aug 2007, Chris Snook wrote:
>
> > > volatile means that there is some vague notion of "read it now". But that
> > > really does not exist. Instead we control visibility via barriers (smp_wmb,
> > > smp_rmb). Would it not be best to not have volatile at all in atomic
> > > operations and let the barriers do the work?
> > From my reply in the other thread...
> >
> > But barriers force a flush of *everything* in scope, which we generally don't
> > want.  On the other hand, we pretty much always want to flush atomic_*
> > operations.  One way or another, we should be restricting the volatile
> > behavior to the thing that needs it.  On most architectures, this patch set
> > just moves that from the declaration, where it is considered harmful, to the
> > use, where it is considered an occasional necessary evil.
> >
> > If you really, *really* distrust the compiler that much, you shouldn't be
> > using barrier, since that uses volatile under the hood too.  You should just
> > go ahead and implement the atomic operations in assembler, like Segher
> > Boessenkool did for powerpc in response to my previous patchset.
>
> From my reply on the other thread:
>
> Maybe we need two read functions? One volatile, one not?

If we're going to do this, and I don't think we need to, I'd prefer that 
atomic_read() be volatile, and something like atomic_read_opt() be non-volatile, 
to discourage premature optimization.

> The atomic_read()s that I have in slub really do not care about when the 
> variables are read. And if volatile creates overhead then I rather not have it.

A single volatile access is no more expensive than a non-volatile access.  It's 
when you have dependencies that you start to see overhead.  If you're doing a 
bunch of atomic operations on the same atomic_t in quick succession, then you 
will see some overhead.  Of course, if you're doing that, I think you have a 
design problem.

On modern, register-rich CPUs with cache latencies of a couple clock cycles, 
volatile generally isn't as much of a performance hit as it used to be.  I think 
that going out of your way to avoid it would be premature optimization on modern 
hardware.

	-- Chris
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