linux-kernel - Re: [PATCH 1/23] document preferred use of volatile with atomic

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Message-ID: <Pine.LNX.4.64.0708141555300.579@schroedinger.engr.sgi.com>
Date:	Tue, 14 Aug 2007 15:56:51 -0700 (PDT)
From:	Christoph Lameter <clameter@....com>
To:	Chris Snook <csnook@...hat.com>
cc:	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	akpm@...ux-foundation.org, paulmck@...ux.vnet.ibm.com,
	Segher Boessenkool <segher@...nel.crashing.org>,
	"Luck, Tony" <tony.luck@...el.com>,
	Chris Friesen <cfriesen@...tel.com>,
	"Robert P. J. Day" <rpjday@...dspring.com>
Subject: Re: [PATCH 1/23] document preferred use of volatile with atomic_t

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?

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.

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