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

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Message-ID: <20070814232835.GH8243@linux.vnet.ibm.com>
Date:	Tue, 14 Aug 2007 16:28:35 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Christoph Lameter <clameter@....com>
Cc:	Chris Snook <csnook@...hat.com>, linux-arch@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	akpm@...ux-foundation.org,
	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, Aug 14, 2007 at 03:56:51PM -0700, 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?
> 
> 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.


The overhead due to volatile access is -way- small.  Not like barrier(),
which can flush out a fair fraction of the machine registers.

						Thanx, Paul
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