linux-kernel - Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures

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Message-ID: <20070817194924.GG8464@linux.vnet.ibm.com>
Date:	Fri, 17 Aug 2007 12:49:24 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	Chris Friesen <cfriesen@...tel.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Nick Piggin <piggin@...erone.com.au>,
	Satyam Sharma <satyam@...radead.org>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Paul Mackerras <paulus@...ba.org>,
	Christoph Lameter <clameter@....com>,
	Chris Snook <csnook@...hat.com>,
	Ilpo Jarvinen <ilpo.jarvinen@...sinki.fi>,
	Stefan Richter <stefanr@...6.in-berlin.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-arch@...r.kernel.org, Netdev <netdev@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>, ak@...e.de,
	heiko.carstens@...ibm.com, David Miller <davem@...emloft.net>,
	schwidefsky@...ibm.com, wensong@...ux-vs.org, horms@...ge.net.au,
	wjiang@...ilience.com, zlynx@....org, rpjday@...dspring.com,
	jesper.juhl@...il.com, segher@...nel.crashing.org
Subject: Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures

On Fri, Aug 17, 2007 at 11:54:33AM -0700, Arjan van de Ven wrote:
> 
> On Fri, 2007-08-17 at 12:50 -0600, Chris Friesen wrote:
> > Linus Torvalds wrote:
> > 
> > >  - in other words, the *only* possible meaning for "volatile" is a purely 
> > >    single-CPU meaning. And if you only have a single CPU involved in the 
> > >    process, the "volatile" is by definition pointless (because even 
> > >    without a volatile, the compiler is required to make the C code appear 
> > >    consistent as far as a single CPU is concerned).
> > 
> > I assume you mean "except for IO-related code and 'random' values like 
> > jiffies" as you mention later on?  I assume other values set in 
> > interrupt handlers would count as "random" from a volatility perspective?
> > 
> > > So anybody who argues for "volatile" fixing bugs is fundamentally 
> > > incorrect. It does NO SUCH THING. By arguing that, such people only show 
> > > that you have no idea what they are talking about.
> > 
> > What about reading values modified in interrupt handlers, as in your 
> > "random" case?  Or is this a bug where the user of atomic_read() is 
> > invalidly expecting a read each time it is called?
> 
> the interrupt handler case is an SMP case since you do not know
> beforehand what cpu your interrupt handler will run on.

With the exception of per-CPU variables, yes.

							Thanx, Paul
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