Date: Fri, 17 Aug 2007 12:49:24 -0700
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Arjan van de Ven <arjan@infradead.org>
Cc: Chris Friesen <cfriesen@nortel.com>,
       Linus Torvalds <torvalds@linux-foundation.org>,
       Nick Piggin <piggin@cyberone.com.au>,
       Satyam Sharma <satyam@infradead.org>,
       Herbert Xu <herbert@gondor.apana.org.au>,
       Paul Mackerras <paulus@samba.org>, Christoph Lameter <clameter@sgi.com>,
       Chris Snook <csnook@redhat.com>,
       Ilpo Jarvinen <ilpo.jarvinen@helsinki.fi>,
       Stefan Richter <stefanr@s5r6.in-berlin.de>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
       linux-arch@vger.kernel.org, Netdev <netdev@vger.kernel.org>,
       Andrew Morton <akpm@linux-foundation.org>, ak@suse.de,
       heiko.carstens@de.ibm.com, David Miller <davem@davemloft.net>,
       schwidefsky@de.ibm.com, wensong@linux-vs.org, horms@verge.net.au,
       wjiang@resilience.com, zlynx@acm.org, rpjday@mindspring.com,
       jesper.juhl@gmail.com, segher@kernel.crashing.org
Subject: Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures
Message-ID: <20070817194924.GG8464@linux.vnet.ibm.com>
Reply-To: paulmck@linux.vnet.ibm.com
References: <20070817035342.GA14744@gondor.apana.org.au> <alpine.LFD.0.999.0708171142550.3666@enigma.security.iitk.ac.in> <46C55E90.7010407@yahoo.com.au> <alpine.LFD.0.999.0708171431150.3666@enigma.security.iitk.ac.in> <46C56ADF.8010501@cyberone.com.au> <alpine.LFD.0.999.0708171612440.3666@enigma.security.iitk.ac.in> <46C59717.4020108@cyberone.com.au> <alpine.LFD.0.999.0708170929580.30176@woody.linux-foundation.org> <46C5EDF9.3090507@nortel.com> <1187376873.2615.2.camel@laptopd505.fenrus.org>
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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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