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

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Message-ID: <20070816023255.GD14613@linux.vnet.ibm.com>
Date:	Wed, 15 Aug 2007 19:32:55 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Christoph Lameter <clameter@....com>
Cc:	Paul Mackerras <paulus@...ba.org>,
	Satyam Sharma <satyam@...radead.org>,
	Stefan Richter <stefanr@...6.in-berlin.de>,
	Chris Snook <csnook@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-arch@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	netdev@...r.kernel.org, Andrew Morton <akpm@...ux-foundation.org>,
	ak@...e.de, heiko.carstens@...ibm.com, davem@...emloft.net,
	schwidefsky@...ibm.com, wensong@...ux-vs.org, horms@...ge.net.au,
	wjiang@...ilience.com, cfriesen@...tel.com, zlynx@....org,
	rpjday@...dspring.com, jesper.juhl@...il.com,
	segher@...nel.crashing.org,
	Herbert Xu <herbert@...dor.apana.org.au>
Subject: Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures

On Wed, Aug 15, 2007 at 06:41:40PM -0700, Christoph Lameter wrote:
> On Wed, 15 Aug 2007, Paul E. McKenney wrote:
> 
> > Understood.  My point is not that the impact is precisely zero, but
> > rather that the impact on optimization is much less hurtful than the
> > problems that could arise otherwise, particularly as compilers become
> > more aggressive in their optimizations.
> 
> The problems arise because barriers are not used as required. Volatile 
> has wishy washy semantics and somehow marries memory barriers with data 
> access. It is clearer to separate the two. Conceptual cleanness usually 
> translates into better code. If one really wants the volatile then lets 
> make it explicit and use
> 
> 	atomic_read_volatile()

There are indeed architectures where you can cause gcc to emit memory
barriers in response to volatile.  I am assuming that we are -not-
making gcc do this.  Given this, then volatiles and memory barrier
instructions are orthogonal -- one controls the compiler, the other
controls the CPU.

						Thanx, Paul
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