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

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Message-ID: <20070816011414.GC9645@linux.vnet.ibm.com>
Date:	Wed, 15 Aug 2007 18:14:14 -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 05:59:41PM -0700, Christoph Lameter wrote:
> On Wed, 15 Aug 2007, Paul E. McKenney wrote:
> 
> > The volatile cast should not disable all that many optimizations,
> > for example, it is much less hurtful than barrier().  Furthermore,
> > the main optimizations disabled (pulling atomic_read() and atomic_set()
> > out of loops) really do need to be disabled.
> 
> In many cases you do not need a barrier. Having volatile there *will* 
> impact optimization because the compiler cannot use a register that may 
> contain the value that was fetched earlier. And the compiler cannot choose 
> freely when to fetch the value. The order of memory accesses are fixed if 
> you use volatile. If the variable is not volatile then the compiler can 
> arrange memory accesses any way they fit and thus generate better code.

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.

						Thanx, Paul
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