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

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Date:	Fri, 17 Aug 2007 13:43:27 +1000
From:	Paul Mackerras <paulus@...ba.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Christoph Lameter <clameter@....com>,
	Chris Snook <csnook@...hat.com>,
	Ilpo J?rvinen <ilpo.jarvinen@...sinki.fi>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Satyam Sharma <satyam@...radead.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	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, cfriesen@...tel.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

Linus Torvalds writes:

> In general, I'd *much* rather we used barriers. Anything that "depends" on 
> volatile is pretty much set up to be buggy. But I'm certainly also willing 
> to have that volatile inside "atomic_read/atomic_set()" if it avoids code 
> that would otherwise break - ie if it hides a bug.

The cost of doing so seems to me to be well down in the noise - 44
bytes of extra kernel text on a ppc64 G5 config, and I don't believe
the extra few cycles for the occasional extra load would be measurable
(they should all hit in the L1 dcache).  I don't mind if x86[-64] have
atomic_read/set be nonvolatile and find all the missing barriers, but
for now on powerpc, I think that not having to find those missing
barriers is worth the 0.00076% increase in kernel text size.

Paul.
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