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Message-ID: <20070816053945.GB32442@gondor.apana.org.au>
Date: Thu, 16 Aug 2007 13:39:45 +0800
From: Herbert Xu <herbert@...dor.apana.org.au>
To: Paul Mackerras <paulus@...ba.org>
Cc: Satyam Sharma <satyam@...radead.org>,
Christoph Lameter <clameter@....com>,
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
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
Subject: Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures
On Thu, Aug 16, 2007 at 02:11:43PM +1000, Paul Mackerras wrote:
>
> The uses of atomic_read where one might want it to allow caching of
> the result seem to me to fall into 3 categories:
>
> 1. Places that are buggy because of a race arising from the way it's
> used.
>
> 2. Places where there is a race but it doesn't matter because we're
> doing some clever trick.
>
> 3. Places where there is some locking in place that eliminates any
> potential race.
Agreed.
> In case 1, adding volatile won't solve the race, of course, but it's
> hard to argue that we shouldn't do something because it will slow down
> buggy code. Case 2 is hopefully pretty rare and accompanied by large
> comment blocks, and in those cases caching the result of atomic_read
> explicitly in a local variable would probably make the code clearer.
> And in case 3 there is no reason to use atomic_t at all; we might as
> well just use an int.
Since adding volatile doesn't help any of the 3 cases, and
takes away optimisations from both 2 and 3, I wonder what
is the point of the addition after all?
Cheers,
--
Visit Openswan at http://www.openswan.org/
Email: Herbert Xu ~{PmV>HI~} <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
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