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Message-ID: <46C4AA26.4060707@redhat.com>
Date: Thu, 16 Aug 2007 15:48:54 -0400
From: Chris Snook <csnook@...hat.com>
To: Herbert Xu <herbert@...dor.apana.org.au>
CC: Stefan Richter <stefanr@...6.in-berlin.de>,
Paul Mackerras <paulus@...ba.org>,
Satyam Sharma <satyam@...radead.org>,
Christoph Lameter <clameter@....com>,
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.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
Herbert Xu wrote:
> On Thu, Aug 16, 2007 at 10:06:31AM +0200, Stefan Richter wrote:
>>> Do you (or anyone else for that matter) have an example of this?
>> The only code I somewhat know, the ieee1394 subsystem, was perhaps
>> authored and is currently maintained with the expectation that each
>> occurrence of atomic_read actually results in a load operation, i.e. is
>> not optimized away. This means all atomic_t (bus generation, packet and
>> buffer refcounts, and some other state variables)* and likewise all
>> atomic bitops in that subsystem.
>
> Can you find an actual atomic_read code snippet there that is
> broken without the volatile modifier?
A whole bunch of atomic_read uses will be broken without the volatile
modifier once we start removing barriers that aren't needed if volatile
behavior is guaranteed.
barrier() clobbers all your registers. volatile atomic_read() only
clobbers one register, and more often than not it's a register you
wanted to clobber anyway.
-- Chris
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