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

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Date:	Thu, 16 Aug 2007 18:35:47 +0800
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	Stefan Richter <stefanr@...6.in-berlin.de>
Cc:	Paul Mackerras <paulus@...ba.org>,
	Satyam Sharma <satyam@...radead.org>,
	Christoph Lameter <clameter@....com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	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 11:54:44AM +0200, Stefan Richter wrote:
> 
> One example was discussed here earlier:  The for (;;) loop in
> nodemgr_host_thread.  There an msleep_interruptible implicitly acted as
> barrier (at the moment because it's in a different translation unit; if
> it were the same, then because it hopefully has own barriers).  So that
> happens to work, although such an implicit barrier is bad style:  Better
> enforce the desired behaviour (== guaranteed load operation) *explicitly*.

Hmm, it's not bad style at all.  Let's assume that everything
is in the same scope.  Such a loop must either call a function
that busy-waits, which should always have a cpu_relax or
something equivalent, or it'll call a function that schedules
away which immediately invalidates any values the compiler might
have cached for the atomic_read.

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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