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

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Message-ID: <20070816235902.GB11594@gondor.apana.org.au>
Date:	Fri, 17 Aug 2007 07:59:02 +0800
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Stefan Richter <stefanr@...6.in-berlin.de>,
	Paul Mackerras <paulus@...ba.org>,
	Satyam Sharma <satyam@...radead.org>,
	Christoph Lameter <clameter@....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 09:34:41AM -0700, Paul E. McKenney wrote:
>
> The compiler can also reorder non-volatile accesses.  For an example
> patch that cares about this, please see:
> 
> 	http://lkml.org/lkml/2007/8/7/280
> 
> This patch uses an ORDERED_WRT_IRQ() in rcu_read_lock() and
> rcu_read_unlock() to ensure that accesses aren't reordered with respect
> to interrupt handlers and NMIs/SMIs running on that same CPU.

Good, finally we have some code to discuss (even though it's
not actually in the kernel yet).

First of all, I think this illustrates that what you want
here has nothing to do with atomic ops.  The ORDERED_WRT_IRQ
macro occurs a lot more times in your patch than atomic
reads/sets.  So *assuming* that it was necessary at all,
then having an ordered variant of the atomic_read/atomic_set
ops could do just as well.

However, I still don't know which atomic_read/atomic_set in
your patch would be broken if there were no volatile.  Could
you please point them out?

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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