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

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Message-ID: <46C50228.4000004@redhat.com>
Date:	Thu, 16 Aug 2007 22:04:24 -0400
From:	Chris Snook <csnook@...hat.com>
To:	Herbert Xu <herbert.xu@...hat.com>
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 03:48:54PM -0400, Chris Snook wrote:
>>> 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.
> 
> Could you please cite the file/function names so we can
> see whether removing the barrier makes sense?
> 
> Thanks,

At a glance, several architectures' implementations of smp_call_function() have 
one or more legitimate atomic_read() busy-waits that shouldn't be using 
CPU-relax.  Some of them do work in the loop.

I'm sure there are plenty more examples that various maintainers could find in 
their own code.

	-- Chris
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