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

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Message-Id: <F5A2B8BD-CD28-44EE-9DB5-5E776279100F@mac.com>
Date:	Mon, 10 Sep 2007 15:59:06 -0400
From:	Kyle Moffett <mrmacman_g4@....com>
To:	Denys Vlasenko <vda.linux@...glemail.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Nick Piggin <piggin@...erone.com.au>,
	Satyam Sharma <satyam@...radead.org>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Paul Mackerras <paulus@...ba.org>,
	Christoph Lameter <clameter@....com>,
	Chris Snook <csnook@...hat.com>,
	Ilpo Jarvinen <ilpo.jarvinen@...sinki.fi>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Stefan Richter <stefanr@...6.in-berlin.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-arch@...r.kernel.org, Netdev <netdev@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>, ak@...e.de,
	heiko.carstens@...ibm.com, David Miller <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 Sep 10, 2007, at 12:46:33, Denys Vlasenko wrote:
> My point is that people are confused as to what atomic_read()   
> exactly means, and this is bad. Same for cpu_relax().  First one  
> says "read", and second one doesn't say "barrier".

Q&A:

Q:  When is it OK to use atomic_read()?
A:  You are asking the question, so never.

Q:  But I need to check the value of the atomic at this point in time...
A:  Your code is buggy if it needs to do that on an atomic_t for  
anything other than debugging or optimization.  Use either  
atomic_*_return() or a lock and some normal integers.

Q:  "So why can't the atomic_read DTRT magically?"
A:  Because "the right thing" depends on the situation and is usually  
best done with something other than atomic_t.

If somebody can post some non-buggy code which is correctly using  
atomic_read() *and* depends on the compiler generating extra  
nonsensical loads due to "volatile" then the issue *might* be  
reconsidered.  This also includes samples of code which uses  
atomic_read() and needs memory barriers (so that we can fix the buggy  
code, not so we can change atomic_read()).  So far the only code  
samples anybody has posted are buggy regardless of whether or not the  
value and/or accessors are flagged "volatile" or not.  And hey, maybe  
the volatile ops *should* be implemented in inline ASM for future- 
proof-ness, but that's a separate issue.

Cheers,
Kyle Moffett

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