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

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Date:	Thu, 16 Aug 2007 07:45:44 +0530 (IST)
From:	Satyam Sharma <satyam@...radead.org>
To:	Christoph Lameter <clameter@....com>
cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Paul Mackerras <paulus@...ba.org>,
	Stefan Richter <stefanr@...6.in-berlin.de>,
	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,
	Herbert Xu <herbert@...dor.apana.org.au>
Subject: Re: [PATCH 0/24] make atomic_read() behave consistently across all
 architectures

On Wed, 15 Aug 2007, Christoph Lameter wrote:

> On Wed, 15 Aug 2007, Paul E. McKenney wrote:
> 
> > Understood.  My point is not that the impact is precisely zero, but
> > rather that the impact on optimization is much less hurtful than the
> > problems that could arise otherwise, particularly as compilers become
> > more aggressive in their optimizations.
> 
> The problems arise because barriers are not used as required. Volatile 
> has wishy washy semantics and somehow marries memory barriers with data 
> access. It is clearer to separate the two. Conceptual cleanness usually 
> translates into better code. If one really wants the volatile then lets 
> make it explicit and use
> 
> 	atomic_read_volatile()

Completely agreed, again. To summarize again (had done so about ~100 mails
earlier in this thread too :-) ...

atomic_{read,set}_volatile() -- guarantees volatility also along with
atomicity (the two _are_ different concepts after all, irrespective of
whether callsites normally want one with the other or not)

atomic_{read,set}_nonvolatile() -- only guarantees atomicity, compiler
free to elid / coalesce / optimize such accesses, can keep the object
in question cached in a local register, leads to smaller text, etc.

As to which one should be the default atomic_read() is a question of
whether majority of callsites (more weightage to important / hot
codepaths, lesser to obscure callsites) want a particular behaviour.

Do we have a consensus here? (hoping against hope, probably :-)

[ This thread has gotten completely out of hand ... for my mail client
  alpine as well, it now seems. Reminds of that 1000+ GPLv3 fest :-) ]
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