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

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Message-ID: <alpine.LFD.0.999.0708171612440.3666@enigma.security.iitk.ac.in>
Date:	Fri, 17 Aug 2007 16:25:14 +0530 (IST)
From:	Satyam Sharma <satyam@...radead.org>
To:	Nick Piggin <piggin@...erone.com.au>
cc:	Herbert Xu <herbert@...dor.apana.org.au>,
	Paul Mackerras <paulus@...ba.org>,
	Linus Torvalds <torvalds@...ux-foundation.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 Fri, 17 Aug 2007, Nick Piggin wrote:

> Satyam Sharma wrote:
> > [...]
> > The point is about *author expecations*. If people do expect atomic_read()
> > (or a variant thereof) to have volatile semantics, why not give them such
> > a variant?
> 
> Because they should be thinking about them in terms of barriers, over
> which the compiler / CPU is not to reorder accesses or cache memory
> operations, rather than "special" "volatile" accesses.

This is obviously just a taste thing. Whether to have that forget(x)
barrier as something author should explicitly sprinkle appropriately
in appropriate places in the code by himself or use a primitive that
includes it itself.

I'm not saying "taste matters aren't important" (they are), but I'm really
skeptical if most folks would find the former tasteful.

> > And by the way, the point is *also* about the fact that cpu_relax(), as
> > of today, implies a full memory clobber, which is not what a lot of such
> > loops want. (due to stuff mentioned elsewhere, summarized in that summary)
> 
> That's not the point,

That's definitely the point, why not. This is why "barrier()", being
heavy-handed, is not the best option.

> because as I also mentioned, the logical extention
> to Linux's barrier API to handle this is the order(x) macro. Again, not
> special volatile accessors.

Sure, that forget(x) macro _is_ proposed to be made part of the generic
API. Doesn't explain why not to define/use primitives that has volatility
semantics in itself, though (taste matters apart).
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