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

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Message-ID: <46C56ADF.8010501@cyberone.com.au>
Date:	Fri, 17 Aug 2007 19:31:11 +1000
From:	Nick Piggin <piggin@...erone.com.au>
To:	Satyam Sharma <satyam@...radead.org>
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

Satyam Sharma wrote:
> 
> On Fri, 17 Aug 2007, Nick Piggin wrote:
> 
> 
>>Satyam Sharma wrote:
>>[...]
>>
>>>Granted, the above IS buggy code. But, the stated objective is to avoid
>>>heisenbugs.
> 
>     ^^^^^^^^^^
> 
> 
>>Anyway, why are you making up code snippets that are buggy in other
>>ways in order to support this assertion being made that lots of kernel
>>code supposedly depends on volatile semantics. Just reference the
>>actual code.
> 
> 
> Because the point is *not* about existing bugs in kernel code. At some
> point Chris Snook (who started this thread) did write that "If I knew
> of the existing bugs in the kernel, I would be sending patches for them,
> not this series" or something to that effect.
> 
> 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. Linux's whole
memory ordering and locking model is completely geared around the
former.


> 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, 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.
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