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

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Date:	Sat, 18 Aug 2007 00:38:12 +0200
From:	Segher Boessenkool <segher@...nel.crashing.org>
To:	Satyam Sharma <satyam@...radead.org>
Cc:	Christoph Lameter <clameter@....com>, heiko.carstens@...ibm.com,
	horms@...ge.net.au, Stefan Richter <stefanr@...6.in-berlin.de>,
	Bill Fink <billfink@...dspring.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	netdev@...r.kernel.org, ak@...e.de, cfriesen@...tel.com,
	rpjday@...dspring.com, jesper.juhl@...il.com,
	linux-arch@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>, zlynx@....org,
	schwidefsky@...ibm.com, Chris Snook <csnook@...hat.com>,
	Herbert Xu <herbert@...dor.apana.org.au>, davem@...emloft.net,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	wensong@...ux-vs.org, wjiang@...ilience.com
Subject: Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures

>>> Here, I should obviously admit that the semantics of *(volatile int 
>>> *)&
>>> aren't any neater or well-defined in the _language standard_ at all. 
>>> The
>>> standard does say (verbatim) "precisely what constitutes as access to
>>> object of volatile-qualified type is implementation-defined", but GCC
>>> does help us out here by doing the right thing.
>>
>> Where do you get that idea?
>
> Try a testcase (experimentally verify).

That doesn't prove anything.  Experiments can only disprove
things.

>> GCC manual, section 6.1, "When
>> is a Volatile Object Accessed?" doesn't say anything of the
>> kind.
>
> True, "implementation-defined" as per the C standard _is_ supposed to 
> mean
> "unspecified behaviour where each implementation documents how the 
> choice
> is made". So ok, probably GCC isn't "documenting" this
> implementation-defined behaviour which it is supposed to, but can't 
> really
> fault them much for this, probably.

GCC _is_ documenting this, namely in this section 6.1.  It doesn't
mention volatile-casted stuff.  Draw your own conclusions.


Segher

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