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

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Date:	Thu, 16 Aug 2007 22:50:38 +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

> Note that "volatile"
> is a type-qualifier, not a type itself, so a cast of the _object_ 
> itself
> to a qualified-type i.e. (volatile int) would not make the access 
> itself
> volatile-qualified.

There is no such thing as "volatile-qualified access" defined
anywhere; there only is the concept of a "volatile-qualified
*object*".

> To serve our purposes, it is necessary for us to take the address of 
> this
> (non-volatile) object, cast the resulting _pointer_ to the 
> corresponding
> volatile-qualified pointer-type, and then dereference it. This makes 
> that
> particular _access_ be volatile-qualified, without the object itself 
> being
> such. Also note that the (dereferenced) result is also a valid lvalue 
> and
> hence can be used in "*(volatile int *)&a = b;" kind of construction
> (which we use for the atomic_set case).

There is a quite convincing argument that such an access _is_ an
access to a volatile object; see GCC PR21568 comment #9.  This
probably isn't the last word on the matter though...


Segher

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