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Message-Id: <4952d3f536060c186f40c277dfc74194@kernel.crashing.org>
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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