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Date: Thu, 16 Aug 2007 10:50:44 +0530 (IST) From: Satyam Sharma <satyam@...radead.org> To: Bill Fink <billfink@...dspring.com> cc: Stefan Richter <stefanr@...6.in-berlin.de>, Christoph Lameter <clameter@....com>, Chris Snook <csnook@...hat.com>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, linux-arch@...r.kernel.org, Linus Torvalds <torvalds@...ux-foundation.org>, netdev@...r.kernel.org, Andrew Morton <akpm@...ux-foundation.org>, ak@...e.de, heiko.carstens@...ibm.com, 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, Herbert Xu <herbert@...dor.apana.org.au>, "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> Subject: Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures Hi Bill, On Wed, 15 Aug 2007, Bill Fink wrote: > On Wed, 15 Aug 2007, Satyam Sharma wrote: > > > (C) > > $ cat tp3.c > > int a; > > > > void func(void) > > { > > *(volatile int *)&a = 10; > > *(volatile int *)&a = 20; > > } > > $ gcc -Os -S tp3.c > > $ cat tp3.s > > ... > > movl $10, a > > movl $20, a > > ... > > I'm curious about one minor tangential point. Why, instead of: > > b = *(volatile int *)&a; > > why can't this just be expressed as: > > b = (volatile int)a; > > Isn't it the contents of a that's volatile, i.e. it's value can change > invisibly to the compiler, and that's why you want to force a read from > memory? Why do you need the "*(volatile int *)&" construct? "b = (volatile int)a;" doesn't help us because a cast to a qualified type has the same effect as a cast to an unqualified version of that type, as mentioned in 6.5.4:4 (footnote 86) of the standard. 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. 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). Satyam - To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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