From mboxrd@z Thu Jan 1 00:00:00 1970 From: Satyam Sharma Subject: Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures Date: Sat, 18 Aug 2007 05:25:41 +0530 (IST) Message-ID: References: <20070816003948.GY9645@linux.vnet.ibm.com> <18115.44462.622801.683446@cargo.ozlabs.ibm.com> <20070816020042.GA30650@gondor.apana.org.au> <18115.45316.702491.681906@cargo.ozlabs.ibm.com> <18115.52863.638655.658466@cargo.ozlabs.ibm.com> <20070816053945.GB32442@gondor.apana.org.au> <18115.62741.807704.969977@cargo.ozlabs.ibm.com> <20070816070907.GA964@gondor.apana.org.au> <46C4ABA5.9010804@redhat.com> Mime-Version: 1.0 Content-Type: TEXT/PLAIN; charset=us-ascii Cc: Christoph Lameter , Paul Mackerras , heiko.carstens@de.ibm.com, horms@verge.net.au, Stefan Richter , Linux Kernel Mailing List , David Miller , "Paul E. McKenney" , =?iso-8859-15?Q?Ilpo_J=E4rvinen?= , ak@suse.de, cfriesen@nortel.com, rpjday@mindspring.com, Netdev , jesper.juhl@gmail.com, linux-arch@vger.kernel.org, zlynx@acm.org, Andrew Morton , schwidefsky@de.ibm.com, Chris Snook , Herbert Xu , Linus Torvalds , wensong@linux-vs.org, wjiang@resilience.com To: Segher Boessenkool Return-path: Received: from pentafluge.infradead.org ([213.146.154.40]:39097 "EHLO pentafluge.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751966AbXHQXnN (ORCPT ); Fri, 17 Aug 2007 19:43:13 -0400 In-Reply-To: Sender: netdev-owner@vger.kernel.org List-Id: netdev.vger.kernel.org On Sat, 18 Aug 2007, Segher Boessenkool wrote: > > > > No it does not have any volatile semantics. atomic_dec() can be > > > > reordered > > > > at will by the compiler within the current basic unit if you do not add > > > > a > > > > barrier. > > > > > > "volatile" has nothing to do with reordering. > > > > If you're talking of "volatile" the type-qualifier keyword, then > > http://lkml.org/lkml/2007/8/16/231 (and sub-thread below it) shows > > otherwise. > > I'm not sure what in that mail you mean, but anyway... > > Yes, of course, the fact that "volatile" creates a side effect > prevents certain things from being reordered wrt the atomic_dec(); > but the atomic_dec() has a side effect *already* so the volatile > doesn't change anything. That's precisely what that sub-thread (read down to the last mail there, and not the first mail only) shows. So yes, "volatile" does have something to do with re-ordering (as guaranteed by the C standard). > > > atomic_dec() writes > > > to memory, so it _does_ have "volatile semantics", implicitly, as > > > long as the compiler cannot optimise the atomic variable away > > > completely -- any store counts as a side effect. > > > > I don't think an atomic_dec() implemented as an inline "asm volatile" > > or one that uses a "forget" macro would have the same re-ordering > > guarantees as an atomic_dec() that uses a volatile access cast. > > The "asm volatile" implementation does have exactly the same > reordering guarantees as the "volatile cast" thing, I don't think so. > if that is > implemented by GCC in the "obvious" way. Even a "plain" asm() > will do the same. Read the relevant GCC documentation. [ of course, if the (latest) GCC documentation is *yet again* wrong, then alright, not much I can do about it, is there. ]