From mboxrd@z Thu Jan 1 00:00:00 1970 From: Denys Vlasenko Subject: Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures Date: Sun, 9 Sep 2007 19:02:54 +0100 Message-ID: <200709091902.55388.vda.linux@googlemail.com> References: <18115.52863.638655.658466@cargo.ozlabs.ibm.com> <46C59717.4020108@cyberone.com.au> Mime-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit Cc: Nick Piggin , Satyam Sharma , Herbert Xu , Paul Mackerras , Christoph Lameter , Chris Snook , Ilpo Jarvinen , "Paul E. McKenney" , Stefan Richter , Linux Kernel Mailing List , linux-arch@vger.kernel.org, Netdev , Andrew Morton , ak@suse.de, heiko.carstens@de.ibm.com, David Miller , schwidefsky@de.ibm.com, wensong@linux-vs.org, horms@verge.net.au, wjiang@resilience.com, cfriesen@nortel.com, zlynx@acm.org, rpjday@mindspring.com, jesper.juhl@gmail.com, segher@kernel.crashing.org To: Linus Torvalds Return-path: In-Reply-To: Content-Disposition: inline Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org On Friday 17 August 2007 17:48, Linus Torvalds wrote: > > On Fri, 17 Aug 2007, Nick Piggin wrote: > > > > That's not obviously just taste to me. Not when the primitive has many > > (perhaps, the majority) of uses that do not require said barriers. And > > this is not solely about the code generation (which, as Paul says, is > > relatively minor even on x86). I prefer people to think explicitly > > about barriers in their lockless code. > > Indeed. > > I think the important issues are: > > - "volatile" itself is simply a badly/weakly defined issue. The semantics > of it as far as the compiler is concerned are really not very good, and > in practice tends to boil down to "I will generate so bad code that > nobody can accuse me of optimizing anything away". > > - "volatile" - regardless of how well or badly defined it is - is purely > a compiler thing. It has absolutely no meaning for the CPU itself, so > it at no point implies any CPU barriers. As a result, even if the > compiler generates crap code and doesn't re-order anything, there's > nothing that says what the CPU will do. > > - in other words, the *only* possible meaning for "volatile" is a purely > single-CPU meaning. And if you only have a single CPU involved in the > process, the "volatile" is by definition pointless (because even > without a volatile, the compiler is required to make the C code appear > consistent as far as a single CPU is concerned). > > So, let's take the example *buggy* code where we use "volatile" to wait > for other CPU's: > > atomic_set(&var, 0); > while (!atomic_read(&var)) > /* nothing */; > > > which generates an endless loop if we don't have atomic_read() imply > volatile. > > The point here is that it's buggy whether the volatile is there or not! > Exactly because the user expects multi-processing behaviour, but > "volatile" doesn't actually give any real guarantees about it. Another CPU > may have done: > > external_ptr = kmalloc(..); > /* Setup is now complete, inform the waiter */ > atomic_inc(&var); > > but the fact is, since the other CPU isn't serialized in any way, the > "while-loop" (even in the presense of "volatile") doesn't actually work > right! Whatever the "atomic_read()" was waiting for may not have > completed, because we have no barriers! Why is all this fixation on "volatile"? I don't think people want "volatile" keyword per se, they want atomic_read(&x) to _always_ compile into an memory-accessing instruction, not register access. -- vda