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Date: Sat, 18 Aug 2007 19:06:33 +0530 (IST) From: Satyam Sharma <satyam@...radead.org> To: Linus Torvalds <torvalds@...ux-foundation.org> cc: Christoph Lameter <clameter@....com>, "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>, Herbert Xu <herbert@...dor.apana.org.au>, Nick Piggin <nickpiggin@...oo.com.au>, Paul Mackerras <paulus@...ba.org>, Segher Boessenkool <segher@...nel.crashing.org>, heiko.carstens@...ibm.com, horms@...ge.net.au, linux-kernel@...r.kernel.org, rpjday@...dspring.com, ak@...e.de, netdev@...r.kernel.org, cfriesen@...tel.com, akpm@...ux-foundation.org, jesper.juhl@...il.com, linux-arch@...r.kernel.org, zlynx@....org, schwidefsky@...ibm.com, Chris Snook <csnook@...hat.com>, davem@...emloft.net, wensong@...ux-vs.org, wjiang@...ilience.com Subject: Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures On Fri, 17 Aug 2007, Linus Torvalds wrote: > On Sat, 18 Aug 2007, Satyam Sharma wrote: > > > > No code does (or would do, or should do): > > > > x.counter++; > > > > on an "atomic_t x;" anyway. > > That's just an example of a general problem. > > No, you don't use "x.counter++". But you *do* use > > if (atomic_read(&x) <= 1) > > and loading into a register is stupid and pointless, when you could just > do it as a regular memory-operand to the cmp instruction. True, but that makes this a bad/poor code generation issue with the compiler, not something that affects the _correctness_ of atomic ops if "volatile" is used for that counter object (as was suggested), because we'd always use the atomic_inc() etc primitives to do increments, which are always (should be!) implemented to be atomic. > And as far as the compiler is concerned, the problem is the 100% same: > combining operations with the volatile memop. > > The fact is, a compiler that thinks that > > movl mem,reg > cmpl $val,reg > > is any better than > > cmpl $val,mem > > is just not a very good compiler. Absolutely, this is definitely a bug report worth opening with gcc. And what you've said to explain this previously sounds definitely correct -- seeing "volatile" for any access does appear to just scare the hell out of gcc and makes it generate such (poor) code. Satyam - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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