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Date: Tue, 21 Aug 2007 00:04:17 +0200 From: Segher Boessenkool <segher@...nel.crashing.org> To: Chris Snook <csnook@...hat.com> Cc: Christoph Lameter <clameter@....com>, Paul Mackerras <paulus@...ba.org>, heiko.carstens@...ibm.com, horms@...ge.net.au, linux-kernel@...r.kernel.org, "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>, ak@...e.de, netdev@...r.kernel.org, cfriesen@...tel.com, akpm@...ux-foundation.org, rpjday@...dspring.com, Nick Piggin <nickpiggin@...oo.com.au>, linux-arch@...r.kernel.org, jesper.juhl@...il.com, satyam@...radead.org, zlynx@....org, schwidefsky@...ibm.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 >> Right. ROTFL... volatile actually breaks atomic_t instead of making >> it safe. x++ becomes a register load, increment and a register store. >> Without volatile we can increment the memory directly. It seems that >> volatile requires that the variable is loaded into a register first >> and then operated upon. Understandable when you think about volatile >> being used to access memory mapped I/O registers where a RMW >> operation could be problematic. > > So, if we want consistent behavior, we're pretty much screwed unless > we use inline assembler everywhere? Nah, this whole argument is flawed -- "without volatile" we still *cannot* "increment the memory directly". On x86, you need a lock prefix; on other archs, some other mechanism to make the memory increment an *atomic* memory increment. And no, RMW on MMIO isn't "problematic" at all, either. An RMW op is a read op, a modify op, and a write op, all rolled into one opcode. But three actual operations. The advantages of asm code for atomic_{read,set} are: 1) all the other atomic ops are implemented that way already; 2) you have full control over the asm insns selected, in particular, you can guarantee you *do* get an atomic op; 3) you don't need to use "volatile <data>" which generates not-all-that-good code on archs like x86, and we want to get rid of it anyway since it is problematic in many ways; 4) you don't need to use *(volatile <type>*)&<data>, which a) doesn't exist in C; b) isn't documented or supported in GCC; c) has a recent history of bugginess; d) _still uses volatile objects_; e) _still_ is problematic in almost all those same ways as in 3); 5) you can mix atomic and non-atomic accesses to the atomic_t, which you cannot with the other alternatives. The only disadvantage I know of is potentially slightly worse instruction scheduling. This is a generic asm() problem: GCC cannot see what actual insns are inside the asm() block. Segher - 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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