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Date: Fri, 24 Aug 2007 15:25:50 +0100 From: Denys Vlasenko <vda.linux@...glemail.com> To: "Kenn Humborg" <kenn@...etree.ie> Cc: "Satyam Sharma" <satyam@...radead.org>, "Heiko Carstens" <heiko.carstens@...ibm.com>, "Herbert Xu" <herbert@...dor.apana.org.au>, "Chris Snook" <csnook@...hat.com>, clameter@....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, 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 Subject: Re: [PATCH] i386: Fix a couple busy loops in mach_wakecpu.h:wait_for_init_deassert() On Friday 24 August 2007 13:12, Kenn Humborg wrote: > > On Thursday 16 August 2007 01:39, Satyam Sharma wrote: > > > static inline void wait_for_init_deassert(atomic_t *deassert) > > > { > > > - while (!atomic_read(deassert)); > > > + while (!atomic_read(deassert)) > > > + cpu_relax(); > > > return; > > > } > > > > For less-than-briliant people like me, it's totally non-obvious that > > cpu_relax() is needed for correctness here, not just to make P4 happy. > > > > IOW: "atomic_read" name quite unambiguously means "I will read > > this variable from main memory". Which is not true and creates > > potential for confusion and bugs. > > To me, "atomic_read" means a read which is synchronized with other > changes to the variable (using the atomic_XXX functions) in such > a way that I will always only see the "before" or "after" > state of the variable - never an intermediate state while a > modification is happening. It doesn't imply that I have to > see the "after" state immediately after another thread modifies > it. So you are ok with compiler propagating n1 to n2 here: n1 += atomic_read(x); other_variable++; n2 += atomic_read(x); without accessing x second time. What's the point? Any sane coder will say that explicitly anyway: tmp = atomic_read(x); n1 += tmp; other_variable++; n2 += tmp; if only for the sake of code readability. Because first code is definitely hinting that it reads RAM twice, and it's actively *bad* for code readability when in fact it's not the case! Locking, compiler and CPU barriers are complicated enough already, please don't make them even harder to understand. -- vda - 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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