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Date: Fri, 24 Aug 2007 13:12:54 +0100 From: "Kenn Humborg" <kenn@...etree.ie> To: "Denys Vlasenko" <vda.linux@...glemail.com>, "Satyam Sharma" <satyam@...radead.org> Cc: "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 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. Perhaps the Linux atomic_XXX functions work like that, or used to work like that, but it's counter-intuitive to me that "atomic" should imply a memory read. Later, Kenn - 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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