From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from cerberus.bluetree.ie ([87.198.132.99]:48977 "EHLO cerberus.bluetree.ie" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757396AbXHXMYK (ORCPT ); Fri, 24 Aug 2007 08:24:10 -0400 From: "Kenn Humborg" Subject: RE: [PATCH] i386: Fix a couple busy loops in mach_wakecpu.h:wait_for_init_deassert() Date: Fri, 24 Aug 2007 13:12:54 +0100 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit In-Reply-To: <200708241259.33659.vda.linux@googlemail.com> Sender: linux-arch-owner@vger.kernel.org To: Denys Vlasenko , Satyam Sharma Cc: Heiko Carstens , Herbert Xu , Chris Snook , clameter@sgi.com, Linux Kernel Mailing List , linux-arch@vger.kernel.org, Linus Torvalds , netdev@vger.kernel.org, Andrew Morton , ak@suse.de, davem@davemloft.net, 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 List-ID: > 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