From mboxrd@z Thu Jan 1 00:00:00 1970 From: Denys Vlasenko Subject: Re: [PATCH] i386: Fix a couple busy loops in mach_wakecpu.h:wait_for_init_deassert() Date: Fri, 24 Aug 2007 15:25:50 +0100 Message-ID: <200708241525.51049.vda.linux@googlemail.com> References: Mime-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit Cc: "Satyam Sharma" , "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 To: "Kenn Humborg" Return-path: Received: from nf-out-0910.google.com ([64.233.182.190]:60403 "EHLO nf-out-0910.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751109AbXHXOZ6 (ORCPT ); Fri, 24 Aug 2007 10:25:58 -0400 Received: by nf-out-0910.google.com with SMTP id g13so632174nfb for ; Fri, 24 Aug 2007 07:25:57 -0700 (PDT) In-Reply-To: Content-Disposition: inline Sender: netdev-owner@vger.kernel.org List-Id: netdev.vger.kernel.org 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