Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751198AbWHXMcn (ORCPT ); Thu, 24 Aug 2006 08:32:43 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751199AbWHXMcn (ORCPT ); Thu, 24 Aug 2006 08:32:43 -0400 Received: from mx2.mail.elte.hu ([157.181.151.9]:65254 "EHLO mx2.mail.elte.hu") by vger.kernel.org with ESMTP id S1751198AbWHXMcm (ORCPT ); Thu, 24 Aug 2006 08:32:42 -0400 Date: Thu, 24 Aug 2006 14:25:27 +0200 From: Ingo Molnar To: Gautham R Shenoy Cc: rusty@rustcorp.com.au, torvalds@osdl.org, akpm@osdl.org, linux-kernel@vger.kernel.org, arjan@linux.intel.com, davej@redhat.com, vatsa@in.ibm.com, dipankar@in.ibm.com, ashok.raj@intel.com Subject: Re: [RFC][PATCH 3/4] (Refcount + Waitqueue) implementation for cpu_hotplug "locking" Message-ID: <20060824122527.GA28275@elte.hu> References: <20060824103233.GD2395@in.ibm.com> <20060824111440.GA19248@elte.hu> <20060824122808.GH2395@in.ibm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20060824122808.GH2395@in.ibm.com> User-Agent: Mutt/1.4.2.1i X-ELTE-SpamScore: -2.9 X-ELTE-SpamLevel: X-ELTE-SpamCheck: no X-ELTE-SpamVersion: ELTE 2.0 X-ELTE-SpamCheck-Details: score=-2.9 required=5.9 tests=ALL_TRUSTED,AWL,BAYES_50 autolearn=no SpamAssassin version=3.0.3 -3.3 ALL_TRUSTED Did not pass through any untrusted hosts 0.5 BAYES_50 BODY: Bayesian spam probability is 40 to 60% [score: 0.4606] -0.1 AWL AWL: From: address is in the auto white-list X-ELTE-VirusStatus: clean Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1520 Lines: 35 * Gautham R Shenoy wrote: > This was the approach I tried to make it cache friendly. > These are the problems I faced. > > - Reader checks the write_active flag. If set, he waits in the global read > queue. else, he gets the lock and increments percpu refcount. > > - the writer would have to check each cpu's read refcount, and ensure that > read refcount =0 on all of them before he sets write_active and > begins a write operation. > This will create a big race window - a writer is checking > for a refcount on cpu(j), a reader comes on cpu(i) where i Let's assume the writer checks refcounts in increasing order of cpus. > Should the reader on cpu(i) wait or go ahead? If we use a global > lock to serialize this operation, we the whole purpose of maintaining > per cpu data is lost. no. The writer first sets the global write_active flag, and _then_ goes on to wait for all readers (if any) to get out of their critical sections. (That's the purpose of the per-cpu waitqueue that readers use to wake up a writer waiting for the refcount to go to 0.) can you still see problems with this scheme? (the 'write_active' flag is probably best implemented as a mutex, where readers check mutex_is_locked(), and writers try to take it.) Ingo - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/