From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751230AbVIYHir (ORCPT ); Sun, 25 Sep 2005 03:38:47 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751232AbVIYHir (ORCPT ); Sun, 25 Sep 2005 03:38:47 -0400 Received: from smtp200.mail.sc5.yahoo.com ([216.136.130.125]:52880 "HELO smtp200.mail.sc5.yahoo.com") by vger.kernel.org with SMTP id S1751230AbVIYHiq (ORCPT ); Sun, 25 Sep 2005 03:38:46 -0400 DomainKey-Signature: a=rsa-sha1; q=dns; c=nofws; s=s1024; d=yahoo.com.au; h=Received:Message-ID:Date:From:User-Agent:X-Accept-Language:MIME-Version:To:CC:Subject:References:In-Reply-To:Content-Type:Content-Transfer-Encoding; b=AhmUTSFFCLYpe2H7RnBNcCFkut5/94I0aclHovxToA93vwnXxaHpeWfob5AEgwrMmtFpiHm63Ivx/DA2uMxfewlz/yKEcclPXHZ7lE9PrjlbYENDbWusbKrcZtkWMnRIb2puMhpScqq0PjgZdFkYjw3Y0vOLbjjav2MdEnyN7LQ= ; Message-ID: <4336542D.4000102@yahoo.com.au> Date: Sun, 25 Sep 2005 17:39:25 +1000 From: Nick Piggin User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.7.10) Gecko/20050802 Debian/1.7.10-1 X-Accept-Language: en MIME-Version: 1.0 To: Tejun Heo CC: zwane@linuxpower.ca, viro@zeniv.linux.org.uk, linux-kernel@vger.kernel.org Subject: Re: [PATCH linux-2.6 04/04] brsem: convert cpucontrol to brsem References: <20050925064218.E7558977@htj.dyndns.org> <20050925064218.642A9DFD@htj.dyndns.org> In-Reply-To: <20050925064218.642A9DFD@htj.dyndns.org> Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Tejun Heo wrote: > +/* > + * cpucontrol is a brsem used to synchronize cpu hotplug events. > + * Invoking lock_cpu_hotplug() read-locks cpucontrol and no > + * hotplugging events will occur until it's released. > + * > + * Unfortunately, brsem itself makes use of lock_cpu_hotplug() and > + * performing brsem write-lock operations on cpucontrol deadlocks. > + * This is avoided by... > + * > + * a. guaranteeing that cpu hotplug events won't occur during the > + * write-lock operations, and > + * > + * b. skipping lock_cpu_hotplug() inside brsem. > + * > + * #a is achieved by acquiring and releasing cpucontrol_mutex outside > + * cpucontrol write-lock. #b is achieved by skipping > + * lock_cpu_hotplug() inside brsem if the current task is > + * cpucontrol_mutex holder (is_cpu_hotplug_holder() test). > + * > + * Also, note that cpucontrol is first initialized with > + * BRSEM_BYPASS_INITIALIZER and then initialized again with > + * __create_brsem() instead of simply using create_brsem(). This is > + * necessary as cpucontrol brsem gets used way before brsem subsystem > + * becomes up and running. > + * > + * Until brsem is properly initialized, all brsem ops succeed > + * unconditionally. cpucontrol becomes operational only after > + * cpucontrol_init() is finished, which should be called after > + * brsem_init_early(). > + */ Mmm, this is just insane IMO. Note that I happen to also think the idea (brsems) have merit, and that cpucontrol may be one of the places where a sane implementation would actually be useful... but at least when you're introducing this kind of complexity anywhere, you *really* need to be able to back it up with numbers. As far as the VFS race fix goes, I guess Al or someone else will comment on its correctness. But I think it might be nicer to first fix it with a regular rwsem and then show some numbers to justify its conversion to a brsem. If you need interruptible rwsems, I almost got an implementation working a while back, and David Howells recently said he was interested in doing them... so that's not an impossibility. Nick -- SUSE Labs, Novell Inc. Send instant messages to your online friends http://au.messenger.yahoo.com