From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752821AbWLOQ1r (ORCPT ); Fri, 15 Dec 2006 11:27:47 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751547AbWLOQ1r (ORCPT ); Fri, 15 Dec 2006 11:27:47 -0500 Received: from mx2.mail.elte.hu ([157.181.151.9]:48239 "EHLO mx2.mail.elte.hu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752821AbWLOQ1q (ORCPT ); Fri, 15 Dec 2006 11:27:46 -0500 Date: Fri, 15 Dec 2006 17:24:16 +0100 From: Ingo Molnar To: Andrew Morton Cc: linux-kernel@vger.kernel.org Subject: Re: + schedule_on_each_cpu-use-preempt_disable.patch added to -mm tree Message-ID: <20061215162416.GB29191@elte.hu> References: <200612150823.kBF8NV2u011171@shell0.pdx.osdl.net> <20061215083112.GB10687@elte.hu> <20061215081138.4c51e7c5.akpm@osdl.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20061215081138.4c51e7c5.akpm@osdl.org> User-Agent: Mutt/1.4.2.2i X-ELTE-VirusStatus: clean X-ELTE-SpamScore: -2.6 X-ELTE-SpamLevel: X-ELTE-SpamCheck: no X-ELTE-SpamVersion: ELTE 2.0 X-ELTE-SpamCheck-Details: score=-2.6 required=5.9 tests=BAYES_00 autolearn=no SpamAssassin version=3.0.3 -2.6 BAYES_00 BODY: Bayesian spam probability is 0 to 1% [score: 0.0000] Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org * Andrew Morton wrote: > > > for_each_online_cpu(cpu) { > > > INIT_WORK(per_cpu_ptr(works, cpu), func); > > > __queue_work(per_cpu_ptr(keventd_wq->cpu_wq, cpu), > > > per_cpu_ptr(works, cpu)); > > > } > > > - mutex_unlock(&workqueue_mutex); > > > + preempt_enable(); > > > > Why not cpu_hotplug_lock()? > > > > Because the workqueue code was explicitly switched over to > per-subsystem cpu-hotplug locking. > > Because lock_cpu_hotplug() is a complete turkey, source of deadlocks > and overall bad idea. not in the locking model i outlined earlier, which would turn it into a read-lock in essence. > This is actually a pretty simple problem. A subsystem has per-cpu > reosurces, and it needs to lock them while using them. duh. We know > how to do that sort of thing. But because the first implementation of > lock_cpu_hotplug() was conceived with magical properties, we seem to > think we need to retain magical properties. We don't... actually, we use two things here: cpu_online_map and the per-cpu keventd workqueues. cpu_online_map is pretty much attached to the CPU hotplug subsystem so it would be quite natural to use cpu_hotplug_read_lock() for that. so i disagree that CPU hotplug locking should be per-subsystem. We should have one lightweight and scalable primitive that protects cpu_online_map use, and that same primitive can be used to protect other per-CPU resources too. Ingo