From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751701AbaDWKmV (ORCPT ); Wed, 23 Apr 2014 06:42:21 -0400 Received: from v094114.home.net.pl ([79.96.170.134]:52959 "HELO v094114.home.net.pl" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with SMTP id S1751206AbaDWKmT (ORCPT ); Wed, 23 Apr 2014 06:42:19 -0400 From: "Rafael J. Wysocki" To: Li Zhong Cc: Tejun Heo , LKML , gregkh@linuxfoundation.org, rafael.j.wysocki@intel.com, toshi.kani@hp.com Subject: Re: [RFC PATCH v5 2/2] Use kernfs_break_active_protection() for device online store callbacks Date: Wed, 23 Apr 2014 12:58:40 +0200 Message-ID: <10378860.X4mJXjBVbJ@vostro.rjw.lan> User-Agent: KMail/4.11.5 (Linux/3.14.0-rc7+; KDE/4.11.5; x86_64; ; ) In-Reply-To: <1398229422.2805.49.camel@ThinkPad-T5421.cn.ibm.com> References: <1397529877.13188.68.camel@ThinkPad-T5421.cn.ibm.com> <20140422204455.GB3615@mtj.dyndns.org> <1398229422.2805.49.camel@ThinkPad-T5421.cn.ibm.com> MIME-Version: 1.0 Content-Transfer-Encoding: 7Bit Content-Type: text/plain; charset="utf-8" Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wednesday, April 23, 2014 01:03:42 PM Li Zhong wrote: > On Tue, 2014-04-22 at 16:44 -0400, Tejun Heo wrote: > > Hello, > > > > On Tue, Apr 22, 2014 at 11:34:39AM +0800, Li Zhong wrote: > > > > Is this assumption true? If so, can we add lockdep assertions in > > > > places to verify and enforce this? If not, aren't we just feeling > > > > good when the reality is broken? > > > > > > It seems not true ... I think there are devices that don't have the > > > online/offline concept, we just need to add it, remove it, like ethernet > > > cards. > > > > > > Maybe we could change the comments above, like: > > > /* We assume device_hotplug_lock must be acquired before > > > * removing devices, which have online/offline sysfs knob, > > > * and some locks are needed to serialize the online/offline > > > * callbacks and device removing. ... > > > ? > > > > > > And we could add lockdep assertions in cpu and memory related code? e.g. > > > remove_memory(), unregister_cpu() > > > > > > Currently, remove_memory() has comments for the function: > > > > > > * NOTE: The caller must call lock_device_hotplug() to serialize hotplug > > > * and online/offline operations before this call, as required by > > > * try_offline_node(). > > > */ > > > > > > maybe it could be removed with the lockdep assertion. > > > > I'm confused about the overall locking scheme. What's the role of > > device_hotplug_lock? Is that solely to prevent the sysfs deadlock > > issue? Or does it serve other synchronization purposes depending on > > the specific subsystem? If the former, the lock no longer needs to > > exist. The only thing necessary would be synchronization between > > device_del() deleting the sysfs file and the unbreak helper invoking > > device-specific callback. If the latter, we probably should change > > that. Sharing hotplug lock across multiple subsystems through driver > > core sounds like a pretty bad idea. > > I think it's the latter. Actually, no, this is not the case if I understand you correctly. > I think device_{on|off}line is better to be > done in some sort of lock which prevents the device from being removed, > including some preparation work that needs be done before device_del(). Quite frankly, you should be confident that you understand the code you're trying to modify or please don't touch it. I'll have a deeper look at this issue later today or tomorrow and will get back to you then. Thanks! -- I speak only for myself. Rafael J. Wysocki, Intel Open Source Technology Center.