From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752427AbaDWFDw (ORCPT ); Wed, 23 Apr 2014 01:03:52 -0400 Received: from e06smtp14.uk.ibm.com ([195.75.94.110]:54414 "EHLO e06smtp14.uk.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750844AbaDWFDu (ORCPT ); Wed, 23 Apr 2014 01:03:50 -0400 Message-ID: <1398229422.2805.49.camel@ThinkPad-T5421.cn.ibm.com> Subject: Re: [RFC PATCH v5 2/2] Use kernfs_break_active_protection() for device online store callbacks From: Li Zhong To: Tejun Heo Cc: LKML , gregkh@linuxfoundation.org, rafael.j.wysocki@intel.com, toshi.kani@hp.com Date: Wed, 23 Apr 2014 13:03:42 +0800 In-Reply-To: <20140422204455.GB3615@mtj.dyndns.org> References: <1397529877.13188.68.camel@ThinkPad-T5421.cn.ibm.com> <20140415145017.GK1863@htj.dyndns.org> <1397612500.13188.83.camel@ThinkPad-T5421.cn.ibm.com> <20140416151749.GE1257@htj.dyndns.org> <1397717444.4034.15.camel@ThinkPad-T5421> <20140417151728.GK15326@htj.dyndns.org> <1398072059.2755.41.camel@ThinkPad-T5421.cn.ibm.com> <1398072230.2755.43.camel@ThinkPad-T5421.cn.ibm.com> <20140421224606.GE22730@htj.dyndns.org> <1398137679.2805.28.camel@ThinkPad-T5421.cn.ibm.com> <20140422204455.GB3615@mtj.dyndns.org> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.2.3-0ubuntu6 Content-Transfer-Encoding: 7bit Mime-Version: 1.0 X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 14042305-1948-0000-0000-0000088C9665 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. 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(). Thanks, Zhong > > Thanks. >