From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753296AbbJOOF1 (ORCPT ); Thu, 15 Oct 2015 10:05:27 -0400 Received: from foss.arm.com ([217.140.101.70]:35980 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752264AbbJOOFV (ORCPT ); Thu, 15 Oct 2015 10:05:21 -0400 Date: Thu, 15 Oct 2015 15:05:18 +0100 From: Javi Merino To: "Chen, Yu C" Cc: "linux-pm@vger.kernel.org" , "edubezval@gmail.com" , "Zhang, Rui" , "linux-kernel@vger.kernel.org" , "stable@vger.kernel.org" , "Pandruvada, Srinivas" Subject: Re: [PATCH 3/3] Thermal: do thermal zone update after a cooling device registered Message-ID: <20151015140517.GC2639@e104805> References: <1443332924-14028-1-git-send-email-yu.c.chen@intel.com> <20150928142902.GA9175@e104805> <36DF59CE26D8EE47B0655C516E9CE64026F22641@SHSMSX101.ccr.corp.intel.com> <20150929160133.GA11588@e104805> <36DF59CE26D8EE47B0655C516E9CE6402865B04D@shsmsx102.ccr.corp.intel.com> <20151014170739.GB2998@e104805> <36DF59CE26D8EE47B0655C516E9CE6402865BCBE@shsmsx102.ccr.corp.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <36DF59CE26D8EE47B0655C516E9CE6402865BCBE@shsmsx102.ccr.corp.intel.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Oct 14, 2015 at 07:23:55PM +0000, Chen, Yu C wrote: > > -----Original Message----- > > From: Javi Merino [mailto:javi.merino@arm.com] > > Sent: Thursday, October 15, 2015 1:08 AM > > To: Chen, Yu C > > Cc: linux-pm@vger.kernel.org; edubezval@gmail.com; Zhang, Rui; linux- > > kernel@vger.kernel.org; stable@vger.kernel.org; Pandruvada, Srinivas > > Subject: Re: [PATCH 3/3] Thermal: do thermal zone update after a cooling > > device registered > > > > On Mon, Oct 12, 2015 at 09:23:28AM +0000, Chen, Yu C wrote: > > > Hi, Javi > > > Sorry for my late response, > > > > > > > -----Original Message----- > > > > From: Javi Merino [mailto:javi.merino@arm.com] > > > > Sent: Wednesday, September 30, 2015 12:02 AM > > > > To: Chen, Yu C > > > > Cc: linux-pm@vger.kernel.org; edubezval@gmail.com; Zhang, Rui; > > > > linux- kernel@vger.kernel.org; stable@vger.kernel.org > > > > Subject: Re: [PATCH 3/3] Thermal: do thermal zone update after a > > > > cooling device registered > > > > > > > > Hi Yu, > > > > > > > > On Mon, Sep 28, 2015 at 06:52:00PM +0100, Chen, Yu C wrote: > > > > > Hi, Javi, > > > > > > > > > > > -----Original Message----- > > > > > > From: Javi Merino [mailto:javi.merino@arm.com] > > > > > > Sent: Monday, September 28, 2015 10:29 PM > > > > > > To: Chen, Yu C > > > > > > Cc: linux-pm@vger.kernel.org; edubezval@gmail.com; Zhang, Rui; > > > > > > linux- kernel@vger.kernel.org; stable@vger.kernel.org > > > > > > Subject: Re: [PATCH 3/3] Thermal: do thermal zone update after a > > > > > > cooling device registered > > > > > > > > > > > > On Sun, Sep 27, 2015 at 06:48:44AM +0100, Chen Yu wrote: > > > > > > > From: Zhang Rui > > > > > > > > > > > > > > > > > > > > > > > > > > I think you need to hold cdev->lock here, to make sure that no > > > > > > thermal zone is added or removed from cdev->thermal_instances > > > > > > while > > > > you are looping. > > > > > > > > > > > Ah right, will add. If I add the cdev ->lock here, will there be a > > > > > AB-BA lock with thermal_zone_unbind_cooling_device? > > > > > > > > You're right, it could lead to a deadlock. The locks can't be > > > > swapped because that won't work in step_wise. > > > > > > > > The best way that I can think of accessing thermal_instances > > > > atomically is by making it RCU protected instead of with mutexes. > > > > What do you think? > > > > > > > RCU would need extra spinlocks to protect the list, and need to > > > sync_rcu after we delete one instance from thermal_instance list, I > > > think it is too complicated for me to rewrite: ( How about using > > thermal_list_lock instead of cdev ->lock? > > > This guy should be big enough to protect the device.thermal_instance list. > > > > thermal_list_lock protects thermal_tz_list and thermal_cdev_list, but it > > doesn't protect the thermal_instances list. For example, > > thermal_zone_bind_cooling_device() adds a cooling device to the > > cdev->thermal_instances list without taking thermal_tz_list. > > > Before thermal_zone_bind_cooling_device is invoked, > the thermal_list_lock will be firstly gripped: > > static void bind_cdev(struct thermal_cooling_device *cdev) > { > mutex_lock(&thermal_list_lock); > either tz->ops->bind : thermal_zone_bind_cooling_device > or __bind() : thermal_zone_bind_cooling_device > mutex_unlock(&thermal_list_lock); > } > > And it is the same as in passive_store. > So when code is trying to add/delete thermal_instance of cdev, > he has already hold thermal_list_lock IMO. Or do I miss anything? thermal_zone_bind_cooling_device() is exported, so you can't really rely on the static thermal_list_lock being acquired in every single call. thermal_list_lock and protects the lists thermal_tz_list and thermal_cdev_list. Making it implicitly protect the cooling device's and thermal zone device's instances list because no sensible code would call thermal_zone_bind_cooling_device() outside of a bind function is just asking for trouble. Locking is hard to understand and easy to get wrong so let's keep it simple. Cheers, Javi