From mboxrd@z Thu Jan 1 00:00:00 1970 From: Javi Merino Subject: Re: [PATCH 3/3] Thermal: do thermal zone update after a cooling device registered Date: Wed, 14 Oct 2015 18:07:39 +0100 Message-ID: <20151014170739.GB2998@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> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Return-path: Received: from foss.arm.com ([217.140.101.70]:59703 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752755AbbJNRHn (ORCPT ); Wed, 14 Oct 2015 13:07:43 -0400 Content-Disposition: inline In-Reply-To: <36DF59CE26D8EE47B0655C516E9CE6402865B04D@shsmsx102.ccr.corp.intel.com> Sender: linux-pm-owner@vger.kernel.org List-Id: linux-pm@vger.kernel.org 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" 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. To sum up, you have to protect accessing the cdev->thermal_instances list but with the current locking scheme, you would create an AB-BA deadlock. As I see it you would have to change the locking scheme to either RCU or add a new mutex that protects the cdev->thermal_instances and tz->thermal_instances lists and change all accesses to them to make sure they comply with the new locking scheme. Is there a better way of solving this? Cheers, Javi