Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751551AbaLXRot (ORCPT ); Wed, 24 Dec 2014 12:44:49 -0500 Received: from mail-ie0-f174.google.com ([209.85.223.174]:55642 "EHLO mail-ie0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751352AbaLXRor (ORCPT ); Wed, 24 Dec 2014 12:44:47 -0500 MIME-Version: 1.0 In-Reply-To: <549AF9C0.7010602@ti.com> References: <1418771379-24369-1-git-send-email-dtor@chromium.org> <1418771379-24369-4-git-send-email-dtor@chromium.org> <549AEE52.9080607@ti.com> <549AF4F3.1090600@ti.com> <549AF9C0.7010602@ti.com> Date: Wed, 24 Dec 2014 09:44:45 -0800 Message-ID: Subject: Re: [PATCH 3/4] PM / OPP: take RCU lock in dev_pm_opp_get_opp_count From: Dmitry Torokhov To: Nishanth Menon Cc: Dmitry Torokhov , "Rafael J. Wysocki" , Viresh Kumar , Thomas Petazzoni , Geert Uytterhoeven , Stefan Wahren , Paul Gortmaker , "linux-pm@vger.kernel.org" , "linux-kernel@vger.kernel.org" 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 Wed, Dec 24, 2014 at 9:37 AM, Nishanth Menon wrote: > On 12/24/2014 11:31 AM, Dmitry Torokhov wrote: >> On Wed, Dec 24, 2014 at 9:16 AM, Nishanth Menon wrote: >>> On 12/24/2014 11:09 AM, Dmitry Torokhov wrote: >>>> On Wed, Dec 24, 2014 at 8:48 AM, Nishanth Menon wrote: >>>>> On 12/16/2014 05:09 PM, Dmitry Torokhov wrote: >>>>>> A lot of callers are missing the fact that dev_pm_opp_get_opp_count >>>>>> needs to be called under RCU lock. Given that RCU locks can safely be >>>>>> nested, instead of providing *_locked() API, let's take RCU lock inside >>>>>> dev_pm_opp_get_opp_count() and leave callers as is. >>>>> >>>>> While it is true that we can safely do nested RCU locks, This also >>>>> encourages wrong usage. >>>>> >>>>> count = dev_pm_opp_get_opp_count(dev) >>>>> ^^ point A >>>>> array = kzalloc(count * sizeof (*array)); >>>>> rcu_read_lock(); >>>>> ^^ point B >>>>> .. work down the list and add OPPs.. >>>>> ... >>>>> >>>>> Between A and B, we might have had list modification (dynamic OPP >>>>> addition or deletion) - which implies that the count is no longer >>>>> accurate between point A and B. instead, enforcing callers to have the >>>>> responsibility of rcu_lock is exactly what we have to do since the OPP >>>>> library has no clue how to enforce pointer or data accuracy. >>>> >>>> No, you seem to have a misconception that rcu_lock protects you past >>>> the point B, but that is also wrong. The only thing rcu "lock" >>>> provides is safe traversing the list and guarantee that elements will >>>> not disappear while you are referencing them, but list can both >>>> contract and expand under you. In that regard code in >>>> drivers/cpufreq/cpufreq_opp.c is utterly wrong. If you want to count >>>> the list and use number of elements you should be taking a mutex. >>>> Luckily all cpufreq drivers at the moment only want to see if OPP >>>> table is empty or not, so as a stop-gap we can take rcu_lock >>>> automatically as we are getting count. We won't get necessarily >>>> accurate result, but at least we will be safe traversing the list. >>> >>> So, instead of a half solution, lets consider this in the realm of >>> dynamic OPPs as well. agreed to the point that we only have safe >>> traversal and pointer validity. the real problem however is with >>> "dynamic OPPs" (one of the original reasons why i did not add dynamic >>> OPPs in the original version was to escape from it's complexity for >>> users - anyways.. we are beyond that now). if OPPs can be removed on >>> the fly, we need the following: >>> a) use OPP notifiers to adequately handle list modification >>> b) lock down list modification (and associated APIs) to ensure that >>> the original cpufreq /devfreq list is correct. >>> >>> I still dont see the need to do this half solution. >> >> The need for half solution at the moment is that you can't safely >> travel the lists and may crash on an invalid pointer. > > So, fix the cpufreq-dt instead of moving the hack inside OPP driver. I started there, but it is not only cpufreq-dt that got it wrong. I considered changing individual drivers (Viresh also suggested adding _locked() variant API), but decided patching opp was less invasive for now. > >> >> Going forward I think (I mentioned that in my other email) that we >> should rework the OPP API so that callers fetch OPP table object for a >> device at init/probe time and then use it to get OPPs. This way won't >> have to travel two lists any time we want to reference an OPP. >> >> And instead of relying notifiers, maybe look into using OPP tables >> directly in cpufreq drivers instead of converting OPP into static-ish >> cpufreq tables. >> > > If you'd like a proper fix for OPP usage, I am all open to see such a > proposal that works not just for cpufreq, but also for devfreq as well. Yeah, let's see what kind of time I have ;) Thanks. -- Dmitry -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/