Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751872AbaLXRhV (ORCPT ); Wed, 24 Dec 2014 12:37:21 -0500 Received: from devils.ext.ti.com ([198.47.26.153]:35577 "EHLO devils.ext.ti.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751369AbaLXRhR (ORCPT ); Wed, 24 Dec 2014 12:37:17 -0500 Message-ID: <549AF9C0.7010602@ti.com> Date: Wed, 24 Dec 2014 11:37:04 -0600 From: Nishanth Menon User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.3.0 MIME-Version: 1.0 To: Dmitry Torokhov CC: "Rafael J. Wysocki" , Viresh Kumar , Thomas Petazzoni , Geert Uytterhoeven , Stefan Wahren , Paul Gortmaker , "linux-pm@vger.kernel.org" , "linux-kernel@vger.kernel.org" Subject: Re: [PATCH 3/4] PM / OPP: take RCU lock in dev_pm_opp_get_opp_count 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> In-Reply-To: Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. > > 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. -- Regards, Nishanth Menon -- 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/