From mboxrd@z Thu Jan 1 00:00:00 1970 From: Viresh Kumar Subject: Re: [PATCH 4/5] cpufreq: Don't drop rwsem before calling CPUFREQ_GOV_POLICY_EXIT Date: Wed, 3 Feb 2016 11:21:26 +0530 Message-ID: <20160203055126.GR31828@vireshk> References: <3d58d4f0bec5e8e941cf96e159c8c21e68956cad.1454410226.git.viresh.kumar@linaro.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Received: from mail-pa0-f51.google.com ([209.85.220.51]:36550 "EHLO mail-pa0-f51.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751341AbcBCFvb (ORCPT ); Wed, 3 Feb 2016 00:51:31 -0500 Received: by mail-pa0-f51.google.com with SMTP id yy13so7494554pab.3 for ; Tue, 02 Feb 2016 21:51:30 -0800 (PST) Content-Disposition: inline In-Reply-To: Sender: linux-pm-owner@vger.kernel.org List-Id: linux-pm@vger.kernel.org To: "Rafael J. Wysocki" Cc: Rafael Wysocki , Juri Lelli , Lists linaro-kernel , "linux-pm@vger.kernel.org" , Saravana Kannan , Peter Zijlstra , Michael Turquette , Steve Muckle , Vincent Guittot , Morten Rasmussen , dietmar.eggemann@arm.com, Linux Kernel Mailing List On 02-02-16, 22:53, Rafael J. Wysocki wrote: > First of all, this is effectively reverting commit 955ef4833574, so > the subject should be "Revert commit 955ef4833574 (cpufreq: Drop rwsem > lock around CPUFREQ_GOV_POLICY_EXIT)". > > There should be a Fixes: tag pointing to commit 955ef4833574 and a > Reported-by: for Juri. > > If there is a link to a bug report related to this, it should be > pointed to by a Link: tag. > > The changelog should say why the original commit was there and why the > way it attempted to solve the problem was incorrect. It also should > say that the original problem was addressed by a previous commit, so > this one can be reverted without consequences. How about this: Revert "cpufreq: Drop rwsem lock around CPUFREQ_GOV_POLICY_EXIT" Earlier, when the struct freq-attr was used to represent governor attributes, the standard cpufreq show/store sysfs attribute callbacks were applied to the governor tunable attributes and they always acquire the policy->rwsem lock before carrying out the operation. That could have resulted in an ABBA deadlock if governor tunable attributes are removed under policy->rwsem while one of them is being accessed concurrently (if sysfs attributes removal wins the race, it will wait for the access to complete with policy->rwsem held while the attribute callback will block on policy->rwsem indefinitely). We attempted to address this issue by dropping policy->rwsem around governor tunable attributes removal (that is, around invocations of the ->governor callback with the event arg equal to CPUFREQ_GOV_POLICY_EXIT) in cpufreq_set_policy(), but that opened up race conditions that had not been possible with policy->rwsem held all the time. The previous commit, "cpufreq: governor: New sysfs show/store callbacks for governor tunables", fixed the original ABBA deadlock by adding new governor specific show/store callbacks. We don't have to drop rwsem around invocations of governor event CPUFREQ_GOV_POLICY_EXIT anymore, and original fix can be reverted now. Fixes: 955ef4833574 ("cpufreq: Drop rwsem lock around CPUFREQ_GOV_POLICY_EXIT") Reported-by: Juri Lelli Signed-off-by: Viresh Kumar > But I'm not going to write that changelog. I actually am not going to > write any changelogs for you any more, because I'm seriously tired of > doing that. Moreover, if I see a patch from you with a changelog > that's not acceptable to me, it will immediately go to the "not > applicable" trash bin no matter what the changes below look like. You > *have* *to* write useful changelogs. This isn't optional or best > effort. This is mandatory and important. Will try to improve, sorry about that (again). > Now, I'm not really sure if the ordering of this patchset is right. > Maybe we should just revert upfront with the "we'll address the > original problem in the following commits" statement in the changelog > and fix it in a different way? Wouldn't that break things like 'git bisect'? People running kernels after the reverted commits may start hitting lockdeps. > It looks like patches [1-3/5] fix a > problem that isn't there even, but would appear after the [4/5] if > they were not applied previously, which doesn't sound really > straightforward to me. I am going to fight hard for it, if you feel 4/5 should be the first patch here, I will do that. -- viresh