From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755608AbaKEO7a (ORCPT ); Wed, 5 Nov 2014 09:59:30 -0500 Received: from mx1.redhat.com ([209.132.183.28]:50939 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755347AbaKEOyT (ORCPT ); Wed, 5 Nov 2014 09:54:19 -0500 From: Prarit Bhargava To: linux-kernel@vger.kernel.org Cc: robert.schoene@tu-dresden.de, sboyd@codeaurora.org, Prarit Bhargava , "Rafael J. Wysocki" , Viresh Kumar , linux-pm@vger.kernel.org Subject: [PATCH 2/5] cpufreq, fix locking around CPUFREQ_GOV_POLICY_EXIT calls Date: Wed, 5 Nov 2014 09:53:56 -0500 Message-Id: <1415199239-19019-3-git-send-email-prarit@redhat.com> In-Reply-To: <1415199239-19019-1-git-send-email-prarit@redhat.com> References: <1415199239-19019-1-git-send-email-prarit@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org commit 955ef4833574636819cd269cfbae12f79cbde63a (" cpufreq: Drop rwsem lock around CPUFREQ_GOV_POLICY_EXIT") opens up a hole in the locking scheme for cpufreq. Simple tests such as rapidly switching the governor between ondemand and performance or attempting to read policy values while a governor switch occurs now fail with very NULL pointer warnings, sysfs namespace collisions, and system hangs. In short, the locking that policy->rwsem is supposed to provide is currently broken. The identified commit attempts to resolve a lockdep warning by removing a lock around a section of code which does a shutdown of the existing policy. The problem is that this is part of the _critical_ section of code that switches the governors and must be protected by the lock; without locking readers may access now NULL or stale data, and writes may collide with each other. With the previous patch, which now returns -EBUSY during times of contention the deadlock reported in 955ef4833574636819cd269cfbae12f79cbde63a (" cpufreq: Drop rwsem lock around CPUFREQ_GOV_POLICY_EXIT") cannot occur, so adding the locks back into this section of code is possible. Cc: "Rafael J. Wysocki" Cc: Viresh Kumar Cc: linux-pm@vger.kernel.org Signed-off-by: Prarit Bhargava --- drivers/cpufreq/cpufreq.c | 4 ---- include/linux/cpufreq.h | 4 ---- 2 files changed, 8 deletions(-) diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c index 3f09ca9..e33cb15 100644 --- a/drivers/cpufreq/cpufreq.c +++ b/drivers/cpufreq/cpufreq.c @@ -2222,9 +2222,7 @@ static int cpufreq_set_policy(struct cpufreq_policy *policy, /* end old governor */ if (old_gov) { __cpufreq_governor(policy, CPUFREQ_GOV_STOP); - up_write(&policy->rwsem); __cpufreq_governor(policy, CPUFREQ_GOV_POLICY_EXIT); - down_write(&policy->rwsem); } /* start new governor */ @@ -2233,9 +2231,7 @@ static int cpufreq_set_policy(struct cpufreq_policy *policy, if (!__cpufreq_governor(policy, CPUFREQ_GOV_START)) goto out; - up_write(&policy->rwsem); __cpufreq_governor(policy, CPUFREQ_GOV_POLICY_EXIT); - down_write(&policy->rwsem); } /* new governor failed, so re-start old one */ diff --git a/include/linux/cpufreq.h b/include/linux/cpufreq.h index 503b085..43909ad 100644 --- a/include/linux/cpufreq.h +++ b/include/linux/cpufreq.h @@ -100,10 +100,6 @@ struct cpufreq_policy { * - Any routine that will write to the policy structure and/or may take away * the policy altogether (eg. CPU hotplug), will hold this lock in write * mode before doing so. - * - * Additional rules: - * - Lock should not be held across - * __cpufreq_governor(data, CPUFREQ_GOV_POLICY_EXIT); */ struct rw_semaphore rwsem; -- 1.7.9.3