linux-kernel - [PATCH 2/5] cpufreq, fix locking around CPUFREQ_GOV_POLICY

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Date:	Wed,  5 Nov 2014 09:53:56 -0500
From:	Prarit Bhargava <prarit@...hat.com>
To:	linux-kernel@...r.kernel.org
Cc:	robert.schoene@...dresden.de, sboyd@...eaurora.org,
	Prarit Bhargava <prarit@...hat.com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	linux-pm@...r.kernel.org
Subject: [PATCH 2/5] cpufreq, fix locking around CPUFREQ_GOV_POLICY_EXIT calls

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" <rjw@...ysocki.net>
Cc: Viresh Kumar <viresh.kumar@...aro.org>
Cc: linux-pm@...r.kernel.org
Signed-off-by: Prarit Bhargava <prarit@...hat.com>
---
 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

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