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Commit 65924187 authored by Junjie Wu's avatar Junjie Wu Committed by Rohit Gupta
Browse files

PM / devfreq: governor_cpufreq: Rewrite locking to avoid deadlocks 



A devfreq governor store in parallel with a cpu freq update can cause
deadlock as shown below.

Assume current devfreq governor is cpufreq, and user tries to change
to some other governor.

Write to sysfs store_governor   | cpufreq driver updating cpu freq
------------------------------- | -----------------------------------
echo bw_hwmon > governor        |
                                | takes rcu_read_lock and calls all
                                | cpufreq transition callbacks for
                                | PRECHANGE or POSTCHANGE
                                |
GOV_STOP on governor_cpufreq.   |
unregister_cpufreq() accquires  |
state_lock mutex.               |
                                | try to accquire same state_lock in
                                | cpufreq_trans_notifier(). Blocked.
unregister from cpufreq         |
transition notifier and wait for|
all rcu_readers to finish.      |
                            Deadlock

A similar deadlock can happen with governor change and policy notifier
callbacks.

The state_lock currently protects multiple unrelated critical
sections: registering/unregistering of cpufreq notifiers, read/writing
the device list, and tracking the cpu states and updating device
frequencies. There is no need for register/unregister of the cpufreq
notifiers to be mutually excluded against the other critical sections
using the same lock.

Split state_lock into two locks to protect the register/unregister of
cpufreq notifiers from the rest of the critical sections.

Change-Id: Id06d326748a5cb0c84c4787da5d0910f44eb5c3c
Signed-off-by: default avatarPan Fang <fangpan@codeaurora.org>
Signed-off-by: default avatarArun KS <arunks@codeaurora.org>
Signed-off-by: default avatarJunjie Wu <junjiew@codeaurora.org>
Suggested-by: default avatarSaravana Kannan <skannan@codeaurora.org>
parent cd8a2954

drivers/devfreq/governor_cpufreq.c

+6 −5
Original line number Diff line number Diff line 
/*

 * Copyright (c) 2014, The Linux Foundation. All rights reserved.

 * Copyright (c) 2014-2015, The Linux Foundation. All rights reserved.

 *

 * This program is free software; you can redistribute it and/or modify

 * it under the terms of the GNU General Public License version 2 and
@@ -53,6 +53,7 @@ struct devfreq_node { 
};

static LIST_HEAD(devfreq_list);

static DEFINE_MUTEX(state_lock);

static DEFINE_MUTEX(cpufreq_reg_lock);



#define show_attr(name) \

static ssize_t show_##name(struct device *dev,				\
@@ -240,7 +241,7 @@ static int register_cpufreq(void) 
	unsigned int cpu;

	struct cpufreq_policy *policy;



	mutex_lock(&state_lock);

	mutex_lock(&cpufreq_reg_lock);



	if (cpufreq_cnt)

		goto cnt_not_zero;
@@ -271,7 +272,7 @@ static int register_cpufreq(void) 
cnt_not_zero:

	if (!ret)

		cpufreq_cnt++;

	mutex_unlock(&state_lock);

	mutex_unlock(&cpufreq_reg_lock);

	return ret;

}
@@ -280,7 +281,7 @@ static int unregister_cpufreq(void) 
	int ret = 0;

	int cpu;



	mutex_lock(&state_lock);

	mutex_lock(&cpufreq_reg_lock);



	if (cpufreq_cnt > 1)

		goto out;
@@ -300,7 +301,7 @@ static int unregister_cpufreq(void) 


out:

	cpufreq_cnt--;

	mutex_unlock(&state_lock);

	mutex_unlock(&cpufreq_reg_lock);

	return ret;

}