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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: Pan Fang <fangpan@codeaurora.org>
Signed-off-by: Arun KS <arunks@codeaurora.org>
Signed-off-by: Junjie Wu <junjiew@codeaurora.org>
Suggested-by: Saravana Kannan <skannan@codeaurora.org>
This commit is contained in:
Junjie Wu 2015-03-20 14:41:50 -07:00
parent 1d3e459d52
commit f28723bccc
1 changed files with 6 additions and 5 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

11
drivers/devfreq/governor_cpufreq.c
View File

@ -1,5 +1,5 @@
/*
* 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
@ -52,6 +52,7 @@ 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, \
@ -239,7 +240,7 @@ 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;
@ -270,7 +271,7 @@ out:
cnt_not_zero:
if (!ret)
cpufreq_cnt++;
mutex_unlock(&state_lock);
mutex_unlock(&cpufreq_reg_lock);
return ret;
}
@ -279,7 +280,7 @@ 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;
@ -299,7 +300,7 @@ static int unregister_cpufreq(void)
out:
cpufreq_cnt--;
mutex_unlock(&state_lock);
mutex_unlock(&cpufreq_reg_lock);
return ret;
}