tree 3d3e9735e2654110afd078b68dfa83ed955260e9
parent cd8a2954001cc4378edc7e62fbff76c24b21cfcf
author Junjie Wu <junjiew@codeaurora.org> 1426887710 -0700
committer Rohit Gupta <rohgup@codeaurora.org> 1490748710 -0700

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>