cpufreq: interactive: fix race on timer restart on governor start

Starting the governor, or restarting on a hotplugged-in CPU, can race
with the timer start in idle, triggering a BUG on timer already pending.
Start the timer before setting the enable flag, and use enable_sem to
protect the sequence (and ensure correct order of the update to the
enable flag).  Delete any existing timer for safety.

Change-Id: Ife77cf9fe099e8fd8543224cbf148c6722c2ffb0
Reported-by: Francisco Franco <francisco.franco@cloudcar.com>
Signed-off-by: Todd Poynor <toddpoynor@google.com>

1 files changed, 3 insertions, 3 deletions

diff --git a/drivers/cpufreq/cpufreq_interactive.c b/drivers/cpufreq/cpufreq_interactive.c
index f3d187604d9..4dc7f04391a 100644
--- a/drivers/cpufreq/cpufreq_interactive.c
+++ b/drivers/cpufreq/cpufreq_interactive.c
@@ -918,17 +918,17 @@ static int cpufreq_governor_interactive(struct cpufreq_policy *policy,
                                 ktime_to_us(ktime_get());
                         pcpu->hispeed_validate_time =
                                 pcpu->floor_validate_time;
-                        pcpu->governor_enabled = 1;
-                        smp_wmb();
+                        down_write(&pcpu->enable_sem);
                         expires = jiffies + usecs_to_jiffies(timer_rate);
                         pcpu->cpu_timer.expires = expires;
                         add_timer_on(&pcpu->cpu_timer, j);
-
                         if (timer_slack_val >= 0) {
                                 expires += usecs_to_jiffies(timer_slack_val);
                                 pcpu->cpu_slack_timer.expires = expires;
                                 add_timer_on(&pcpu->cpu_slack_timer, j);
                         }
+                        pcpu->governor_enabled = 1;
+                        up_write(&pcpu->enable_sem);
                 }
 
                 /*