linux-kernel - [PATCH 36/70] cpufreq: interactive: fix race on timer restart on governor start

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Date:	Tue, 27 Oct 2015 18:30:24 +0100
From:	Bálint Czobor <czoborbalint@...il.com>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Viresh Kumar <viresh.kumar@...aro.org>
Cc:	linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
	Todd Poynor <toddpoynor@...gle.com>,
	Bálint Czobor <czoborbalint@...il.com>
Subject: [PATCH 36/70] cpufreq: interactive: fix race on timer restart on governor start

From: Todd Poynor <toddpoynor@...gle.com>

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@...udcar.com>
Signed-off-by: Todd Poynor <toddpoynor@...gle.com>
Signed-off-by: Bálint Czobor <czoborbalint@...il.com>
---
 drivers/cpufreq/cpufreq_interactive.c |    6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/cpufreq/cpufreq_interactive.c b/drivers/cpufreq/cpufreq_interactive.c
index 74f5609..d22d162 100644
--- a/drivers/cpufreq/cpufreq_interactive.c
+++ b/drivers/cpufreq/cpufreq_interactive.c
@@ -919,17 +919,17 @@ 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);
 		}
 
 		/*
-- 
1.7.9.5

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