[PATCH v2] cpufreq: fix governor start/stop race condition

[Xiaoguang Chen]

cpufreq governor stop and start should be kept in sequence.
If not, there will be unexpected behavior, for example:

we have 4 cpus and policy->cpu=cpu0, cpu1/2/3 are linked to cpu0.
the normal sequence is as below:

1) Current governor is userspace, one application tries to set
governor to ondemand. it will call __cpufreq_set_policy in which it
will stop userspace governor and then start ondemand governor.

2) Current governor is userspace, now cpu0 hotplugs in cpu3, it will
call cpufreq_add_policy_cpu. on which it first stops userspace
governor, and then starts userspace governor.

Now if the sequence of above two cases interleaves, it becames
below sequence:

1) application stops userspace governor
2)                                      hotplug stops userspace governor
3) application starts ondemand governor
4)                                      hotplug starts a governor

in step 4, hotplug is supposed to start userspace governor, but now
the governor has been changed by application to ondemand, so hotplug
starts ondemand governor again !!!!

The solution is: do not allow stop governor multi-times
Governor stop should only do once, after it is stopped,
no other governor stop should be executed.

Signed-off-by: Xiaoguang Chen <chenxg@xxxxxxxxxxx>
---
 drivers/cpufreq/cpufreq.c | 10 +++++++++-
 include/linux/cpufreq.h   |  1 +
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
index 2d53f47..c8d7cb2 100644
--- a/drivers/cpufreq/cpufreq.c
+++ b/drivers/cpufreq/cpufreq.c
@@ -1562,6 +1562,11 @@ static int __cpufreq_governor(struct cpufreq_policy *policy,
 
 	pr_debug("__cpufreq_governor for CPU %u, event %u\n",
 						policy->cpu, event);
+
+	if ((!policy->governor->enabled && (event == CPUFREQ_GOV_STOP)) ||
+	    (policy->governor->enabled && (event == CPUFREQ_GOV_START)))
+		return 0;
+
 	ret = policy->governor->governor(policy, event);
 
 	if (!ret) {
@@ -1569,6 +1574,10 @@ static int __cpufreq_governor(struct cpufreq_policy *policy,
 			policy->governor->initialized++;
 		else if (event == CPUFREQ_GOV_POLICY_EXIT)
 			policy->governor->initialized--;
+		else if (event == CPUFREQ_GOV_STOP)
+			policy->governor->enabled = 0;
+		else if (event == CPUFREQ_GOV_START)
+			policy->governor->enabled = 1;
 	}
 
 	/* we keep one module reference alive for
@@ -1581,7 +1590,6 @@ static int __cpufreq_governor(struct cpufreq_policy *policy,
 	return ret;
 }
 
-
 int cpufreq_register_governor(struct cpufreq_governor *governor)
 {
 	int err;
diff --git a/include/linux/cpufreq.h b/include/linux/cpufreq.h
index 037d36a..16c5b70 100644
--- a/include/linux/cpufreq.h
+++ b/include/linux/cpufreq.h
@@ -199,6 +199,7 @@ struct cpufreq_governor {
 			will fallback to performance governor */
 	struct list_head	governor_list;
 	struct module		*owner;
+	int enabled;
 };
 
 /*
-- 
1.8.0

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