linux-kernel - Re: [PATCH v2 2/2] cpufreq: Specify default governor on command line

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Date:   Thu, 25 Jun 2020 14:20:52 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Quentin Perret <qperret@...gle.com>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Arnd Bergmann <arnd@...db.de>,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        "Cc: Android Kernel" <kernel-team@...roid.com>,
        Todd Kjos <tkjos@...gle.com>, adharmap@...eaurora.org
Subject: Re: [PATCH v2 2/2] cpufreq: Specify default governor on command line

On 24-06-20, 16:32, Quentin Perret wrote:
> Right, but I must admit that, looking at this more, I'm getting a bit
> confused with the overall locking for governors :/
> 
> When in cpufreq_init_policy() we find a governor using
> find_governor(policy->last_governor), what guarantees this governor is
> not concurrently unregistered? That is, what guarantees this governor
> doesn't go away between that find_governor() call, and the subsequent
> call to try_module_get() in cpufreq_set_policy() down the line?
> 
> Can we somewhat assume that whatever governor is referred to by
> policy->last_governor will have a non-null refcount? Or are the
> cpufreq_online() and cpufreq_unregister_governor() path mutually
> exclusive? Or is there something else?

This should be sufficient to fix pending issues I believe. Based over your
patches.

-- 
viresh

-------------------------8<-------------------------
From: Viresh Kumar <viresh.kumar@...aro.org>
Date: Thu, 25 Jun 2020 13:15:23 +0530
Subject: [PATCH] cpufreq: Fix locking issues with governors

The locking around governors handling isn't adequate currently. The list
of governors should never be traversed without locking in place. Also we
must make sure the governor isn't removed while it is still referenced
by code.

Reported-by: Quentin Perret <qperret@...gle.com>
Signed-off-by: Viresh Kumar <viresh.kumar@...aro.org>
---
 drivers/cpufreq/cpufreq.c | 59 ++++++++++++++++++++++++---------------
 1 file changed, 36 insertions(+), 23 deletions(-)

diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
index 4b1a5c0173cf..dad6b85f4c89 100644
--- a/drivers/cpufreq/cpufreq.c
+++ b/drivers/cpufreq/cpufreq.c
@@ -624,6 +624,24 @@ static struct cpufreq_governor *find_governor(const char *str_governor)
 	return NULL;
 }
 
+static struct cpufreq_governor *get_governor(const char *str_governor)
+{
+	struct cpufreq_governor *t;
+
+	mutex_lock(&cpufreq_governor_mutex);
+	t = find_governor(str_governor);
+	if (!t)
+		goto unlock;
+
+	if (!try_module_get(t->owner))
+		t = NULL;
+
+unlock:
+	mutex_unlock(&cpufreq_governor_mutex);
+
+	return t;
+}
+
 static unsigned int cpufreq_parse_policy(char *str_governor)
 {
 	if (!strncasecmp(str_governor, "performance", CPUFREQ_NAME_LEN))
@@ -643,28 +661,14 @@ static struct cpufreq_governor *cpufreq_parse_governor(char *str_governor)
 {
 	struct cpufreq_governor *t;
 
-	mutex_lock(&cpufreq_governor_mutex);
-
-	t = find_governor(str_governor);
-	if (!t) {
-		int ret;
-
-		mutex_unlock(&cpufreq_governor_mutex);
-
-		ret = request_module("cpufreq_%s", str_governor);
-		if (ret)
-			return NULL;
-
-		mutex_lock(&cpufreq_governor_mutex);
+	t = get_governor(str_governor);
+	if (t)
+		return t;
 
-		t = find_governor(str_governor);
-	}
-	if (t && !try_module_get(t->owner))
-		t = NULL;
-
-	mutex_unlock(&cpufreq_governor_mutex);
+	if (request_module("cpufreq_%s", str_governor))
+		return NULL;
 
-	return t;
+	return get_governor(str_governor);
 }
 
 /**
@@ -818,12 +822,14 @@ static ssize_t show_scaling_available_governors(struct cpufreq_policy *policy,
 		goto out;
 	}
 
+	mutex_lock(&cpufreq_governor_mutex);
 	for_each_governor(t) {
 		if (i >= (ssize_t) ((PAGE_SIZE / sizeof(char))
 		    - (CPUFREQ_NAME_LEN + 2)))
-			goto out;
+			break;
 		i += scnprintf(&buf[i], CPUFREQ_NAME_PLEN, "%s ", t->name);
 	}
+	mutex_unlock(&cpufreq_governor_mutex);
 out:
 	i += sprintf(&buf[i], "\n");
 	return i;
@@ -1060,11 +1066,14 @@ static int cpufreq_init_policy(struct cpufreq_policy *policy)
 {
 	struct cpufreq_governor *gov = NULL;
 	unsigned int pol = CPUFREQ_POLICY_UNKNOWN;
+	bool put_governor = false;
+	int ret;
 
 	if (has_target()) {
 		/* Update policy governor to the one used before hotplug. */
-		gov = find_governor(policy->last_governor);
+		gov = get_governor(policy->last_governor);
 		if (gov) {
+			put_governor = true;
 			pr_debug("Restoring governor %s for cpu %d\n",
 				 policy->governor->name, policy->cpu);
 		} else if (default_governor) {
@@ -1091,7 +1100,11 @@ static int cpufreq_init_policy(struct cpufreq_policy *policy)
 			return -ENODATA;
 	}
 
-	return cpufreq_set_policy(policy, gov, pol);
+	ret = cpufreq_set_policy(policy, gov, pol);
+	if (put_governor)
+		module_put(gov->owner);
+
+	return ret;
 }
 
 static int cpufreq_add_policy_cpu(struct cpufreq_policy *policy, unsigned int cpu)