linux-kernel - [PATCH V3 1/2] base/drivers/arch_topology: Replace mutex with READ_ONCE / WRITE

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Message-Id: <1543221866-19671-1-git-send-email-daniel.lezcano@linaro.org>
Date:   Mon, 26 Nov 2018 09:44:20 +0100
From:   Daniel Lezcano <daniel.lezcano@...aro.org>
To:     rjw@...ysocki.net
Cc:     linux-kernel@...r.kernel.org, viresh.kumar@...aro.org,
        Sudeep Holla <sudeep.holla@....com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Juri Lelli <juri.lelli@....com>
Subject: [PATCH V3 1/2] base/drivers/arch_topology: Replace mutex with READ_ONCE / WRITE_ONCE

The mutex protects a per_cpu variable access. The potential race can
happen only when the cpufreq governor module is loaded and at the same
time the cpu capacity is changed in the sysfs.

There is no real interest of using a mutex to protect a variable
assignation when there is no situation where a task can take the lock
and block.

Replace the mutex by READ_ONCE / WRITE_ONCE.

Signed-off-by: Daniel Lezcano <daniel.lezcano@...aro.org>
Cc: Sudeep Holla <sudeep.holla@....com>
Reviewed-by: Viresh Kumar <viresh.kumar@...aro.org>
---
 drivers/base/arch_topology.c  | 7 +------
 include/linux/arch_topology.h | 2 +-
 2 files changed, 2 insertions(+), 7 deletions(-)

diff --git a/drivers/base/arch_topology.c b/drivers/base/arch_topology.c
index edfcf8d..fd5325b 100644
--- a/drivers/base/arch_topology.c
+++ b/drivers/base/arch_topology.c
@@ -31,12 +31,11 @@ void arch_set_freq_scale(struct cpumask *cpus, unsigned long cur_freq,
 		per_cpu(freq_scale, i) = scale;
 }
 
-static DEFINE_MUTEX(cpu_scale_mutex);
 DEFINE_PER_CPU(unsigned long, cpu_scale) = SCHED_CAPACITY_SCALE;
 
 void topology_set_cpu_scale(unsigned int cpu, unsigned long capacity)
 {
-	per_cpu(cpu_scale, cpu) = capacity;
+	WRITE_ONCE(per_cpu(cpu_scale, cpu), capacity);
 }
 
 static ssize_t cpu_capacity_show(struct device *dev,
@@ -71,10 +70,8 @@ static ssize_t cpu_capacity_store(struct device *dev,
 	if (new_capacity > SCHED_CAPACITY_SCALE)
 		return -EINVAL;
 
-	mutex_lock(&cpu_scale_mutex);
 	for_each_cpu(i, &cpu_topology[this_cpu].core_sibling)
 		topology_set_cpu_scale(i, new_capacity);
-	mutex_unlock(&cpu_scale_mutex);
 
 	schedule_work(&update_topology_flags_work);
 
@@ -141,7 +138,6 @@ void topology_normalize_cpu_scale(void)
 		return;
 
 	pr_debug("cpu_capacity: capacity_scale=%u\n", capacity_scale);
-	mutex_lock(&cpu_scale_mutex);
 	for_each_possible_cpu(cpu) {
 		pr_debug("cpu_capacity: cpu=%d raw_capacity=%u\n",
 			 cpu, raw_capacity[cpu]);
@@ -151,7 +147,6 @@ void topology_normalize_cpu_scale(void)
 		pr_debug("cpu_capacity: CPU%d cpu_capacity=%lu\n",
 			cpu, topology_get_cpu_scale(NULL, cpu));
 	}
-	mutex_unlock(&cpu_scale_mutex);
 }
 
 bool __init topology_parse_cpu_capacity(struct device_node *cpu_node, int cpu)
diff --git a/include/linux/arch_topology.h b/include/linux/arch_topology.h
index d9bdc1a..12c439f 100644
--- a/include/linux/arch_topology.h
+++ b/include/linux/arch_topology.h
@@ -20,7 +20,7 @@ struct sched_domain;
 static inline
 unsigned long topology_get_cpu_scale(struct sched_domain *sd, int cpu)
 {
-	return per_cpu(cpu_scale, cpu);
+	return READ_ONCE(per_cpu(cpu_scale, cpu));
 }
 
 void topology_set_cpu_scale(unsigned int cpu, unsigned long capacity);
-- 
2.7.4