PM / devfreq: memlat: Prevent deadlock with hotplug in start_hwmon

When start_hwmon() runs with another thread trying to hotplug a
CPU the two threads can enter a deadlock situation as follows:

Thread A (start_hwmon())		Thread B (CPU down)
get_online_cpus()
	|
atomic_inc(&cpu_hotplug.refcount)
					     CPU down
						|
				     mutex_lock(&cpu_add_remove_lock)
						|
				     cpu_hotplug_begin() waits on
				     cpu_hotplug.refcount to reset
register_cpu_notifier()
	|
mutex_lock(&cpu_add_remove_lock)

With this change the notifers are registered and unregistered per
device rather than having a common notifier block for all the memlat
devices and unregistration only happens on stop_hwmon. This makes it
possible to move the registration outside the hotplug lock without
any race between multiple memlat devices.

Change-Id: I6ad561fe4967042e45190aea2c9b7fcfe05bafdd
Signed-off-by: Rohit Gupta <rohgup@codeaurora.org>

1 files changed, 7 insertions, 39 deletions

diff --git a/drivers/devfreq/arm-memlat-mon.c b/drivers/devfreq/arm-memlat-mon.c
index d69c2186c5da..370d7d95042b 100644
--- a/drivers/devfreq/arm-memlat-mon.c
+++ b/drivers/devfreq/arm-memlat-mon.c
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2014-2015, The Linux Foundation. All rights reserved.
+ * Copyright (c) 2014-2016, The Linux Foundation. All rights reserved.
  *
  * This program is free software; you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 2 and
@@ -57,14 +57,9 @@ static DEFINE_PER_CPU(struct memlat_hwmon_data, pm_data);
 struct cpu_grp_info {
 	cpumask_t cpus;
 	struct memlat_hwmon hw;
+	struct notifier_block arm_memlat_cpu_notif;
 };
 
-static DEFINE_MUTEX(notif_mutex);
-static unsigned int notif_refcount;
-static void notif_unregister_work_fn(struct work_struct *work);
-static void hotplug_notif_unregister(void);
-static DECLARE_WORK(notif_unregister_work, notif_unregister_work_fn);
-
 static unsigned long compute_freq(struct memlat_hwmon_data *hw_data,
 						unsigned long cyc_cnt)
 {
@@ -155,7 +150,7 @@ static void stop_hwmon(struct memlat_hwmon *hw)
 	for_each_cpu(cpu, &cpu_grp->cpus) {
 		hw_data = &per_cpu(pm_data, cpu);
 		if (hw_data->init_pending)
-			hotplug_notif_unregister();
+			hw_data->init_pending = false;
 		else
 			delete_events(hw_data);
 
@@ -167,6 +162,7 @@ static void stop_hwmon(struct memlat_hwmon *hw)
 	}
 	put_online_cpus();
 
+	unregister_cpu_notifier(&cpu_grp->arm_memlat_cpu_notif);
 }
 
 static struct perf_event_attr *alloc_attr(void)
@@ -239,40 +235,10 @@ static int arm_memlat_cpu_callback(struct notifier_block *nb,
 		pr_warn("Failed to create perf event for CPU%lu\n", cpu);
 
 	hw_data->init_pending = false;
-	hotplug_notif_unregister();
 
 	return NOTIFY_OK;
 }
 
-static struct notifier_block arm_memlat_cpu_notifier = {
-	.notifier_call = arm_memlat_cpu_callback,
-};
-
-static void notif_unregister_work_fn(struct work_struct *work)
-{
-	unregister_cpu_notifier(&arm_memlat_cpu_notifier);
-}
-
-static void hotplug_notif_register(void)
-{
-	mutex_lock(&notif_mutex);
-	if (!notif_refcount) {
-		flush_work(&notif_unregister_work);
-		register_cpu_notifier(&arm_memlat_cpu_notifier);
-	}
-	notif_refcount++;
-	mutex_unlock(&notif_mutex);
-}
-
-static void hotplug_notif_unregister(void)
-{
-	mutex_lock(&notif_mutex);
-	notif_refcount--;
-	if (!notif_refcount)
-		schedule_work(&notif_unregister_work);
-	mutex_unlock(&notif_mutex);
-}
-
 static int start_hwmon(struct memlat_hwmon *hw)
 {
 	int cpu, ret = 0;
@@ -280,13 +246,14 @@ static int start_hwmon(struct memlat_hwmon *hw)
 	struct cpu_grp_info *cpu_grp = container_of(hw,
 					struct cpu_grp_info, hw);
 
+	register_cpu_notifier(&cpu_grp->arm_memlat_cpu_notif);
+
 	get_online_cpus();
 	for_each_cpu(cpu, &cpu_grp->cpus) {
 		hw_data = &per_cpu(pm_data, cpu);
 		ret = set_events(hw_data, cpu);
 		if (ret) {
 			if (!cpu_online(cpu)) {
-				hotplug_notif_register();
 				hw_data->init_pending = true;
 				ret = 0;
 			} else {
@@ -337,6 +304,7 @@ static int arm_memlat_mon_driver_probe(struct platform_device *pdev)
 	cpu_grp = devm_kzalloc(dev, sizeof(*cpu_grp), GFP_KERNEL);
 	if (!cpu_grp)
 		return -ENOMEM;
+	cpu_grp->arm_memlat_cpu_notif.notifier_call = arm_memlat_cpu_callback;
 	hw = &cpu_grp->hw;
 
 	hw->dev = dev;