PM / Domains: Implement subdomain counters as atomic fields

Currently, pm_genpd_poweron() and pm_genpd_poweroff() need to take
the parent PM domain's lock in order to modify the parent's counter
of active subdomains in a nonracy way.  This causes the locking to be
considerably complex and in fact is not necessary, because the
subdomain counters may be implemented as atomic fields and they
won't have to be modified under a lock.

Replace the unsigned in sd_count field in struct generic_pm_domain
by an atomic_t one and modify the code in drivers/base/power/domain.c
to take this change into account.

This patch doesn't change the locking yet, that is going to be done
in a separate subsequent patch.
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>

drivers/base/power/domain.c

@@ -29,10 +29,20 @@ static struct generic_pm_domain *dev_to_genpd(struct device *dev)
 	return pd_to_genpd(dev->pm_domain);
 }
 
-static void genpd_sd_counter_dec(struct generic_pm_domain *genpd)
+static bool genpd_sd_counter_dec(struct generic_pm_domain *genpd)
 {
-	if (!WARN_ON(genpd->sd_count == 0))
-			genpd->sd_count--;
+	bool ret = false;
+
+	if (!WARN_ON(atomic_read(&genpd->sd_count) == 0))
+		ret = !!atomic_dec_and_test(&genpd->sd_count);
+
+	return ret;
+}
+
+static void genpd_sd_counter_inc(struct generic_pm_domain *genpd)
+{
+	atomic_inc(&genpd->sd_count);
+	smp_mb__after_atomic_inc();
 }
 
 static void genpd_acquire_lock(struct generic_pm_domain *genpd)
@@ -118,7 +128,7 @@ int pm_genpd_poweron(struct generic_pm_domain *genpd)
 
 	genpd_set_active(genpd);
 	if (parent)
-		parent->sd_count++;
+		genpd_sd_counter_inc(parent);
 
  out:
 	mutex_unlock(&genpd->lock);
@@ -254,7 +264,7 @@ static int pm_genpd_poweroff(struct generic_pm_domain *genpd)
 	    || genpd->resume_count > 0)
 		return 0;
 
-	if (genpd->sd_count > 0)
+	if (atomic_read(&genpd->sd_count) > 0)
 		return -EBUSY;
 
 	not_suspended = 0;
@@ -325,8 +335,7 @@ static int pm_genpd_poweroff(struct generic_pm_domain *genpd)
 	genpd->status = GPD_STATE_POWER_OFF;
 
 	if (parent) {
-		genpd_sd_counter_dec(parent);
-		if (parent->sd_count == 0)
+		if (genpd_sd_counter_dec(parent))
 			genpd_queue_power_off_work(parent);
 
 		genpd_release_lock(parent);
@@ -506,7 +515,8 @@ static void pm_genpd_sync_poweroff(struct generic_pm_domain *genpd)
 	if (genpd->status == GPD_STATE_POWER_OFF)
 		return;
 
-	if (genpd->suspended_count != genpd->device_count || genpd->sd_count > 0)
+	if (genpd->suspended_count != genpd->device_count
+	    || atomic_read(&genpd->sd_count) > 0)
 		return;
 
 	if (genpd->power_off)
@@ -1167,7 +1177,7 @@ int pm_genpd_add_subdomain(struct generic_pm_domain *genpd,
 	list_add_tail(&new_subdomain->sd_node, &genpd->sd_list);
 	new_subdomain->parent = genpd;
 	if (subdomain->status != GPD_STATE_POWER_OFF)
-		genpd->sd_count++;
+		genpd_sd_counter_inc(genpd);
 
  out:
 	mutex_unlock(&new_subdomain->lock);
@@ -1242,7 +1252,7 @@ void pm_genpd_init(struct generic_pm_domain *genpd,
 	genpd->gov = gov;
 	INIT_WORK(&genpd->power_off_work, genpd_power_off_work_fn);
 	genpd->in_progress = 0;
-	genpd->sd_count = 0;
+	atomic_set(&genpd->sd_count, 0);
 	genpd->status = is_off ? GPD_STATE_POWER_OFF : GPD_STATE_ACTIVE;
 	init_waitqueue_head(&genpd->status_wait_queue);
 	genpd->poweroff_task = NULL;

include/linux/pm_domain.h

@@ -33,7 +33,7 @@ struct generic_pm_domain {
 	struct dev_power_governor *gov;
 	struct work_struct power_off_work;
 	unsigned int in_progress;	/* Number of devices being suspended now */
-	unsigned int sd_count;	/* Number of subdomains with power "on" */
+	atomic_t sd_count;	/* Number of subdomains with power "on" */
 	enum gpd_status status;	/* Current state of the domain */
 	wait_queue_head_t status_wait_queue;
 	struct task_struct *poweroff_task;	/* Powering off task */