[PATCH 2/2] driver core: Fix possible supplier PM-usage counter imbalance

["Rafael J. Wysocki" <rjw@rjwysocki.net>]

From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

If a stateless device link to a certain supplier with
DL_FLAG_PM_RUNTIME set in the flags is added and then removed by the
consumer driver's probe callback, the supplier's PM-runtime usage
counter will be nonzero after that which effectively causes the
supplier to remain "always on" going forward.

Namely, device_link_add() called to add the link invokes
device_link_rpm_prepare() which notices that the consumer driver is
probing, so it increments the supplier's PM-runtime usage counter
with the assumption that the link will stay around until
pm_runtime_put_suppliers() is called by driver_probe_device(),
but if the link goes away before that point, the supplier's
PM-runtime usage counter will remain nonzero.

To prevent that from happening, first rework pm_runtime_get_suppliers()
and pm_runtime_put_suppliers() to use the rpm_active refounts of device
links and make the latter only drop rpm_active and the supplier's
PM-runtime usage counter for each link by one, unless rpm_active is
one already for it.  Next, modify device_link_add() to bump up the
new link's rpm_active refcount and the suppliers PM-runtime usage
counter by two, to prevent pm_runtime_put_suppliers(), if it is
called subsequently, from suspending the supplier prematurely (in
case its PM-runtime usage counter goes down to 0 in there).

Due to the way rpm_put_suppliers() works, this change does not
affect runtime suspend of the consumer ends of new device links (or,
generally, device links for which DL_FLAG_PM_RUNTIME has just been
set).

Fixes: e2f3cd831a28 ("driver core: Fix handling of runtime PM flags in device_link_add()")
Reported-by: Ulf Hansson <ulf.hansson@linaro.org> 
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
---

Note that the issue had been there before commit e2f3cd831a28, but it was
overlooked by that commit and this change is a fix on top of it, so make
the Fixes: tag point to commit e2f3cd831a28 (instead of an earlier one
that the patch will not be applicable to).

---
 drivers/base/core.c          |   21 ++++-----------------
 drivers/base/power/runtime.c |   27 +++++++++++++++++++++++++--
 include/linux/pm_runtime.h   |    4 ++++
 3 files changed, 33 insertions(+), 19 deletions(-)

Index: linux-pm/drivers/base/power/runtime.c
===================================================================
--- linux-pm.orig/drivers/base/power/runtime.c
+++ linux-pm/drivers/base/power/runtime.c
@@ -1655,8 +1655,10 @@ void pm_runtime_get_suppliers(struct dev
 	idx = device_links_read_lock();
 
 	list_for_each_entry_rcu(link, &dev->links.suppliers, c_node)
-		if (link->flags & DL_FLAG_PM_RUNTIME)
+		if (link->flags & DL_FLAG_PM_RUNTIME) {
+			refcount_inc(&link->rpm_active);
 			pm_runtime_get_sync(link->supplier);
+		}
 
 	device_links_read_unlock(idx);
 }
@@ -1673,7 +1675,8 @@ void pm_runtime_put_suppliers(struct dev
 	idx = device_links_read_lock();
 
 	list_for_each_entry_rcu(link, &dev->links.suppliers, c_node)
-		if (link->flags & DL_FLAG_PM_RUNTIME)
+		if (link->flags & DL_FLAG_PM_RUNTIME &&
+		    refcount_dec_not_one(&link->rpm_active))
 			pm_runtime_put(link->supplier);
 
 	device_links_read_unlock(idx);
@@ -1686,6 +1689,26 @@ void pm_runtime_new_link(struct device *
 	spin_unlock_irq(&dev->power.lock);
 }
 
+/**
+ * pm_runtime_active_link - Set up new device link as active for PM-runtime.
+ * @link: Device link to be set up as active.
+ * @supplier: Supplier end of the link.
+ *
+ * Add 2 to the rpm_active refcount of @link and increment the PM-runtime
+ * usage counter of @supplier once more in case the link is being added while
+ * the consumer driver is probing and pm_runtime_put_suppliers() will be called
+ * subsequently.
+ *
+ * Note that this doesn't prevent rpm_put_suppliers() from decreasing the link's
+ * rpm_active refcount down to one, so runtime suspend of the consumer end of
+ * @link is not affected.
+ */
+void pm_runtime_active_link(struct device_link *link, struct device *supplier)
+{
+	refcount_add(2, &link->rpm_active);
+	pm_runtime_get_noresume(supplier);
+}
+
 void pm_runtime_drop_link(struct device *dev)
 {
 	spin_lock_irq(&dev->power.lock);
Index: linux-pm/drivers/base/core.c
===================================================================
--- linux-pm.orig/drivers/base/core.c
+++ linux-pm/drivers/base/core.c
@@ -165,19 +165,6 @@ void device_pm_move_to_tail(struct devic
 	device_links_read_unlock(idx);
 }
 
-static void device_link_rpm_prepare(struct device *consumer,
-				    struct device *supplier)
-{
-	pm_runtime_new_link(consumer);
-	/*
-	 * If the link is being added by the consumer driver at probe time,
-	 * balance the decrementation of the supplier's runtime PM usage counter
-	 * after consumer probe in driver_probe_device().
-	 */
-	if (consumer->links.status == DL_DEV_PROBING)
-		pm_runtime_get_noresume(supplier);
-}
-
 /**
  * device_link_add - Create a link between two devices.
  * @consumer: Consumer end of the link.
@@ -286,11 +273,11 @@ struct device_link *device_link_add(stru
 
 		if (flags & DL_FLAG_PM_RUNTIME) {
 			if (!(link->flags & DL_FLAG_PM_RUNTIME)) {
-				device_link_rpm_prepare(consumer, supplier);
+				pm_runtime_new_link(consumer);
 				link->flags |= DL_FLAG_PM_RUNTIME;
 			}
 			if (flags & DL_FLAG_RPM_ACTIVE)
-				refcount_inc(&link->rpm_active);
+				pm_runtime_active_link(link, supplier);
 		}
 
 		if (flags & DL_FLAG_STATELESS) {
@@ -323,9 +310,9 @@ struct device_link *device_link_add(stru
 
 	if (flags & DL_FLAG_PM_RUNTIME) {
 		if (flags & DL_FLAG_RPM_ACTIVE)
-			refcount_inc(&link->rpm_active);
+			pm_runtime_active_link(link, supplier);
 
-		device_link_rpm_prepare(consumer, supplier);
+		pm_runtime_new_link(consumer);
 	}
 
 	get_device(supplier);
Index: linux-pm/include/linux/pm_runtime.h
===================================================================
--- linux-pm.orig/include/linux/pm_runtime.h
+++ linux-pm/include/linux/pm_runtime.h
@@ -59,6 +59,8 @@ extern void pm_runtime_clean_up_links(st
 extern void pm_runtime_get_suppliers(struct device *dev);
 extern void pm_runtime_put_suppliers(struct device *dev);
 extern void pm_runtime_new_link(struct device *dev);
+extern void pm_runtime_active_link(struct device_link *link,
+				   struct device *supplier);
 extern void pm_runtime_drop_link(struct device *dev);
 
 static inline void pm_suspend_ignore_children(struct device *dev, bool enable)
@@ -178,6 +180,8 @@ static inline void pm_runtime_clean_up_l
 static inline void pm_runtime_get_suppliers(struct device *dev) {}
 static inline void pm_runtime_put_suppliers(struct device *dev) {}
 static inline void pm_runtime_new_link(struct device *dev) {}
+static inline void pm_runtime_active_link(struct device_link *link,
+					  struct device *supplier) {}
 static inline void pm_runtime_drop_link(struct device *dev) {}
 
 #endif /* !CONFIG_PM */