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Subject[PATCH 4.19 195/639] driver core: Fix possible supplier PM-usage counter imbalance
Date
From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

[ Upstream commit 4c06c4e6cf63d7f3d5dfe62593a073253d750a59 ]

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>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Tested-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
drivers/base/core.c | 21 ++++-----------------
drivers/base/power/runtime.c | 27 +++++++++++++++++++++++++--
include/linux/pm_runtime.h | 4 ++++
3 files changed, 33 insertions(+), 19 deletions(-)

diff --git a/drivers/base/core.c b/drivers/base/core.c
index 7599147d5f83c..ab08211ba5d2d 100644
--- a/drivers/base/core.c
+++ b/drivers/base/core.c
@@ -165,19 +165,6 @@ void device_pm_move_to_tail(struct device *dev)
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.
@@ -262,11 +249,11 @@ struct device_link *device_link_add(struct device *consumer,

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);
}

kref_get(&link->kref);
@@ -281,9 +268,9 @@ struct device_link *device_link_add(struct device *consumer,

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);
diff --git a/drivers/base/power/runtime.c b/drivers/base/power/runtime.c
index ab454c4533ba1..0527890b4c191 100644
--- a/drivers/base/power/runtime.c
+++ b/drivers/base/power/runtime.c
@@ -1570,8 +1570,10 @@ void pm_runtime_get_suppliers(struct device *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);
}
@@ -1588,7 +1590,8 @@ void pm_runtime_put_suppliers(struct device *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);
@@ -1601,6 +1604,26 @@ void pm_runtime_new_link(struct device *dev)
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);
diff --git a/include/linux/pm_runtime.h b/include/linux/pm_runtime.h
index f0fc4700b6ff5..bace7df51af4d 100644
--- a/include/linux/pm_runtime.h
+++ b/include/linux/pm_runtime.h
@@ -59,6 +59,8 @@ extern void pm_runtime_clean_up_links(struct device *dev);
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)
@@ -176,6 +178,8 @@ static inline void pm_runtime_clean_up_links(struct device *dev) {}
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 */
--
2.20.1


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