[PATCH 5/9] driver core: do not always lock parent in shutdown

[David Jeffery]

Don't lock a parent device unless it is needed in device_shutdown. This
is in preparation for making device shutdown asynchronous, when it will
be needed to allow children of a common parent to shut down
simultaneously.

And only acquire a reference to the parent device if the parent is to be
locked.

Signed-off-by: Stuart Hayes <stuart.w.hayes@xxxxxxxxx>
Signed-off-by: David Jeffery <djeffery@xxxxxxxxxx>
Signed-off-by: Tarun Sahu <tarunsahu@xxxxxxxxxx>
Tested-by: Laurence Oberman <loberman@xxxxxxxxxx>
---

The sashiko patch analysis may complain about the parent locking and
possible use of device_move. This issue cannot currently happen as
devices with need_parent_lock do not use device_move. An earlier patch
also adds a warning for this condition should some future code change
violate the incompatibility between need_parent_lock and device_move.


 drivers/base/core.c | 44 ++++++++++++++++++++++++++++----------------
 1 file changed, 28 insertions(+), 16 deletions(-)

diff --git a/drivers/base/core.c b/drivers/base/core.c
index a94d24b30c30..cd725466c32d 100644
--- a/drivers/base/core.c
+++ b/drivers/base/core.c
@@ -4910,12 +4910,10 @@ int device_change_owner(struct device *dev, kuid_t kuid, kgid_t kgid)
 	return error;
 }
 
-static void shutdown_one_device(struct device *dev, struct device *parent)
+static void __shutdown_one_device(struct device *dev)
 {
-	/* hold lock to avoid race with probe/release */
-	if (parent)
-		device_lock(parent);
-	device_lock(dev);
+	if (!dev->p || dev->p->dead)
+		return;
 
 	/* Don't allow any more runtime suspends */
 	pm_runtime_get_noresume(dev);
@@ -4935,13 +4933,33 @@ static void shutdown_one_device(struct device *dev, struct device *parent)
 			dev_info(dev, "shutdown\n");
 		dev->driver->shutdown(dev);
 	}
+}
 
-	device_unlock(dev);
-	if (parent)
+static void shutdown_one_device(struct device *dev)
+{
+	struct device *parent;
+
+	device_lock(dev);
+
+	/* use parent lock if needed to avoid race with probe/release */
+	if (dev->bus && dev->bus->need_parent_lock && dev->p && !dev->p->dead &&
+	    (parent = get_device(dev->parent))) {
+		/* the parent lock needs to be acquired first, so re-lock */
+		device_unlock(dev);
+
+		device_lock(parent);
+		device_lock(dev);
+
+		__shutdown_one_device(dev);
+		device_unlock(dev);
 		device_unlock(parent);
+		put_device(parent);
+	} else {
+		__shutdown_one_device(dev);
+		device_unlock(dev);
+	}
 
 	put_device(dev);
-	put_device(parent);
 }
 
 /**
@@ -4949,7 +4967,7 @@ static void shutdown_one_device(struct device *dev, struct device *parent)
  */
 void device_shutdown(void)
 {
-	struct device *dev, *parent;
+	struct device *dev;
 
 	wait_for_device_probe();
 	device_block_probing();
@@ -4966,12 +4984,6 @@ void device_shutdown(void)
 		dev = list_entry(devices_kset->list.prev, struct device,
 				kobj.entry);
 
-		/*
-		 * hold reference count of device's parent to
-		 * prevent it from being freed because parent's
-		 * lock is to be held
-		 */
-		parent = get_device(dev->parent);
 		get_device(dev);
 		/*
 		 * Make sure the device is off the kset list, in the
@@ -4980,7 +4992,7 @@ void device_shutdown(void)
 		list_del_init(&dev->kobj.entry);
 		spin_unlock(&devices_kset->list_lock);
 
-		shutdown_one_device(dev, parent);
+		shutdown_one_device(dev);
 
 		spin_lock(&devices_kset->list_lock);
 	}
-- 
2.55.0