[PATCH v19 4/7] driver core: do not always lock parent in shutdown

[Tarun Sahu <tarunsahu@google.com>]

From: David Jeffery <djeffery@redhat.com>

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@gmail.com>
Signed-off-by: David Jeffery <djeffery@redhat.com>
Signed-off-by: Tarun Sahu <tarunsahu@google.com>
Tested-by: Laurence Oberman <loberman@redhat.com>
---
 drivers/base/core.c | 42 ++++++++++++++++++++++++++----------------
 1 file changed, 26 insertions(+), 16 deletions(-)

diff --git a/drivers/base/core.c b/drivers/base/core.c
index 82a7af1f3ba2..31f95e86856e 100644
--- a/drivers/base/core.c
+++ b/drivers/base/core.c
@@ -4898,14 +4898,10 @@ int device_change_owner(struct device *dev, kuid_t kuid, kgid_t kgid)
 	return error;
 }
 
-static void shutdown_one_device(struct device *dev)
+static void __shutdown_one_device(struct device *dev)
 {
-	struct device *parent = dev->parent;
-
-	/* 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);
@@ -4925,12 +4921,32 @@ static void shutdown_one_device(struct device *dev)
 			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(parent);
 	put_device(dev);
 }
 
@@ -4956,12 +4972,6 @@ 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
-		 */
-		get_device(dev->parent);
 		get_device(dev);
 		/*
 		 * Make sure the device is off the kset list, in the
-- 
2.55.0.229.g6434b31f56-goog