xenbus/backend: Protect xenbus callback with lock

A driver's 'reclaim_memory' callback can race with 'probe' or 'remove'
because it will be called whenever memory pressure is detected.  To
avoid such race, this commit embeds a spinlock in each 'xenbus_device'
and make 'xenbus' to hold the lock while the corresponded callbacks are
running.

Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: SeongJae Park <sjpark@amazon.de>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>

drivers/xen/xenbus/xenbus_probe.c

@@ -239,7 +239,9 @@ int xenbus_dev_probe(struct device *_dev)
 		goto fail;
 	}
 
+	spin_lock(&dev->reclaim_lock);
 	err = drv->probe(dev, id);
+	spin_unlock(&dev->reclaim_lock);
 	if (err)
 		goto fail_put;
 
@@ -268,8 +270,11 @@ int xenbus_dev_remove(struct device *_dev)
 
 	free_otherend_watch(dev);
 
-	if (drv->remove)
+	if (drv->remove) {
+		spin_lock(&dev->reclaim_lock);
 		drv->remove(dev);
+		spin_unlock(&dev->reclaim_lock);
+	}
 
 	module_put(drv->driver.owner);
 
@@ -468,6 +473,7 @@ int xenbus_probe_node(struct xen_bus_type *bus,
 		goto fail;
 
 	dev_set_name(&xendev->dev, "%s", devname);
+	spin_lock_init(&xendev->reclaim_lock);
 
 	/* Register with generic device framework. */
 	err = device_register(&xendev->dev);

drivers/xen/xenbus/xenbus_probe_backend.c

@@ -250,12 +250,18 @@ static int backend_probe_and_watch(struct notifier_block *notifier,
 static int backend_reclaim_memory(struct device *dev, void *data)
 {
 	const struct xenbus_driver *drv;
+	struct xenbus_device *xdev;
 
 	if (!dev->driver)
 		return 0;
 	drv = to_xenbus_driver(dev->driver);
-	if (drv && drv->reclaim_memory)
-		drv->reclaim_memory(to_xenbus_device(dev));
+	if (drv && drv->reclaim_memory) {
+		xdev = to_xenbus_device(dev);
+		if (!spin_trylock(&xdev->reclaim_lock))
+			return 0;
+		drv->reclaim_memory(xdev);
+		spin_unlock(&xdev->reclaim_lock);
+	}
 	return 0;
 }
 

include/xen/xenbus.h

@@ -76,6 +76,7 @@ struct xenbus_device {
 	enum xenbus_state state;
 	struct completion down;
 	struct work_struct work;
+	spinlock_t reclaim_lock;
 };
 
 static inline struct xenbus_device *to_xenbus_device(struct device *dev)