[PATCH v12 2/5] xenbus/backend: Protect xenbus callback with lock

[SeongJae Park <sjpark@amazon.com>]

From: SeongJae Park <sjpark@amazon.de>

'reclaim_memory' callback can race with a driver code as this callback
will be called from any memory pressure detected context.  To deal with
the case, this commit adds a spinlock in the 'xenbus_device'.  Whenever
'reclaim_memory' callback is called, the lock of the device which passed
to the callback as its argument is locked.  Thus, drivers registering
their 'reclaim_memory' callback should protect the data that might race
with the callback with the lock by themselves.

Signed-off-by: SeongJae Park <sjpark@amazon.de>
---
 drivers/xen/xenbus/xenbus_probe.c         |  1 +
 drivers/xen/xenbus/xenbus_probe_backend.c | 11 +++++++++--
 include/xen/xenbus.h                      |  2 ++
 3 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/drivers/xen/xenbus/xenbus_probe.c b/drivers/xen/xenbus/xenbus_probe.c
index 5b471889d723..b86393f172e6 100644
--- a/drivers/xen/xenbus/xenbus_probe.c
+++ b/drivers/xen/xenbus/xenbus_probe.c
@@ -472,6 +472,7 @@ 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);
diff --git a/drivers/xen/xenbus/xenbus_probe_backend.c b/drivers/xen/xenbus/xenbus_probe_backend.c
index 7e78ebef7c54..e862cb932cc4 100644
--- a/drivers/xen/xenbus/xenbus_probe_backend.c
+++ b/drivers/xen/xenbus/xenbus_probe_backend.c
@@ -251,12 +251,19 @@ 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;
+	unsigned long flags;
 
 	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_irqsave(&xdev->reclaim_lock, flags))
+			return 0;
+		drv->reclaim_memory(xdev);
+		spin_unlock_irqrestore(&xdev->reclaim_lock, flags);
+	}
 	return 0;
 }
 
diff --git a/include/xen/xenbus.h b/include/xen/xenbus.h
index c861cfb6f720..d9468313061d 100644
--- a/include/xen/xenbus.h
+++ b/include/xen/xenbus.h
@@ -76,6 +76,8 @@ struct xenbus_device {
 	enum xenbus_state state;
 	struct completion down;
 	struct work_struct work;
+	/* 'reclaim_memory' callback is called while this lock is acquired */
+	spinlock_t reclaim_lock;
 };
 
 static inline struct xenbus_device *to_xenbus_device(struct device *dev)
-- 
2.17.1