xen/xenbus: fix locking

Commit 060eabe8fb ("xenbus/backend: Protect xenbus callback with
lock") introduced a bug by holding a lock while calling a function
which might schedule.

Fix that by using a semaphore instead.

Fixes: 060eabe8fb ("xenbus/backend: Protect xenbus callback with lock")
Signed-off-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20200305100323.16736-1-jgross@suse.com
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>

drivers/xen/xenbus/xenbus_probe.c

@@ -239,9 +239,9 @@ int xenbus_dev_probe(struct device *_dev)
 		goto fail;
 	}
 
-	spin_lock(&dev->reclaim_lock);
+	down(&dev->reclaim_sem);
 	err = drv->probe(dev, id);
-	spin_unlock(&dev->reclaim_lock);
+	up(&dev->reclaim_sem);
 	if (err)
 		goto fail_put;
 
@@ -271,9 +271,9 @@ int xenbus_dev_remove(struct device *_dev)
 	free_otherend_watch(dev);
 
 	if (drv->remove) {
-		spin_lock(&dev->reclaim_lock);
+		down(&dev->reclaim_sem);
 		drv->remove(dev);
-		spin_unlock(&dev->reclaim_lock);
+		up(&dev->reclaim_sem);
 	}
 
 	module_put(drv->driver.owner);
@@ -473,7 +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);
+	sema_init(&xendev->reclaim_sem, 1);
 
 	/* Register with generic device framework. */
 	err = device_register(&xendev->dev);

drivers/xen/xenbus/xenbus_probe_backend.c

@@ -45,6 +45,7 @@
 #include <linux/mm.h>
 #include <linux/notifier.h>
 #include <linux/export.h>
+#include <linux/semaphore.h>
 
 #include <asm/page.h>
 #include <asm/pgtable.h>
@@ -257,10 +258,10 @@ static int backend_reclaim_memory(struct device *dev, void *data)
 	drv = to_xenbus_driver(dev->driver);
 	if (drv && drv->reclaim_memory) {
 		xdev = to_xenbus_device(dev);
-		if (!spin_trylock(&xdev->reclaim_lock))
+		if (down_trylock(&xdev->reclaim_sem))
 			return 0;
 		drv->reclaim_memory(xdev);
-		spin_unlock(&xdev->reclaim_lock);
+		up(&xdev->reclaim_sem);
 	}
 	return 0;
 }

include/xen/xenbus.h

@@ -42,6 +42,7 @@
 #include <linux/completion.h>
 #include <linux/init.h>
 #include <linux/slab.h>
+#include <linux/semaphore.h>
 #include <xen/interface/xen.h>
 #include <xen/interface/grant_table.h>
 #include <xen/interface/io/xenbus.h>
@@ -76,7 +77,7 @@ struct xenbus_device {
 	enum xenbus_state state;
 	struct completion down;
 	struct work_struct work;
-	spinlock_t reclaim_lock;
+	struct semaphore reclaim_sem;
 };
 
 static inline struct xenbus_device *to_xenbus_device(struct device *dev)