[PATCH v2 1/2] xen/xenbus: reference count registered modules

[Paul Durrant <pdurrant@amazon.com>]

To prevent a module being removed whilst attached to a frontend, and
hence xenbus calling into potentially invalid text, take a reference on
the module before calling the probe() method (dropping it if unsuccessful)
and drop the reference after returning from the remove() method.

NOTE: This allows the ad-hoc reference counting in xen-netback to be
      removed. This will be done in a subsequent patch.

Suggested-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Paul Durrant <pdurrant@amazon.com>
---
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>

v2:
 - New in v2
---
 drivers/xen/xenbus/xenbus_probe.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/drivers/xen/xenbus/xenbus_probe.c b/drivers/xen/xenbus/xenbus_probe.c
index 5b471889d723..5a4947690500 100644
--- a/drivers/xen/xenbus/xenbus_probe.c
+++ b/drivers/xen/xenbus/xenbus_probe.c
@@ -232,9 +232,11 @@ int xenbus_dev_probe(struct device *_dev)
 		return err;
 	}
 
+	__module_get(drv->driver.owner);
+
 	err = drv->probe(dev, id);
 	if (err)
-		goto fail;
+		goto fail_put;
 
 	err = watch_otherend(dev);
 	if (err) {
@@ -244,6 +246,8 @@ int xenbus_dev_probe(struct device *_dev)
 	}
 
 	return 0;
+fail_put:
+	module_put(drv->driver.owner);
 fail:
 	xenbus_dev_error(dev, err, "xenbus_dev_probe on %s", dev->nodename);
 	xenbus_switch_state(dev, XenbusStateClosed);
@@ -263,6 +267,8 @@ int xenbus_dev_remove(struct device *_dev)
 	if (drv->remove)
 		drv->remove(dev);
 
+	module_put(drv->driver.owner);
+
 	free_otherend_details(dev);
 
 	xenbus_switch_state(dev, XenbusStateClosed);
-- 
2.20.1