lockd: fix race in nlm_release()

The sm_count is decremented to zero but left on the nsm_handles list.
So in the space between decrementing sm_count and acquiring nsm_mutex,
it is possible for another task to find this nsm_handle, increment the
use count and then enter nsm_release itself.

Thus there's nothing to prevent the nsm being freed before we acquire
nsm_mutex here.
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>

fs/lockd/host.c

@@ -529,12 +529,10 @@ nsm_release(struct nsm_handle *nsm)
 {
 	if (!nsm)
 		return;
+	mutex_lock(&nsm_mutex);
 	if (atomic_dec_and_test(&nsm->sm_count)) {
-		mutex_lock(&nsm_mutex);
-		if (atomic_read(&nsm->sm_count) == 0) {
-			list_del(&nsm->sm_link);
-			kfree(nsm);
-		}
-		mutex_unlock(&nsm_mutex);
+		list_del(&nsm->sm_link);
+		kfree(nsm);
 	}
+	mutex_unlock(&nsm_mutex);
 }