ceph: get snap_rwsem read lock in handle_cap_export for ceph_add_cap

ceph_add_cap says in its function documentation that the caller should
hold the read lock on the session snap_rwsem. Furthermore, not only
ceph_add_cap needs that lock, when it calls to ceph_lookup_snap_realm it
eventually calls ceph_get_snap_realm which states via lockdep that
snap_rwsem needs to be held. handle_cap_export calls ceph_add_cap
without that mdsc->snap_rwsem held. Thus, since ceph_get_snap_realm
and ceph_add_cap both need the lock, the common place to acquire that
lock is inside handle_cap_export.

Signed-off-by: Niels Dossche <dossche.niels@gmail.com>
Reviewed-by: Xiubo Li <xiubli@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

fs/ceph/caps.c

@@ -3870,6 +3870,7 @@ static void handle_cap_export(struct inode *inode, struct ceph_mds_caps *ex,
 	dout("handle_cap_export inode %p ci %p mds%d mseq %d target %d\n",
 	     inode, ci, mds, mseq, target);
 retry:
+	down_read(&mdsc->snap_rwsem);
 	spin_lock(&ci->i_ceph_lock);
 	cap = __get_cap_for_mds(ci, mds);
 	if (!cap || cap->cap_id != le64_to_cpu(ex->cap_id))
@@ -3933,6 +3934,7 @@ retry:
 	}
 
 	spin_unlock(&ci->i_ceph_lock);
+	up_read(&mdsc->snap_rwsem);
 	mutex_unlock(&session->s_mutex);
 
 	/* open target session */
@@ -3958,6 +3960,7 @@ retry:
 
 out_unlock:
 	spin_unlock(&ci->i_ceph_lock);
+	up_read(&mdsc->snap_rwsem);
 	mutex_unlock(&session->s_mutex);
 	if (tsession) {
 		mutex_unlock(&tsession->s_mutex);