ceph: set caps count after composing cap reconnect message

It's possible that some caps get released while composing the cap
reconnect message.
Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
Reviewed-by: Sage Weil <sage@inktank.com>

fs/ceph/mds_client.c

@@ -43,6 +43,7 @@
  */
 
 struct ceph_reconnect_state {
+	int nr_caps;
 	struct ceph_pagelist *pagelist;
 	bool flock;
 };
@@ -2549,6 +2550,8 @@ static int encode_caps_cb(struct inode *inode, struct ceph_cap *cap,
 	} else {
 		err = ceph_pagelist_append(pagelist, &rec, reclen);
 	}
+
+	recon_state->nr_caps++;
 out_free:
 	kfree(path);
 out_dput:
@@ -2576,6 +2579,7 @@ static void send_mds_reconnect(struct ceph_mds_client *mdsc,
 	struct rb_node *p;
 	int mds = session->s_mds;
 	int err = -ENOMEM;
+	int s_nr_caps;
 	struct ceph_pagelist *pagelist;
 	struct ceph_reconnect_state recon_state;
 
@@ -2611,10 +2615,12 @@ static void send_mds_reconnect(struct ceph_mds_client *mdsc,
 	discard_cap_releases(mdsc, session);
 
 	/* traverse this session's caps */
-	err = ceph_pagelist_encode_32(pagelist, session->s_nr_caps);
+	s_nr_caps = session->s_nr_caps;
+	err = ceph_pagelist_encode_32(pagelist, s_nr_caps);
 	if (err)
 		goto fail;
 
+	recon_state.nr_caps = 0;
 	recon_state.pagelist = pagelist;
 	recon_state.flock = session->s_con.peer_features & CEPH_FEATURE_FLOCK;
 	err = iterate_session_caps(session, encode_caps_cb, &recon_state);
@@ -2643,11 +2649,18 @@ static void send_mds_reconnect(struct ceph_mds_client *mdsc,
 
 	if (recon_state.flock)
 		reply->hdr.version = cpu_to_le16(2);
-	if (pagelist->length) {
-		/* set up outbound data if we have any */
+
+	/* raced with cap release? */
+	if (s_nr_caps != recon_state.nr_caps) {
+		struct page *page = list_first_entry(&pagelist->head,
+						     struct page, lru);
+		__le32 *addr = kmap_atomic(page);
+		*addr = cpu_to_le32(recon_state.nr_caps);
+		kunmap_atomic(addr);
+	}
+
 	reply->hdr.data_len = cpu_to_le32(pagelist->length);
 	ceph_msg_data_add_pagelist(reply, pagelist);
-	}
 	ceph_con_send(&session->s_con, reply);
 
 	mutex_unlock(&session->s_mutex);