[sheepdog] [PATCH 2/2] sheep: add a description about treatment of current_vnode_info()

[Hitoshi Mitake]

Some part of group.c use main_thread_get() instead of
get_vnode_info(). It is a little bit tricky so this patch adds
comments as a description.

Signed-off-by: Hitoshi Mitake <mitake.hitoshi at lab.ntt.co.jp>
---
 sheep/group.c |   14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/sheep/group.c b/sheep/group.c
index e37b198..ad64a8d 100644
--- a/sheep/group.c
+++ b/sheep/group.c
@@ -652,6 +652,12 @@ static void update_cluster_info(const struct cluster_info *cinfo,
 		sockfd_cache_add_group(nroot);
 	sockfd_cache_add(&joined->nid);
 
+	/*
+	 * We can use main_thread_get() to obtain current_vnode_info. The
+	 * reference count of old_vnode_info is decremented at the last of this
+	 * function, but it doesn't cause an illegal release. The counter part
+	 * of this reference decrement is alloc_vnode_info().
+	 */
 	old_vnode_info = main_thread_get(current_vnode_info);
 	main_thread_set(current_vnode_info, alloc_vnode_info(nroot));
 
@@ -932,6 +938,10 @@ main_fn void sd_leave_handler(const struct sd_node *left,
 		/* Mark leave node as gateway only node */
 		sys->this_node.nr_vnodes = 0;
 
+	/*
+	 * Using main_thread_get() instead of get_vnode_info() is allowed
+	 * because of the same reason of update_cluster_info()
+	 */
 	old_vnode_info = main_thread_get(current_vnode_info);
 	main_thread_set(current_vnode_info, alloc_vnode_info(nroot));
 	if (sys->cinfo.status == SD_STATUS_OK) {
@@ -961,6 +971,10 @@ static void update_node_size(struct sd_node *node)
 
 static void kick_node_recover(void)
 {
+	/*
+	 * Using main_thread_get() instead of get_vnode_info() is allowed
+	 * because of the same reason of update_cluster_info()
+	 */
 	struct vnode_info *old = main_thread_get(current_vnode_info);
 	int ret;
 
-- 
1.7.10.4