[PATCH v4 -rcu 1/4] rcu/segcblist: Do not depend on rcl->len to store the segcb len during merge

["Joel Fernandes (Google)" <joel@joelfernandes.org>]

The donecbs's ->len field is used to store the total count of the segmented
callback list's length. This ->len field is then added to the destination segcb
list.

However, this presents a problem for per-segment length counting which is added
in a future patch. This future patch sets the rcl->len field as we move
segments of callbacks between source and destination lists, thus becoming
incompatible with the donecb's ->len field.

This commit therefore avoids depending on the ->len field in this way. IMHO,
this is also less error-prone and is more accurate - the donecb's ->len field
should be the length of the done segment and not just used as a temporarily
variable.

Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
---
 kernel/rcu/rcu_segcblist.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/rcu_segcblist.c b/kernel/rcu/rcu_segcblist.c
index 2d2a6b6b9dfb..b70d4154433c 100644
--- a/kernel/rcu/rcu_segcblist.c
+++ b/kernel/rcu/rcu_segcblist.c
@@ -513,14 +513,18 @@ void rcu_segcblist_merge(struct rcu_segcblist *dst_rsclp,
 {
 	struct rcu_cblist donecbs;
 	struct rcu_cblist pendcbs;
+	long src_len;
 
 	rcu_cblist_init(&donecbs);
 	rcu_cblist_init(&pendcbs);
-	rcu_segcblist_extract_count(src_rsclp, &donecbs);
+
+	src_len = rcu_segcblist_xchg_len(src_rsclp, 0);
 	rcu_segcblist_extract_done_cbs(src_rsclp, &donecbs);
 	rcu_segcblist_extract_pend_cbs(src_rsclp, &pendcbs);
-	rcu_segcblist_insert_count(dst_rsclp, &donecbs);
+
+	rcu_segcblist_add_len(dst_rsclp, src_len);
 	rcu_segcblist_insert_done_cbs(dst_rsclp, &donecbs);
 	rcu_segcblist_insert_pend_cbs(dst_rsclp, &pendcbs);
+
 	rcu_segcblist_init(src_rsclp);
 }
-- 
2.28.0.297.g1956fa8f8d-goog