From e6060b41c9955374079926a7612b857a8458ed1f Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Mon, 13 May 2019 15:57:50 -0700 Subject: [PATCH] rcu/nocb: Allow lockless use of rcu_segcblist_empty() Currently, rcu_segcblist_empty() assumes that the callback list is not being changed by other CPUs, but upcoming changes will require it to operate locklessly. This commit therefore adds the needed READ_ONCE() call, along with the WRITE_ONCE() calls when updating the callback list's ->head field. Signed-off-by: Paul E. McKenney --- kernel/rcu/rcu_segcblist.c | 4 ++-- kernel/rcu/rcu_segcblist.h | 2 +- 2 files changed, 3 insertions(+), 3 deletions(-) diff --git a/kernel/rcu/rcu_segcblist.c b/kernel/rcu/rcu_segcblist.c index 0e7fe67..06435a3 100644 --- a/kernel/rcu/rcu_segcblist.c +++ b/kernel/rcu/rcu_segcblist.c @@ -213,7 +213,7 @@ void rcu_segcblist_extract_done_cbs(struct rcu_segcblist *rsclp, if (!rcu_segcblist_ready_cbs(rsclp)) return; /* Nothing to do. */ *rclp->tail = rsclp->head; - rsclp->head = *rsclp->tails[RCU_DONE_TAIL]; + WRITE_ONCE(rsclp->head, *rsclp->tails[RCU_DONE_TAIL]); WRITE_ONCE(*rsclp->tails[RCU_DONE_TAIL], NULL); rclp->tail = rsclp->tails[RCU_DONE_TAIL]; for (i = RCU_CBLIST_NSEGS - 1; i >= RCU_DONE_TAIL; i--) @@ -268,7 +268,7 @@ void rcu_segcblist_insert_done_cbs(struct rcu_segcblist *rsclp, if (!rclp->head) return; /* No callbacks to move. */ *rclp->tail = rsclp->head; - rsclp->head = rclp->head; + WRITE_ONCE(rsclp->head, rclp->head); for (i = RCU_DONE_TAIL; i < RCU_CBLIST_NSEGS; i++) if (&rsclp->head == rsclp->tails[i]) WRITE_ONCE(rsclp->tails[i], rclp->tail); diff --git a/kernel/rcu/rcu_segcblist.h b/kernel/rcu/rcu_segcblist.h index f74960f..d9142b3 100644 --- a/kernel/rcu/rcu_segcblist.h +++ b/kernel/rcu/rcu_segcblist.h @@ -36,7 +36,7 @@ struct rcu_head *rcu_cblist_dequeue(struct rcu_cblist *rclp); */ static inline bool rcu_segcblist_empty(struct rcu_segcblist *rsclp) { - return !rsclp->head; + return !READ_ONCE(rsclp->head); } /* Return number of callbacks in segmented callback list. */ -- 2.7.4