linux-kernel - [PATCH tip/core/rcu 08/18] rcu/nocb: Allow lockless use of rcu_segcblist

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Message-Id: <20190801230810.21570-8-paulmck@linux.ibm.com>
Date:   Thu,  1 Aug 2019 16:08:00 -0700
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     rcu@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, mingo@...nel.org,
        jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
        rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
        fweisbec@...il.com, oleg@...hat.com, joel@...lfernandes.org,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>
Subject: [PATCH tip/core/rcu 08/18] 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 <paulmck@...ux.ibm.com>
---
 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 0e7fe678b6ac..06435a368be5 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 f74960f0305c..d9142b3590a8 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. */
-- 
2.17.1