[PATCH tip/core/rcu 4/6] rcu/tree: segcblist: Remove redundant smp_mb()s

paulmck
2021-01-06T01:27:35Z

The full memory barriers in rcu_segcblist_enqueue() and in rcu_do_batch()
are not needed because rcu_segcblist_add_len(), and thus also
rcu_segcblist_inc_len(), already includes a memory barrier *before*
and *after* the length of the list is updated.

This commit therefore removes these redundant smp_mb() invocations.

Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
kernel/rcu/rcu_segcblist.c | 1 -
kernel/rcu/tree.c | 1 -
2 files changed, 2 deletions(-)

diff --git a/kernel/rcu/rcu_segcblist.c b/kernel/rcu/rcu_segcblist.c
index 0504fc83..5059b61 100644
--- a/kernel/rcu/rcu_segcblist.c
+++ b/kernel/rcu/rcu_segcblist.c
@@ -327,7 +327,6 @@ void rcu_segcblist_enqueue(struct rcu_segcblist *rsclp,
struct rcu_head *rhp)
{
rcu_segcblist_inc_len(rsclp);
- smp_mb(); /* Ensure counts are updated before callback is enqueued. */
rcu_segcblist_inc_seglen(rsclp, RCU_NEXT_TAIL);
rhp->next = NULL;
WRITE_ONCE(*rsclp->tails[RCU_NEXT_TAIL], rhp);
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index cc6f379..b0fb654 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2523,7 +2523,6 @@ static void rcu_do_batch(struct rcu_data *rdp)

/* Update counts and requeue any remaining callbacks. */
rcu_segcblist_insert_done_cbs(&rdp->cblist, &rcl);
- smp_mb(); /* List handling before counting for rcu_barrier(). */
rcu_segcblist_add_len(&rdp->cblist, -count);

/* Reinstate batch limit if we have worked down the excess. */
--
2.9.5