From 8d672fa6bf68ffc36a0c5e4868499f86bbea2308 Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Wed, 2 May 2018 14:46:43 -0700 Subject: [PATCH] rcu: Make rcu_init_new_rnp() stop upon already-set bit Currently, rcu_init_new_rnp() walks up the rcu_node combining tree, setting bits in the ->qsmaskinit fields on the way up. It walks up unconditionally, regardless of the initial state of these bits. This is OK because only the corresponding RCU grace-period kthread ever tests or sets these bits during runtime. However, it is also pointless, and it increases both memory and lock contention (albeit only slightly), so this commit stops the walk as soon as an already-set bit is encountered. Signed-off-by: Paul E. McKenney --- kernel/rcu/tree.c | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-) diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index 05f69b7..3fe854a 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -3479,9 +3479,10 @@ EXPORT_SYMBOL_GPL(rcu_barrier_sched); static void rcu_init_new_rnp(struct rcu_node *rnp_leaf) { long mask; + long oldmask; struct rcu_node *rnp = rnp_leaf; - raw_lockdep_assert_held_rcu_node(rnp); + raw_lockdep_assert_held_rcu_node(rnp_leaf); WARN_ON_ONCE(rnp->wait_blkd_tasks); for (;;) { mask = rnp->grpmask; @@ -3489,8 +3490,11 @@ static void rcu_init_new_rnp(struct rcu_node *rnp_leaf) if (rnp == NULL) return; raw_spin_lock_rcu_node(rnp); /* Interrupts already disabled. */ + oldmask = rnp->qsmaskinit; rnp->qsmaskinit |= mask; raw_spin_unlock_rcu_node(rnp); /* Interrupts remain disabled. */ + if (oldmask) + return; } } -- 2.7.4