From 9fa46fb8c9c6dfad30487fb3d905c2ff04b379b7 Mon Sep 17 00:00:00 2001 From: Paul E. McKenney Date: Mon, 26 Jun 2017 15:43:27 -0700 Subject: rcu: Advance outgoing CPU's callbacks before migrating them It is possible that the outgoing CPU is unaware of recent grace periods, and so it is also possible that some of its pending callbacks are actually ready to be invoked. The current callback-migration code would needlessly force these callbacks to pass through another grace period. This commit therefore invokes rcu_advance_cbs() on the outgoing CPU's callbacks in order to give them full credit for having passed through any recent grace periods. This also fixes an odd theoretical bug where there are no callbacks in the system except for those on the outgoing CPU, none of those callbacks have yet been associated with a grace-period number, there is never again another callback registered, and the surviving CPU never again takes a scheduling-clock interrupt, never goes idle, and never enters nohz_full userspace execution. Yes, this is (just barely) possible. It requires that the surviving CPU be a nohz_full CPU, that its scheduler-clock interrupt be shut off, and that it loop forever in the kernel. You get bonus points if you can make this one happen! ;-) Signed-off-by: Paul E. McKenney --- kernel/rcu/tree.c | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-) (limited to 'kernel/rcu') diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index 4ea28e820f4a..c080c6ed66af 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -3926,6 +3926,7 @@ static void rcu_migrate_callbacks(int cpu, struct rcu_state *rsp) struct rcu_data *my_rdp; struct rcu_data *rdp = per_cpu_ptr(rsp->rda, cpu); struct rcu_node *rnp = rdp->mynode; /* Outgoing CPU's rdp & rnp. */ + struct rcu_node *rnp_root = rcu_get_root(rdp->rsp); if (rcu_is_nocb_cpu(cpu) || rcu_segcblist_empty(&rdp->cblist)) return; /* No callbacks to migrate. */ @@ -3936,7 +3937,11 @@ static void rcu_migrate_callbacks(int cpu, struct rcu_state *rsp) local_irq_restore(flags); return; } - raw_spin_lock(&rsp->orphan_lock); /* irqs already disabled. */ + raw_spin_lock_rcu_node(rnp_root); /* irqs already disabled. */ + rcu_advance_cbs(rsp, rnp_root, rdp); /* Leverage recent GPs. */ + raw_spin_unlock_rcu_node(rnp_root); + + raw_spin_lock(&rsp->orphan_lock); rcu_send_cbs_to_orphanage(cpu, rsp, rnp, rdp); rcu_adopt_orphan_cbs(rsp, flags); raw_spin_unlock_irqrestore(&rsp->orphan_lock, flags); -- cgit v1.2.3