[PATCH tip/core/rcu 2/2] rcu: Make expedited GPs handle CPU 0 being offline

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

From: Boqun Feng <boqun.feng@gmail.com>

Currently, the parallelized initialization of expedited grace periods uses
the workqueue associated with each rcu_node structure's ->grplo field.
This works fine unless that CPU is offline.  This commit therefore
uses the CPU corresponding to the lowest-numbered online CPU, or just
reports the quiescent states if there are no online CPUs on this rcu_node
structure.

Note that this patch uses cpu_is_offline() instead of the usual
approach of checking bits in the rcu_node structure's ->qsmaskinitnext
field.  This is safe because preemption is disabled across both the
cpu_is_offline() check and the call to queue_work_on().

Not-Yet-Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
[ paulmck: Disable preemption to close offline race window. ]
Not-Yet-Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 kernel/rcu/tree_exp.h | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
index c6385ee1af65..6acac74092cb 100644
--- a/kernel/rcu/tree_exp.h
+++ b/kernel/rcu/tree_exp.h
@@ -472,6 +472,7 @@ static void sync_rcu_exp_select_node_cpus(struct work_struct *wp)
 static void sync_rcu_exp_select_cpus(struct rcu_state *rsp,
 				     smp_call_func_t func)
 {
+	int cpu;
 	struct rcu_node *rnp;
 
 	trace_rcu_exp_grace_period(rsp->name, rcu_exp_gp_seq_endval(rsp), TPS("reset"));
@@ -493,8 +494,19 @@ static void sync_rcu_exp_select_cpus(struct rcu_state *rsp,
 			continue;
 		}
 		INIT_WORK(&rnp->rew.rew_work, sync_rcu_exp_select_node_cpus);
-		queue_work_on(rnp->grplo, rcu_par_gp_wq, &rnp->rew.rew_work);
-		rnp->exp_need_flush = true;
+		preempt_disable();
+		for_each_leaf_node_possible_cpu(rnp, cpu) {
+			if (cpu_is_offline(cpu)) /* Preemption disabled. */
+				continue;
+			queue_work_on(cpu, rcu_par_gp_wq, &rnp->rew.rew_work);
+			rnp->exp_need_flush = true;
+			break;
+		}
+		preempt_enable();
+		if (!rnp->exp_need_flush) { /* All offline, report QSes. */
+			queue_work(rcu_par_gp_wq, &rnp->rew.rew_work);
+			rnp->exp_need_flush = true;
+		}
 	}
 
 	/* Wait for workqueue jobs (if any) to complete. */
-- 
2.17.1