[PATCH] rcu: Use cpus_read_lock() while looking at cpu_online_mask

[Sebastian Andrzej Siewior]

It was possible that sync_rcu_exp_select_cpus() enqueued something on
CPU0 while CPU0 was offline. Such a work item wouldn't be processed
until CPU0 gets back online. This problem was addressed in commit
fcc6354365015 ("rcu: Make expedited GPs handle CPU 0 being offline"). I
don't think the issue fully addressed.

Assume grplo = 0 and grphi = 7 and sync_rcu_exp_select_cpus() is invoked
on CPU1. The preempt_disable() section on CPU1 won't ensure that CPU0
remains online between looking at cpu_online_mask and invoking
queue_work_on() on CPU1.

Use cpus_read_lock() to ensure that `cpu' is not going down between
looking at cpu_online_mask at invoking queue_work_on() and waiting for
its completion. It is added around the loop + flush_work() which is
similar to work_on_cpu_safe() (and we can have multiple jobs running on
NUMA systems).

Fixes: fcc6354365015 ("rcu: Make expedited GPs handle CPU 0 being
		      offline")
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
---
 kernel/rcu/tree_exp.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
index 01b6ddeb4f050..a104cf91e6b90 100644
--- a/kernel/rcu/tree_exp.h
+++ b/kernel/rcu/tree_exp.h
@@ -479,6 +479,7 @@ static void sync_rcu_exp_select_cpus(struct rcu_state *rsp,
 	sync_exp_reset_tree(rsp);
 	trace_rcu_exp_grace_period(rsp->name, rcu_exp_gp_seq_endval(rsp), TPS("select"));
 
+	cpus_read_lock();
 	/* Schedule work for each leaf rcu_node structure. */
 	rcu_for_each_leaf_node(rsp, rnp) {
 		rnp->exp_need_flush = false;
@@ -493,13 +494,11 @@ static void sync_rcu_exp_select_cpus(struct rcu_state *rsp,
 			continue;
 		}
 		INIT_WORK(&rnp->rew.rew_work, sync_rcu_exp_select_node_cpus);
-		preempt_disable();
 		cpu = cpumask_next(rnp->grplo - 1, cpu_online_mask);
 		/* If all offline, queue the work on an unbound CPU. */
 		if (unlikely(cpu > rnp->grphi))
 			cpu = WORK_CPU_UNBOUND;
 		queue_work_on(cpu, rcu_par_gp_wq, &rnp->rew.rew_work);
-		preempt_enable();
 		rnp->exp_need_flush = true;
 	}
 
@@ -507,6 +506,7 @@ static void sync_rcu_exp_select_cpus(struct rcu_state *rsp,
 	rcu_for_each_leaf_node(rsp, rnp)
 		if (rnp->exp_need_flush)
 			flush_work(&rnp->rew.rew_work);
+	cpus_read_unlock();
 }
 
 static void synchronize_sched_expedited_wait(struct rcu_state *rsp)
-- 
2.19.0.rc2