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

From: Paul E. McKenney
Date: Mon Jun 25 2018 - 18:45:05 EST


From: Boqun Feng <boqun.feng@xxxxxxxxx>

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@xxxxxxxxx>
[ paulmck: Disable preemption to close offline race window. ]
Not-Yet-Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
---
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