[PATCH rcu 3/3] rcu: Allow expedited RCU grace periods on incoming CPUs

["Paul E. McKenney" <paulmck@kernel.org>]

Although it is usually safe to invoke synchronize_rcu_expedited() from a
preemption-enabled CPU-hotplug notifier, if it is invoked from a notifier
between CPUHP_AP_RCUTREE_ONLINE and CPUHP_AP_ACTIVE, its attempts to
invoke a workqueue handler will hang due to RCU waiting on a CPU that
the scheduler is not paying attention to.  This commit therefore expands
use of the existing workqueue-independent synchronize_rcu_expedited()
from early boot to also include CPUs that are being hotplugged.

Link: https://lore.kernel.org/lkml/7359f994-8aaf-3cea-f5cf-c0d3929689d6@quicinc.com/
Reported-by: Mukesh Ojha <quic_mojha@quicinc.com>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
 kernel/rcu/tree_exp.h | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
index 60197ea24ceb9..1a45667402260 100644
--- a/kernel/rcu/tree_exp.h
+++ b/kernel/rcu/tree_exp.h
@@ -816,7 +816,7 @@ static int rcu_print_task_exp_stall(struct rcu_node *rnp)
  */
 void synchronize_rcu_expedited(void)
 {
-	bool boottime = (rcu_scheduler_active == RCU_SCHEDULER_INIT);
+	bool no_wq;
 	struct rcu_exp_work rew;
 	struct rcu_node *rnp;
 	unsigned long s;
@@ -841,9 +841,15 @@ void synchronize_rcu_expedited(void)
 	if (exp_funnel_lock(s))
 		return;  /* Someone else did our work for us. */
 
+	/* Don't use workqueue during boot or from an incoming CPU. */
+	preempt_disable();
+	no_wq = rcu_scheduler_active == RCU_SCHEDULER_INIT ||
+		!cpumask_test_cpu(smp_processor_id(), cpu_active_mask);
+	preempt_enable();
+
 	/* Ensure that load happens before action based on it. */
-	if (unlikely(boottime)) {
-		/* Direct call during scheduler init and early_initcalls(). */
+	if (unlikely(no_wq)) {
+		/* Direct call for scheduler init, early_initcall()s, and incoming CPUs. */
 		rcu_exp_sel_wait_wake(s);
 	} else {
 		/* Marshall arguments & schedule the expedited grace period. */
@@ -861,7 +867,7 @@ void synchronize_rcu_expedited(void)
 	/* Let the next expedited grace period start. */
 	mutex_unlock(&rcu_state.exp_mutex);
 
-	if (likely(!boottime))
+	if (likely(!no_wq))
 		destroy_work_on_stack(&rew.rew_work);
 }
 EXPORT_SYMBOL_GPL(synchronize_rcu_expedited);
-- 
2.31.1.189.g2e36527f23