From: Scott Wood <swood@redhat.com>
Date: Wed, 11 Sep 2019 17:57:28 +0100
Subject: [PATCH] rcu: Use rcuc threads on PREEMPT_RT as we did
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-rt-devel.git
Git-commit: da77ceac3d20f27310a07a7c346a4ee6b40d6c28
Patch-mainline: Queued in subsystem maintainer repository
References: SLE Realtime Extension

While switching to the reworked RCU-thread code, it has been forgotten
to enable the thread processing on -RT.
Besides restoring behavior that used to be default on RT, this avoids
a deadlock on scheduler locks.

Signed-off-by: Scott Wood <swood@redhat.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Daniel Wagner <dwagner@suse.de>
---
 kernel/rcu/tree.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -100,8 +100,10 @@ struct rcu_state rcu_state = {
 static bool dump_tree;
 module_param(dump_tree, bool, 0444);
 /* By default, use RCU_SOFTIRQ instead of rcuc kthreads. */
-static bool use_softirq = 1;
+static bool use_softirq = !IS_ENABLED(CONFIG_PREEMPT_RT);
+#ifndef CONFIG_PREEMPT_RT
 module_param(use_softirq, bool, 0444);
+#endif
 /* Control rcu_node-tree auto-balancing at boot time. */
 static bool rcu_fanout_exact;
 module_param(rcu_fanout_exact, bool, 0444);