From: Juri Lelli <juri.lelli@redhat.com>
Date: Fri, 19 Jul 2019 15:59:59 +0200
Subject: rcu/tree: Call setschedule() gp ktread to SCHED_FIFO outside of
 atomic region
Git-commit: 1a763fd7c6335e3122c1cc09576ef6c99ada4267
Patch-mainline: 5.4-rc1
References: bnc#1151927 5.3.4

sched_setscheduler() needs to acquire cpuset_rwsem, but it is currently
called from an invalid (atomic) context by rcu_spawn_gp_kthread().

Fix that by simply moving sched_setscheduler_nocheck() call outside of
the atomic region, as it doesn't actually require to be guarded by
rcu_node lock.

Suggested-by: Peter Zijlstra <peterz@infradead.org>
Tested-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Signed-off-by: Juri Lelli <juri.lelli@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: bristot@redhat.com
Cc: claudio@evidence.eu.com
Cc: lizefan@huawei.com
Cc: longman@redhat.com
Cc: luca.abeni@santannapisa.it
Cc: mathieu.poirier@linaro.org
Cc: rostedt@goodmis.org
Cc: tj@kernel.org
Cc: tommaso.cucinotta@santannapisa.it
Link: https://lkml.kernel.org/r/20190719140000.31694-8-juri.lelli@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
---
 kernel/rcu/tree.c |    6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3234,13 +3234,13 @@ static int __init rcu_spawn_gp_kthread(v
 	t = kthread_create(rcu_gp_kthread, NULL, "%s", rcu_state.name);
 	if (WARN_ONCE(IS_ERR(t), "%s: Could not start grace-period kthread, OOM is now expected behavior\n", __func__))
 		return 0;
+	if (kthread_prio)
+		sched_setscheduler_nocheck(t, SCHED_FIFO, &sp);
 	rnp = rcu_get_root();
 	raw_spin_lock_irqsave_rcu_node(rnp, flags);
 	rcu_state.gp_kthread = t;
-	if (kthread_prio) {
+	if (kthread_prio)
 		sp.sched_priority = kthread_prio;
-		sched_setscheduler_nocheck(t, SCHED_FIFO, &sp);
-	}
 	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 	wake_up_process(t);
 	rcu_spawn_nocb_kthreads();