From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Date: Fri, 16 Feb 2018 11:45:13 +0100
Subject: [PATCH] RCU: skip the "schedule() in RCU section" warning on UP,
 too
Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/4.16/older/patches-4.16.7-rt1.tar.xz

In "RCU: we need to skip that warning but only on sleeping locks" we
skipped a warning on SMP systems in case we schedule out in a RCU
section while attempt to obtain a sleeping lock. This is also required
on UP systems.
In order to do so, I introduce a tiny version of migrate_disable() +
_enable() which only update the counters which we then can check against
on RT && !SMP.

Cc: stable-rt@vger.kernel.org
Reported-by: Grygorii Strashko <grygorii.strashko@ti.com>
Tested-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 include/linux/preempt.h  |    9 +++++++++
 include/linux/sched.h    |    6 ++++++
 kernel/rcu/tree_plugin.h |    2 +-
 kernel/sched/core.c      |   45 +++++++++++++++++++++++++++++++++++++++++++++
 4 files changed, 61 insertions(+), 1 deletion(-)

--- a/include/linux/preempt.h
+++ b/include/linux/preempt.h
@@ -211,6 +211,15 @@ extern void migrate_enable(void);
 
 int __migrate_disabled(struct task_struct *p);
 
+#elif !defined(CONFIG_SMP) && defined(CONFIG_PREEMPT_RT_BASE)
+
+extern void migrate_disable(void);
+extern void migrate_enable(void);
+static inline int __migrate_disabled(struct task_struct *p)
+{
+	return 0;
+}
+
 #else
 #define migrate_disable()		barrier()
 #define migrate_enable()		barrier()
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -604,6 +604,12 @@ struct task_struct {
 # ifdef CONFIG_SCHED_DEBUG
 	int				migrate_disable_atomic;
 # endif
+
+#elif !defined(CONFIG_SMP) && defined(CONFIG_PREEMPT_RT_BASE)
+	int				migrate_disable;
+# ifdef CONFIG_SCHED_DEBUG
+	int				migrate_disable_atomic;
+# endif
 #endif
 
 #ifdef CONFIG_PREEMPT_RCU
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -328,7 +328,7 @@ static void rcu_preempt_note_context_swi
 	int mg_counter = 0;
 
 	lockdep_assert_irqs_disabled();
-#if defined(CONFIG_PREEMPT_COUNT) && defined(CONFIG_SMP)
+#if defined(CONFIG_PREEMPT_RT_BASE)
 	mg_counter = t->migrate_disable;
 #endif
 	WARN_ON_ONCE(!preempt && t->rcu_read_lock_nesting > 0 && !mg_counter);
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7278,4 +7278,49 @@ void migrate_enable(void)
 	preempt_enable();
 }
 EXPORT_SYMBOL(migrate_enable);
+
+#elif !defined(CONFIG_SMP) && defined(CONFIG_PREEMPT_RT_BASE)
+void migrate_disable(void)
+{
+	struct task_struct *p = current;
+
+	if (in_atomic() || irqs_disabled()) {
+#ifdef CONFIG_SCHED_DEBUG
+		p->migrate_disable_atomic++;
+#endif
+		return;
+	}
+#ifdef CONFIG_SCHED_DEBUG
+	if (unlikely(p->migrate_disable_atomic)) {
+		tracing_off();
+		WARN_ON_ONCE(1);
+	}
+#endif
+
+	p->migrate_disable++;
+}
+EXPORT_SYMBOL(migrate_disable);
+
+void migrate_enable(void)
+{
+	struct task_struct *p = current;
+
+	if (in_atomic() || irqs_disabled()) {
+#ifdef CONFIG_SCHED_DEBUG
+		p->migrate_disable_atomic--;
+#endif
+		return;
+	}
+
+#ifdef CONFIG_SCHED_DEBUG
+	if (unlikely(p->migrate_disable_atomic)) {
+		tracing_off();
+		WARN_ON_ONCE(1);
+	}
+#endif
+
+	WARN_ON_ONCE(p->migrate_disable <= 0);
+	p->migrate_disable--;
+}
+EXPORT_SYMBOL(migrate_enable);
 #endif