From 1b8fe659df64e431894e6b47a3b18a228f622bb1 Mon Sep 17 00:00:00 2001
Message-Id: <1b8fe659df64e431894e6b47a3b18a228f622bb1.1599166691.git.zanussi@kernel.org>
In-Reply-To: <56457dc415803c8abc5acb513ada877a79596f05.1599166690.git.zanussi@kernel.org>
References: <56457dc415803c8abc5acb513ada877a79596f05.1599166690.git.zanussi@kernel.org>
From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Date: Thu, 5 Jul 2018 14:44:51 +0200
Subject: [PATCH 155/333] sched/migrate_disable: fallback to preempt_disable()
 instead barrier()
Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/4.19/older/patches-4.19.142-rt63.tar.xz

On SMP + !RT migrate_disable() is still around. It is not part of spin_lock()
anymore so it has almost no users. However the futex code has a workaround for
the !in_atomic() part of migrate disable which fails because the matching
migrade_disable() is no longer part of spin_lock().

On !SMP + !RT migrate_disable() is reduced to barrier(). This is not optimal
because we few spots where a "preempt_disable()" statement was replaced with
"migrate_disable()".

We also used the migration_disable counter to figure out if a sleeping lock is
acquired so RCU does not complain about schedule() during rcu_read_lock() while
a sleeping lock is held. This changed, we no longer use it, we have now a
sleeping_lock counter for the RCU purpose.

This means we can now:
- for SMP + RT_BASE
  full migration program, nothing changes here

- for !SMP + RT_BASE
  the migration counting is no longer required. It used to ensure that the task
  is not migrated to another CPU and that this CPU remains online. !SMP ensures
  that already.
  Move it to CONFIG_SCHED_DEBUG so the counting is done for debugging purpose
  only.

- for all other cases including !RT
  fallback to preempt_disable(). The only remaining users of migrate_disable()
  are those which were converted from preempt_disable() and the futex
  workaround which is already in the preempt_disable() section due to the
  spin_lock that is held.

Cc: stable-rt@vger.kernel.org
Reported-by: joe.korty@concurrent-rt.com
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 include/linux/preempt.h |  6 +++---
 include/linux/sched.h   |  4 ++--
 kernel/sched/core.c     | 23 +++++++++++------------
 kernel/sched/debug.c    |  2 +-
 4 files changed, 17 insertions(+), 18 deletions(-)

diff --git a/include/linux/preempt.h b/include/linux/preempt.h
index 9eafc34898b4..ed8413e7140f 100644
--- a/include/linux/preempt.h
+++ b/include/linux/preempt.h
@@ -204,7 +204,7 @@ do { \
 
 #define preemptible()	(preempt_count() == 0 && !irqs_disabled())
 
-#ifdef CONFIG_SMP
+#if defined(CONFIG_SMP) && defined(CONFIG_PREEMPT_RT_BASE)
 
 extern void migrate_disable(void);
 extern void migrate_enable(void);
@@ -221,8 +221,8 @@ static inline int __migrate_disabled(struct task_struct *p)
 }
 
 #else
-#define migrate_disable()		barrier()
-#define migrate_enable()		barrier()
+#define migrate_disable()		preempt_disable()
+#define migrate_enable()		preempt_enable()
 static inline int __migrate_disabled(struct task_struct *p)
 {
 	return 0;
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 50197f5c4506..4a4f03a8d8d4 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -667,7 +667,7 @@ struct task_struct {
 	int				nr_cpus_allowed;
 	const cpumask_t			*cpus_ptr;
 	cpumask_t			cpus_mask;
-#if defined(CONFIG_PREEMPT_COUNT) && defined(CONFIG_SMP)
+#if defined(CONFIG_SMP) && defined(CONFIG_PREEMPT_RT_BASE)
 	int				migrate_disable;
 	int				migrate_disable_update;
 # ifdef CONFIG_SCHED_DEBUG
@@ -675,8 +675,8 @@ struct task_struct {
 # endif
 
 #elif !defined(CONFIG_SMP) && defined(CONFIG_PREEMPT_RT_BASE)
-	int				migrate_disable;
 # ifdef CONFIG_SCHED_DEBUG
+	int				migrate_disable;
 	int				migrate_disable_atomic;
 # endif
 #endif
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index d10698fa8cad..453e1a45dcbf 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1031,7 +1031,7 @@ void set_cpus_allowed_common(struct task_struct *p, const struct cpumask *new_ma
 	p->nr_cpus_allowed = cpumask_weight(new_mask);
 }
 
-#if defined(CONFIG_PREEMPT_COUNT) && defined(CONFIG_SMP)
+#if defined(CONFIG_SMP) && defined(CONFIG_PREEMPT_RT_BASE)
 int __migrate_disabled(struct task_struct *p)
 {
 	return p->migrate_disable;
@@ -1071,7 +1071,7 @@ static void __do_set_cpus_allowed_tail(struct task_struct *p,
 
 void do_set_cpus_allowed(struct task_struct *p, const struct cpumask *new_mask)
 {
-#if defined(CONFIG_PREEMPT_COUNT) && defined(CONFIG_SMP)
+#if defined(CONFIG_SMP) && defined(CONFIG_PREEMPT_RT_BASE)
 	if (__migrate_disabled(p)) {
 		lockdep_assert_held(&p->pi_lock);
 
@@ -1145,7 +1145,7 @@ static int __set_cpus_allowed_ptr(struct task_struct *p,
 	if (cpumask_test_cpu(task_cpu(p), new_mask) || __migrate_disabled(p))
 		goto out;
 
-#if defined(CONFIG_PREEMPT_COUNT) && defined(CONFIG_SMP)
+#if defined(CONFIG_SMP) && defined(CONFIG_PREEMPT_RT_BASE)
 	if (__migrate_disabled(p)) {
 		p->migrate_disable_update = 1;
 		goto out;
@@ -7205,7 +7205,7 @@ const u32 sched_prio_to_wmult[40] = {
 
 #undef CREATE_TRACE_POINTS
 
-#if defined(CONFIG_PREEMPT_COUNT) && defined(CONFIG_SMP)
+#if defined(CONFIG_SMP) && defined(CONFIG_PREEMPT_RT_BASE)
 
 static inline void
 update_nr_migratory(struct task_struct *p, long delta)
@@ -7353,45 +7353,44 @@ EXPORT_SYMBOL(migrate_enable);
 #elif !defined(CONFIG_SMP) && defined(CONFIG_PREEMPT_RT_BASE)
 void migrate_disable(void)
 {
+#ifdef CONFIG_SCHED_DEBUG
 	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++;
+#endif
+	barrier();
 }
 EXPORT_SYMBOL(migrate_disable);
 
 void migrate_enable(void)
 {
+#ifdef CONFIG_SCHED_DEBUG
 	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--;
+#endif
+	barrier();
 }
 EXPORT_SYMBOL(migrate_enable);
 #endif
diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
index 5027158d3908..dd6c364d6f01 100644
--- a/kernel/sched/debug.c
+++ b/kernel/sched/debug.c
@@ -982,7 +982,7 @@ void proc_sched_show_task(struct task_struct *p, struct pid_namespace *ns,
 		P(dl.runtime);
 		P(dl.deadline);
 	}
-#if defined(CONFIG_PREEMPT_COUNT) && defined(CONFIG_SMP)
+#if defined(CONFIG_SMP) && defined(CONFIG_PREEMPT_RT_BASE)
 	P(migrate_disable);
 #endif
 	P(nr_cpus_allowed);
-- 
2.17.1

