[PATCH 4/4] sched: Exclude CPU boot code from PF_IDLE area

[Frederic Weisbecker <frederic@xxxxxxxxxx>]

The commit:

	cff9b2332ab7 ("kernel/sched: Modify initial boot task idle setup")

has changed the semantics of what is to be considered an idle task in
such a way that only the actual idle loop is accounted as PF_IDLE. The
intent is to exclude the CPU boot code from that coverage.

However this doesn't clear the flag when the CPU goes down. Therefore
when the CPU goes up again, its boot code is part of the PF_IDLE zone.

Make sure this flag behave consistently and clear the flag when a CPU
exits from the idle loop. If anything, RCU-tasks relies on it to exclude
CPU boot code from its quiescent states.

Fixes: cff9b2332ab7 ("kernel/sched: Modify initial boot task idle setup")
Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
---
 include/linux/sched.h | 2 +-
 kernel/cpu.c          | 4 ++++
 kernel/sched/idle.c   | 1 -
 3 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 8885be2c143e..ad18962b921d 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1945,7 +1945,7 @@ extern struct task_struct *idle_task(int cpu);
  */
 static __always_inline bool is_idle_task(const struct task_struct *p)
 {
-	return !!(p->flags & PF_IDLE);
+	return !!(READ_ONCE(p->flags) & PF_IDLE);
 }
 
 extern struct task_struct *curr_task(int cpu);
diff --git a/kernel/cpu.c b/kernel/cpu.c
index 3b9d5c7eb4a2..3a1991010f4e 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -1394,7 +1394,9 @@ void cpuhp_report_idle_dead(void)
 {
 	struct cpuhp_cpu_state *st = this_cpu_ptr(&cpuhp_state);
 
+	WRITE_ONCE(current->flags, current->flags & ~PF_IDLE);
 	BUG_ON(st->state != CPUHP_AP_OFFLINE);
+
 	rcutree_report_cpu_dead();
 	st->state = CPUHP_AP_IDLE_DEAD;
 	/*
@@ -1642,6 +1644,8 @@ void cpuhp_online_idle(enum cpuhp_state state)
 {
 	struct cpuhp_cpu_state *st = this_cpu_ptr(&cpuhp_state);
 
+	WRITE_ONCE(current->flags, current->flags | PF_IDLE);
+
 	/* Happens for the boot cpu */
 	if (state != CPUHP_AP_ONLINE_IDLE)
 		return;
diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
index 5007b25c5bc6..342f58a329f5 100644
--- a/kernel/sched/idle.c
+++ b/kernel/sched/idle.c
@@ -373,7 +373,6 @@ EXPORT_SYMBOL_GPL(play_idle_precise);
 
 void cpu_startup_entry(enum cpuhp_state state)
 {
-	current->flags |= PF_IDLE;
 	arch_cpu_idle_prepare();
 	cpuhp_online_idle(state);
 	while (1)
-- 
2.41.0