[PATCH 4.19] hrtimer: Report offline hrtimer enqueue

[Thomas Gleixner <tglx@xxxxxxxxxxxxx>]

From:  Frederic Weisbecker <frederic@xxxxxxxxxx>

commit dad6a09f3148257ac1773cd90934d721d68ab595 upstream.

The hrtimers migration on CPU-down hotplug process has been moved
earlier, before the CPU actually goes to die. This leaves a small window
of opportunity to queue an hrtimer in a blind spot, leaving it ignored.

For example a practical case has been reported with RCU waking up a
SCHED_FIFO task right before the CPUHP_AP_IDLE_DEAD stage, queuing that
way a sched/rt timer to the local offline CPU.

Make sure such situations never go unnoticed and warn when that happens.

Fixes: 5c0930ccaad5 ("hrtimers: Push pending hrtimers away from outgoing CPU earlier")
Reported-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Link: https://lore.kernel.org/r/20240129235646.3171983-4-boqun.feng@xxxxxxxxx
---

Backport to 4.19 as tglx has too much spare time...

---
 include/linux/hrtimer.h |    4 +++-
 kernel/time/hrtimer.c   |    3 +++
 2 files changed, 6 insertions(+), 1 deletion(-)

--- a/include/linux/hrtimer.h
+++ b/include/linux/hrtimer.h
@@ -182,6 +182,7 @@ enum  hrtimer_base_type {
  * @hang_detected:	The last hrtimer interrupt detected a hang
  * @softirq_activated:	displays, if the softirq is raised - update of softirq
  *			related settings is not required then.
+ * @online:		CPU is online from an hrtimers point of view
  * @nr_events:		Total number of hrtimer interrupt events
  * @nr_retries:		Total number of hrtimer interrupt retries
  * @nr_hangs:		Total number of hrtimer interrupt hangs
@@ -206,7 +207,8 @@ struct hrtimer_cpu_base {
 	unsigned int			hres_active		: 1,
 					in_hrtirq		: 1,
 					hang_detected		: 1,
-					softirq_activated       : 1;
+					softirq_activated       : 1,
+					online			: 1;
 #ifdef CONFIG_HIGH_RES_TIMERS
 	unsigned int			nr_events;
 	unsigned short			nr_retries;
--- a/kernel/time/hrtimer.c
+++ b/kernel/time/hrtimer.c
@@ -970,6 +970,7 @@ static int enqueue_hrtimer(struct hrtime
 			   enum hrtimer_mode mode)
 {
 	debug_activate(timer, mode);
+	WARN_ON_ONCE(!base->cpu_base->online);
 
 	base->cpu_base->active_bases |= 1 << base->index;
 
@@ -1887,6 +1888,7 @@ int hrtimers_prepare_cpu(unsigned int cp
 	cpu_base->softirq_next_timer = NULL;
 	cpu_base->expires_next = KTIME_MAX;
 	cpu_base->softirq_expires_next = KTIME_MAX;
+	cpu_base->online = 1;
 	return 0;
 }
 
@@ -1953,6 +1955,7 @@ int hrtimers_cpu_dying(unsigned int dyin
 	smp_call_function_single(ncpu, retrigger_next_event, NULL, 0);
 
 	raw_spin_unlock(&new_base->lock);
+	old_base->online = 0;
 	raw_spin_unlock(&old_base->lock);
 
 	return 0;