rt,ntp: Move call to schedule_delayed_work() to helper thread

The ntp code for notify_cmos_timer() is called from a hard interrupt
context. schedule_delayed_work() under PREEMPT_RT_FULL calls spinlocks
that have been converted to mutexes, thus calling schedule_delayed_work()
from interrupt is not safe.

Add a helper thread that does the call to schedule_delayed_work and wake
up that thread instead of calling schedule_delayed_work() directly.
This is only for CONFIG_PREEMPT_RT_FULL, otherwise the code still calls
schedule_delayed_work() directly in irq context.

Note: There's a few places in the kernel that do this. Perhaps the RT
code should have a dedicated thread that does the checks. Just register
a notifier on boot up for your check and wake up the thread when
needed. This will be a todo.

Signed-off-by: Steven Rostedt <[email protected]>
[bigeasy: use swork_queue() instead a helper thread]
Signed-off-by: Sebastian Andrzej Siewior <[email protected]>

Author: rostedt

kernel/time/ntp.c

@@ -17,6 +17,7 @@
 #include <linux/module.h>
 #include <linux/rtc.h>
 #include <linux/math64.h>
+#include <linux/swork.h>
 
 #include "ntp_internal.h"
 #include "timekeeping_internal.h"
@@ -568,10 +569,35 @@ static void sync_cmos_clock(struct work_struct *work)
 			   &sync_cmos_work, timespec64_to_jiffies(&next));
 }
 
+#ifdef CONFIG_PREEMPT_RT_FULL
+
+static void run_clock_set_delay(struct swork_event *event)
+{
+	queue_delayed_work(system_power_efficient_wq, &sync_cmos_work, 0);
+}
+
+static struct swork_event ntp_cmos_swork;
+
+void ntp_notify_cmos_timer(void)
+{
+	swork_queue(&ntp_cmos_swork);
+}
+
+static __init int create_cmos_delay_thread(void)
+{
+	WARN_ON(swork_get());
+	INIT_SWORK(&ntp_cmos_swork, run_clock_set_delay);
+	return 0;
+}
+early_initcall(create_cmos_delay_thread);
+
+#else
+
 void ntp_notify_cmos_timer(void)
 {
 	queue_delayed_work(system_power_efficient_wq, &sync_cmos_work, 0);
 }
+#endif /* CONFIG_PREEMPT_RT_FULL */
 
 #else
 void ntp_notify_cmos_timer(void) { }