posix-timers: Use a callback for cancel synchronization on PREEMPT_RT

Posix timer delete retry loops are affected by the same priority inversion
and live lock issues as the other timers.
    
Provide a RT specific synchronization function which keeps a reference to
the timer by holding rcu read lock to prevent the timer from being freed,
dropping the timer lock and invoking the timer specific wait function via a
new callback.
    
This does not yet cover posix CPU timers because they need more special
treatment on PREEMPT_RT.

[ This is folded into the original attempt which did not use a callback. ]
Originally-by: Anna-Maria Gleixenr <anna-maria@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Link: https://lkml.kernel.org/r/20190819143801.656864506@linutronix.de

kernel/time/alarmtimer.c

@@ -605,6 +605,19 @@ static int alarm_timer_try_to_cancel(struct k_itimer *timr)
 	return alarm_try_to_cancel(&timr->it.alarm.alarmtimer);
 }
 
+/**
+ * alarm_timer_wait_running - Posix timer callback to wait for a timer
+ * @timr:	Pointer to the posixtimer data struct
+ *
+ * Called from the core code when timer cancel detected that the callback
+ * is running. @timr is unlocked and rcu read lock is held to prevent it
+ * from being freed.
+ */
+static void alarm_timer_wait_running(struct k_itimer *timr)
+{
+	hrtimer_cancel_wait_running(&timr->it.alarm.alarmtimer.timer);
+}
+
 /**
  * alarm_timer_arm - Posix timer callback to arm a timer
  * @timr:	Pointer to the posixtimer data struct
@@ -834,6 +847,7 @@ const struct k_clock alarm_clock = {
 	.timer_forward		= alarm_timer_forward,
 	.timer_remaining	= alarm_timer_remaining,
 	.timer_try_to_cancel	= alarm_timer_try_to_cancel,
+	.timer_wait_running	= alarm_timer_wait_running,
 	.nsleep			= alarm_timer_nsleep,
 };
 #endif /* CONFIG_POSIX_TIMERS */

kernel/time/posix-timers.c

@@ -805,13 +805,25 @@ static int common_hrtimer_try_to_cancel(struct k_itimer *timr)
 	return hrtimer_try_to_cancel(&timr->it.real.timer);
 }
 
+static void common_timer_wait_running(struct k_itimer *timer)
+{
+	hrtimer_cancel_wait_running(&timer->it.real.timer);
+}
+
 static struct k_itimer *timer_wait_running(struct k_itimer *timer,
 					   unsigned long *flags)
 {
+	const struct k_clock *kc = READ_ONCE(timer->kclock);
 	timer_t timer_id = READ_ONCE(timer->it_id);
 
+	/* Prevent kfree(timer) after dropping the lock */
+	rcu_read_lock();
 	unlock_timer(timer, *flags);
-	cpu_relax();
+
+	if (!WARN_ON_ONCE(!kc->timer_wait_running))
+		kc->timer_wait_running(timer);
+
+	rcu_read_unlock();
 	/* Relock the timer. It might be not longer hashed. */
 	return lock_timer(timer_id, flags);
 }
@@ -1255,6 +1267,7 @@ static const struct k_clock clock_realtime = {
 	.timer_forward		= common_hrtimer_forward,
 	.timer_remaining	= common_hrtimer_remaining,
 	.timer_try_to_cancel	= common_hrtimer_try_to_cancel,
+	.timer_wait_running	= common_timer_wait_running,
 	.timer_arm		= common_hrtimer_arm,
 };
 
@@ -1270,6 +1283,7 @@ static const struct k_clock clock_monotonic = {
 	.timer_forward		= common_hrtimer_forward,
 	.timer_remaining	= common_hrtimer_remaining,
 	.timer_try_to_cancel	= common_hrtimer_try_to_cancel,
+	.timer_wait_running	= common_timer_wait_running,
 	.timer_arm		= common_hrtimer_arm,
 };
 
@@ -1300,6 +1314,7 @@ static const struct k_clock clock_tai = {
 	.timer_forward		= common_hrtimer_forward,
 	.timer_remaining	= common_hrtimer_remaining,
 	.timer_try_to_cancel	= common_hrtimer_try_to_cancel,
+	.timer_wait_running	= common_timer_wait_running,
 	.timer_arm		= common_hrtimer_arm,
 };
 
@@ -1315,6 +1330,7 @@ static const struct k_clock clock_boottime = {
 	.timer_forward		= common_hrtimer_forward,
 	.timer_remaining	= common_hrtimer_remaining,
 	.timer_try_to_cancel	= common_hrtimer_try_to_cancel,
+	.timer_wait_running	= common_timer_wait_running,
 	.timer_arm		= common_hrtimer_arm,
 };
 

kernel/time/posix-timers.h

@@ -24,6 +24,7 @@ struct k_clock {
 	int	(*timer_try_to_cancel)(struct k_itimer *timr);
 	void	(*timer_arm)(struct k_itimer *timr, ktime_t expires,
 			     bool absolute, bool sigev_none);
+	void	(*timer_wait_running)(struct k_itimer *timr);
 };
 
 extern const struct k_clock clock_posix_cpu;