timers: Reinitialize per cpu bases on hotplug

commit 26456f87aca7157c057de65c9414b37f1ab881d1 upstream.

The timer wheel bases are not (re)initialized on CPU hotplug. That leaves
them with a potentially stale clk and next_expiry valuem, which can cause
trouble then the CPU is plugged.

Add a prepare callback which forwards the clock, sets next_expiry to far in
the future and reset the control flags to a known state.

Set base->must_forward_clk so the first timer which is queued will try to
forward the clock to current jiffies.

Fixes: 500462a9de ("timers: Switch to a non-cascading wheel")
Reported-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Sebastian Siewior <bigeasy@linutronix.de>
Cc: Anna-Maria Gleixner <anna-maria@linutronix.de>
Link: https://lkml.kernel.org/r/alpine.DEB.2.20.1712272152200.2431@nanos
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

kernel/cpu.c

@@ -1309,9 +1309,9 @@ static struct cpuhp_step cpuhp_bp_states[] = {
 	 * before blk_mq_queue_reinit_notify() from notify_dead(),
 	 * otherwise a RCU stall occurs.
 	 */
-	[CPUHP_TIMERS_DEAD] = {
+	[CPUHP_TIMERS_PREPARE] = {
 		.name			= "timers:dead",
-		.startup.single		= NULL,
+		.startup.single		= timers_prepare_cpu,
 		.teardown.single	= timers_dead_cpu,
 	},
 	/* Kicks the plugged cpu into life */

kernel/time/timer.c

@@ -1851,6 +1851,21 @@ static void migrate_timer_list(struct timer_base *new_base, struct hlist_head *h
 	}
 }
 
+int timers_prepare_cpu(unsigned int cpu)
+{
+	struct timer_base *base;
+	int b;
+
+	for (b = 0; b < NR_BASES; b++) {
+		base = per_cpu_ptr(&timer_bases[b], cpu);
+		base->clk = jiffies;
+		base->next_expiry = base->clk + NEXT_TIMER_MAX_DELTA;
+		base->is_idle = false;
+		base->must_forward_clk = true;
+	}
+	return 0;
+}
+
 int timers_dead_cpu(unsigned int cpu)
 {
 	struct timer_base *old_base;