[PATCH 2/3] rcu/nocb: Fix rcuog wake-up from offline softirq

[Frederic Weisbecker]

After a CPU has set itself offline and before it eventually calls
rcutree_report_cpu_dead(), there are still opportunities for callbacks
to be enqueued, for example from an IRQ. When that happens on NOCB, the
rcuog wake-up is deferred through an IPI to an online CPU in order not
to call into the scheduler and risk arming the RT-bandwidth after
hrtimers have been migrated out and disabled.

But performing a synchronized IPI from an IRQ is buggy as reported in
the following scenario:

	WARNING: CPU: 1 PID: 26 at kernel/smp.c:633 smp_call_function_single
	Modules linked in: rcutorture torture
	CPU: 1 UID: 0 PID: 26 Comm: migration/1 Not tainted 6.11.0-rc1-00012-g9139f93209d1 #1
	Stopper: multi_cpu_stop+0x0/0x320 <- __stop_cpus+0xd0/0x120
	RIP: 0010:smp_call_function_single
	<IRQ>
	swake_up_one_online
	__call_rcu_nocb_wake
	__call_rcu_common
	? rcu_torture_one_read
	call_timer_fn
	__run_timers
	run_timer_softirq
	handle_softirqs
	irq_exit_rcu
	? tick_handle_periodic
	sysvec_apic_timer_interrupt
	</IRQ>

The periodic tick must be shutdown when the CPU is offline, just like is
done for oneshot tick. This must be fixed but this is not enough:
softirqs can happen on any hardirq tail and reproduce the above scenario.

Fix this with introducing a special deferred rcuog wake up mode when the
CPU is offline. This deferred wake up doesn't arm any timer and simply
wait for rcu_report_cpu_dead() to be called in order to flush any
pending rcuog wake up.

Reported-by: kernel test robot <oliver.sang@xxxxxxxxx>
Closes: https://lore.kernel.org/oe-lkp/202409231644.4c55582d-lkp@xxxxxxxxx
Fixes: 9139f93209d1 ("rcu/nocb: Fix RT throttling hrtimer armed from offline CPU")
Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
---
 kernel/rcu/tree.h      |  1 +
 kernel/rcu/tree_nocb.h | 14 ++++++++++++--
 2 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index a9a811d9d7a3..7ed060edd12b 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -290,6 +290,7 @@ struct rcu_data {
 #define RCU_NOCB_WAKE_LAZY	2
 #define RCU_NOCB_WAKE		3
 #define RCU_NOCB_WAKE_FORCE	4
+#define RCU_NOCB_WAKE_OFFLINE   5
 
 #define RCU_JIFFIES_TILL_FORCE_QS (1 + (HZ > 250) + (HZ > 500))
 					/* For jiffies_till_first_fqs and */
diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index 2fb803f863da..8648233e1717 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -295,6 +295,8 @@ static void wake_nocb_gp_defer(struct rcu_data *rdp, int waketype,
 	case RCU_NOCB_WAKE_FORCE:
 		if (rdp_gp->nocb_defer_wakeup < RCU_NOCB_WAKE)
 			mod_timer(&rdp_gp->nocb_timer, jiffies + 1);
+		fallthrough;
+	case RCU_NOCB_WAKE_OFFLINE:
 		if (rdp_gp->nocb_defer_wakeup < waketype)
 			WRITE_ONCE(rdp_gp->nocb_defer_wakeup, waketype);
 		break;
@@ -562,8 +564,16 @@ static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_alldone,
 	lazy_len = READ_ONCE(rdp->lazy_len);
 	if (was_alldone) {
 		rdp->qlen_last_fqs_check = len;
-		// Only lazy CBs in bypass list
-		if (lazy_len && bypass_len == lazy_len) {
+		if (cpu_is_offline(rdp->cpu)) {
+			/*
+			 * Offline CPUs can't call swake_up_one_online() from IRQs. Rely
+			 * on the final deferred wake-up rcutree_report_cpu_dead()
+			 */
+			rcu_nocb_unlock(rdp);
+			wake_nocb_gp_defer(rdp, RCU_NOCB_WAKE_OFFLINE,
+					   TPS("WakeEmptyIsDeferredOffline"));
+		} else if (lazy_len && bypass_len == lazy_len) {
+			// Only lazy CBs in bypass list
 			rcu_nocb_unlock(rdp);
 			wake_nocb_gp_defer(rdp, RCU_NOCB_WAKE_LAZY,
 					   TPS("WakeLazy"));
-- 
2.46.0