cpumask/hotplug: Fix cpu_dying() state tracking

Vincent reported that for states with a NULL startup/teardown function
we do not call cpuhp_invoke_callback() (because there is none) and as
such we'll not update the cpu_dying() state.

The stale cpu_dying() can eventually lead to triggering BUG().

Rectify this by updating cpu_dying() in the exact same places the
hotplug machinery tracks its directional state, namely
cpuhp_set_state() and cpuhp_reset_state().

Reported-by: Vincent Donnefort <[email protected]>
Suggested-by: Vincent Donnefort <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Reviewed-by: Vincent Donnefort <[email protected]>
Reviewed-by: Valentin Schneider <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]

Author: Peter Zijlstra

kernel/cpu.c

@@ -63,6 +63,7 @@ struct cpuhp_cpu_state {
 	bool			rollback;
 	bool			single;
 	bool			bringup;
+	int			cpu;
 	struct hlist_node	*node;
 	struct hlist_node	*last;
 	enum cpuhp_state	cb_state;
@@ -160,9 +161,6 @@ static int cpuhp_invoke_callback(unsigned int cpu, enum cpuhp_state state,
 	int (*cb)(unsigned int cpu);
 	int ret, cnt;
 
-	if (cpu_dying(cpu) != !bringup)
-		set_cpu_dying(cpu, !bringup);
-
 	if (st->fail == state) {
 		st->fail = CPUHP_INVALID;
 		return -EAGAIN;
@@ -467,20 +465,25 @@ static inline enum cpuhp_state
 cpuhp_set_state(struct cpuhp_cpu_state *st, enum cpuhp_state target)
 {
 	enum cpuhp_state prev_state = st->state;
+	bool bringup = st->state < target;
 
 	st->rollback = false;
 	st->last = NULL;
 
 	st->target = target;
 	st->single = false;
-	st->bringup = st->state < target;
+	st->bringup = bringup;
+	if (cpu_dying(st->cpu) != !bringup)
+		set_cpu_dying(st->cpu, !bringup);
 
 	return prev_state;
 }
 
 static inline void
 cpuhp_reset_state(struct cpuhp_cpu_state *st, enum cpuhp_state prev_state)
 {
+	bool bringup = !st->bringup;
+
 	st->target = prev_state;
 
 	/*
@@ -503,7 +506,9 @@ cpuhp_reset_state(struct cpuhp_cpu_state *st, enum cpuhp_state prev_state)
 			st->state++;
 	}
 
-	st->bringup = !st->bringup;
+	st->bringup = bringup;
+	if (cpu_dying(st->cpu) != !bringup)
+		set_cpu_dying(st->cpu, !bringup);
 }
 
 /* Regular hotplug invocation of the AP hotplug thread */
@@ -693,6 +698,7 @@ static void cpuhp_create(unsigned int cpu)
 
 	init_completion(&st->done_up);
 	init_completion(&st->done_down);
+	st->cpu = cpu;
 }
 
 static int cpuhp_should_run(unsigned int cpu)