cpu/hotplug: Remove the 'cpu' member of cpuhp_cpu_state

Currently the setting of the 'cpu' member of struct cpuhp_cpu_state in
cpuhp_create() is too late as it is used earlier in _cpu_up().

If kzalloc_node() in __smpboot_create_thread() fails then the rollback will
be done with st->cpu==0 causing CPU0 to be erroneously set to be dying,
causing the scheduler to get mightily confused and throw its toys out of
the pram.

However the cpu number is actually available directly, so simply remove
the 'cpu' member and avoid the problem in the first place.

Fixes: 2ea46c6fc945 ("cpumask/hotplug: Fix cpu_dying() state tracking")
Signed-off-by: Steven Price <[e-mail]>
Signed-off-by: Thomas Gleixner <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

kernel/cpu.c

···
 	bool			rollback;
 	bool			single;
 	bool			bringup;
-	int			cpu;
 	struct hlist_node	*node;
 	struct hlist_node	*last;
 	enum cpuhp_state	cb_state;
···
 #endif
 
 static inline enum cpuhp_state
-cpuhp_set_state(struct cpuhp_cpu_state *st, enum cpuhp_state target)
+cpuhp_set_state(int cpu, struct cpuhp_cpu_state *st, enum cpuhp_state target)
 {
 	enum cpuhp_state prev_state = st->state;
 	bool bringup = st->state < target;
···
 	st->target = target;
 	st->single = false;
 	st->bringup = bringup;
-	if (cpu_dying(st->cpu) != !bringup)
-		set_cpu_dying(st->cpu, !bringup);
+	if (cpu_dying(cpu) != !bringup)
+		set_cpu_dying(cpu, !bringup);
 
 	return prev_state;
 }
 
 static inline void
-cpuhp_reset_state(struct cpuhp_cpu_state *st, enum cpuhp_state prev_state)
+cpuhp_reset_state(int cpu, struct cpuhp_cpu_state *st,
+		  enum cpuhp_state prev_state)
 {
 	bool bringup = !st->bringup;
 
···
 	}
 
 	st->bringup = bringup;
-	if (cpu_dying(st->cpu) != !bringup)
-		set_cpu_dying(st->cpu, !bringup);
+	if (cpu_dying(cpu) != !bringup)
+		set_cpu_dying(cpu, !bringup);
 }
 
 /* Regular hotplug invocation of the AP hotplug thread */
···
 	wait_for_ap_thread(st, st->bringup);
 }
 
-static int cpuhp_kick_ap(struct cpuhp_cpu_state *st, enum cpuhp_state target)
+static int cpuhp_kick_ap(int cpu, struct cpuhp_cpu_state *st,
+			 enum cpuhp_state target)
 {
 	enum cpuhp_state prev_state;
 	int ret;
 
-	prev_state = cpuhp_set_state(st, target);
+	prev_state = cpuhp_set_state(cpu, st, target);
 	__cpuhp_kick_ap(st);
 	if ((ret = st->result)) {
-		cpuhp_reset_state(st, prev_state);
+		cpuhp_reset_state(cpu, st, prev_state);
 		__cpuhp_kick_ap(st);
 	}
 
···
 	if (st->target <= CPUHP_AP_ONLINE_IDLE)
 		return 0;
 
-	return cpuhp_kick_ap(st, st->target);
+	return cpuhp_kick_ap(cpu, st, st->target);
 }
 
 static int bringup_cpu(unsigned int cpu)
···
 			 ret, cpu, cpuhp_get_step(st->state)->name,
 			 st->state);
 
-		cpuhp_reset_state(st, prev_state);
+		cpuhp_reset_state(cpu, st, prev_state);
 		if (can_rollback_cpu(st))
 			WARN_ON(cpuhp_invoke_callback_range(false, cpu, st,
 							    prev_state));
···
 
 	init_completion(&st->done_up);
 	init_completion(&st->done_down);
-	st->cpu = cpu;
 }
 
 static int cpuhp_should_run(unsigned int cpu)
···
 	cpuhp_lock_release(true);
 
 	trace_cpuhp_enter(cpu, st->target, prev_state, cpuhp_kick_ap_work);
-	ret = cpuhp_kick_ap(st, st->target);
+	ret = cpuhp_kick_ap(cpu, st, st->target);
 	trace_cpuhp_exit(cpu, st->state, prev_state, ret);
 
 	return ret;
···
 			 ret, cpu, cpuhp_get_step(st->state)->name,
 			 st->state);
 
-		cpuhp_reset_state(st, prev_state);
+		cpuhp_reset_state(cpu, st, prev_state);
 
 		if (st->state < prev_state)
 			WARN_ON(cpuhp_invoke_callback_range(true, cpu, st,
···
 
 	cpuhp_tasks_frozen = tasks_frozen;
 
-	prev_state = cpuhp_set_state(st, target);
+	prev_state = cpuhp_set_state(cpu, st, target);
 	/*
 	 * If the current CPU state is in the range of the AP hotplug thread,
 	 * then we need to kick the thread.
···
 	ret = cpuhp_down_callbacks(cpu, st, target);
 	if (ret && st->state < prev_state) {
 		if (st->state == CPUHP_TEARDOWN_CPU) {
-			cpuhp_reset_state(st, prev_state);
+			cpuhp_reset_state(cpu, st, prev_state);
 			__cpuhp_kick_ap(st);
 		} else {
 			WARN(1, "DEAD callback error for CPU%d", cpu);
···
 
 	cpuhp_tasks_frozen = tasks_frozen;
 
-	cpuhp_set_state(st, target);
+	cpuhp_set_state(cpu, st, target);
 	/*
 	 * If the current CPU state is in the range of the AP hotplug thread,
 	 * then we need to kick the thread once more.