[PATCH REPOST v2] cpu: Make atomic callbacks run on UP with disabled interrupts

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On SMP callbacks in the "starting" range are invoked while the CPU is
brought up and interrupts are still disabled. Callbacks which are added
later ar invoked via the hotplug-thread on the target CPU and interrupts
are explicitly disabled.
In the UP case callbacks which are added later are invoked "directly"
without the thread. This is okay since there is just one CPU but with
enabled interrupts debug code, such as smp_processor_id(), will issue
warnings.

Disable interrupts before invoking the calback on UP if the state is
atomic and interrupts are expected to be disabled.
The "save" part is required because this is also invoked early in the
boot process while interrupts are disabled and must not be enabled.

Fixes: 06ddd17521bf1 ("sched/smp: Always define is_percpu_thread() and scheduler_ipi()")
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
Not in -next, repost of
	https://lore.kernel.org/all/20251103132820.0llS_QPg@linutronix.de/

 kernel/cpu.c | 27 ++++++++++++++++++---------
 1 file changed, 18 insertions(+), 9 deletions(-)

diff --git a/kernel/cpu.c b/kernel/cpu.c
index b674fdf96208b..9c5ba57955a3d 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -249,6 +249,14 @@ static int cpuhp_invoke_callback(unsigned int cpu, enum cpuhp_state state,
 	return ret;
 }
 
+/*
+ * The former STARTING/DYING states, ran with IRQs disabled and must not fail.
+ */
+static bool cpuhp_is_atomic_state(enum cpuhp_state state)
+{
+	return CPUHP_AP_IDLE_DEAD <= state && state < CPUHP_AP_ONLINE;
+}
+
 #ifdef CONFIG_SMP
 static bool cpuhp_is_ap_state(enum cpuhp_state state)
 {
@@ -271,14 +279,6 @@ static inline void complete_ap_thread(struct cpuhp_cpu_state *st, bool bringup)
 	complete(done);
 }
 
-/*
- * The former STARTING/DYING states, ran with IRQs disabled and must not fail.
- */
-static bool cpuhp_is_atomic_state(enum cpuhp_state state)
-{
-	return CPUHP_AP_IDLE_DEAD <= state && state < CPUHP_AP_ONLINE;
-}
-
 /* Synchronization state management */
 enum cpuhp_sync_state {
 	SYNC_STATE_DEAD,
@@ -2364,7 +2364,16 @@ static int cpuhp_issue_call(int cpu, enum cpuhp_state state, bool bringup,
 	else
 		ret = cpuhp_invoke_callback(cpu, state, bringup, node, NULL);
 #else
-	ret = cpuhp_invoke_callback(cpu, state, bringup, node, NULL);
+	if (cpuhp_is_atomic_state(state)) {
+		guard(irqsave)();
+		ret = cpuhp_invoke_callback(cpu, state, bringup, node, NULL);
+		/*
+		 * STARTING/DYING must not fail!
+		 */
+		WARN_ON_ONCE(ret);
+	} else {
+		ret = cpuhp_invoke_callback(cpu, state, bringup, node, NULL);
+	}
 #endif
 	BUG_ON(ret && !bringup);
 	return ret;
-- 
2.51.0