[PATCH v3 10/13] sched: Guard sched_tick_start/stop against uninitialized tick_work_cpu

[Jing Wu <realwujing@gmail.com>]

sched_tick_start() and sched_tick_stop() are called during CPU hotplug
for CPUs not in the HK_TYPE_KERNEL_NOISE set.  They dereference
tick_work_cpu, which is allocated by sched_tick_offload_init() and only
called from housekeeping_init() when nohz_full= is present at boot.

When the DHM subsystem first-enables HK_TYPE_KERNEL_NOISE at runtime via
housekeeping_update_types(), tick_work_cpu remains NULL because
sched_tick_offload_init() is __init-only and cannot be re-invoked.  A
subsequent CPU offline/online cycle for an isolated CPU triggers
WARN_ON_ONCE(!tick_work_cpu) followed by a NULL-pointer dereference in
per_cpu_ptr(tick_work_cpu, cpu), crashing the kernel.

Since nohz_full= was not active at boot, tick_nohz_full_running remains
false and the tick-offload infrastructure is never activated; isolated
CPUs continue to receive their own ticks.  Guard both helpers with an
additional !tick_work_cpu check so they become no-ops in this case.

Signed-off-by: Jing Wu <realwujing@gmail.com>
Signed-off-by: Qiliang Yuan <yuanql9@chinatelecom.cn>
---
 kernel/sched/core.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 371b509d92164..df004e3efca70 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -5778,7 +5778,7 @@ static void sched_tick_start(int cpu)
 	int os;
 	struct tick_work *twork;
 
-	if (housekeeping_cpu(cpu, HK_TYPE_KERNEL_NOISE))
+	if (housekeeping_cpu(cpu, HK_TYPE_KERNEL_NOISE) || !tick_work_cpu)
 		return;
 
 	WARN_ON_ONCE(!tick_work_cpu);
@@ -5799,7 +5799,7 @@ static void sched_tick_stop(int cpu)
 	struct tick_work *twork;
 	int os;
 
-	if (housekeeping_cpu(cpu, HK_TYPE_KERNEL_NOISE))
+	if (housekeeping_cpu(cpu, HK_TYPE_KERNEL_NOISE) || !tick_work_cpu)
 		return;
 
 	WARN_ON_ONCE(!tick_work_cpu);

-- 
2.43.0