[PATCH] arm: smp: Avoid false positive CPU hotplug Lockdep-RCU splat

[Stefan Wiehler]

With CONFIG_PROVE_RCU_LIST=y and by executing

  $ echo 0 > /sys/devices/system/cpu/cpu1/online

one can trigger the following Lockdep-RCU splat on ARM:

  =============================
  WARNING: suspicious RCU usage
  6.8.0-rc7-00001-g0db1d0ed8958 #10 Not tainted
  -----------------------------
  kernel/locking/lockdep.c:3762 RCU-list traversed in non-reader section!!

  other info that might help us debug this:

  RCU used illegally from offline CPU!
  rcu_scheduler_active = 2, debug_locks = 1
  no locks held by swapper/1/0.

  stack backtrace:
  CPU: 1 PID: 0 Comm: swapper/1 Not tainted 6.8.0-rc7-00001-g0db1d0ed8958 #10
  Hardware name: Allwinner sun8i Family
   unwind_backtrace from show_stack+0x10/0x14
   show_stack from dump_stack_lvl+0x60/0x90
   dump_stack_lvl from lockdep_rcu_suspicious+0x150/0x1a0
   lockdep_rcu_suspicious from __lock_acquire+0x11fc/0x29f8
   __lock_acquire from lock_acquire+0x10c/0x348
   lock_acquire from _raw_spin_lock_irqsave+0x50/0x6c
   _raw_spin_lock_irqsave from check_and_switch_context+0x7c/0x4a8
   check_and_switch_context from arch_cpu_idle_dead+0x10/0x7c
   arch_cpu_idle_dead from do_idle+0xbc/0x138
   do_idle from cpu_startup_entry+0x28/0x2c
   cpu_startup_entry from secondary_start_kernel+0x11c/0x124
   secondary_start_kernel from 0x401018a0

The CPU is already reported as offline from RCU perspective in
cpuhp_report_idle_dead() before arch_cpu_idle_dead() is invoked. Above
RCU-Lockdep splat is then triggered by check_and_switch_context() acquiring the
ASID spinlock.

Avoid the false-positive Lockdep-RCU splat by briefly reporting the CPU as
online again while the spinlock is held.

Signed-off-by: Stefan Wiehler <stefan.wiehler at nokia.com>
---
 arch/arm/kernel/smp.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/arch/arm/kernel/smp.c b/arch/arm/kernel/smp.c
index 3431c0553f45..6875e2c5dd50 100644
--- a/arch/arm/kernel/smp.c
+++ b/arch/arm/kernel/smp.c
@@ -319,7 +319,14 @@ void __noreturn arch_cpu_idle_dead(void)
 {
 	unsigned int cpu = smp_processor_id();
 
+	/*
+	 * Briefly report CPU as online again to avoid false positive
+	 * Lockdep-RCU splat when check_and_switch_context() acquires ASID
+	 * spinlock.
+	 */
+	rcutree_report_cpu_starting(cpu);
 	idle_task_exit();
+	rcutree_report_cpu_dead();
 
 	local_irq_disable();
 
-- 
2.42.0