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@xxxxxxxxx>
---
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
