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

[Joel Fernandes]

On 3/7/2024 11:09 AM, Stefan Wiehler wrote:
> 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();

I agree with the problem but disagree with the patch because it feels like a
terrible workaround.

Can we just use arch_spin_lock() for the cpu_asid_lock? This might require
acquiring the raw_lock within the raw_spinlock_t, but there is precedent:

arch/powerpc/kvm/book3s_hv_rm_mmu.c:245:
arch_spin_lock(&kvm->mmu_lock.rlock.raw_lock);

IMO, lockdep tracking of this lock is not necessary or possible considering the
hotplug situation.

Or is there a reason you need lockdep working for the cpu_asid_lock?

thanks,

 - Joel