Re: [tip: locking/core] lockdep: Fix lockdep recursion

[Qian Cai]

On Fri, 2020-10-09 at 13:36 -0400, Qian Cai wrote:
> Back to x86, we have:
> 
> start_secondary()
>   smp_callin()
>     apic_ap_setup()
>       setup_local_APIC()
>         printk() in certain conditions.
> 
> which is before smp_store_cpu_info().
> 
> Can't we add a rcu_cpu_starting() at the very top for each start_secondary(),
> secondary_start_kernel(), smp_start_secondary() etc, so we don't worry about
> any printk() later?

This is rather irony. rcu_cpu_starting() is taking a lock and then reports
itself.

[    8.826732][    T0]  __lock_acquire.cold.76+0x2ad/0x3e0
[    8.826732][    T0]  lock_acquire+0x1c8/0x820
[    8.826732][    T0]  _raw_spin_lock_irqsave+0x30/0x50
[    8.826732][    T0]  rcu_cpu_starting+0xd0/0x2c0
[    8.826732][    T0]  start_secondary+0x10/0x2a0
[    8.826732][    T0]  secondary_startup_64_no_verify+0xb8/0xbb