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

Paul E. McKenney
2020-10-09T18:21:52Z

On Fri, Oct 09, 2020 at 01:54:34PM -0400, Qian Cai wrote:
> 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

Fun!!!

There should be some way around this. I cannot safely record the
offline-to-online transition without acquiring a lock. I suppose
I could trick lockdep into thinking that it was a recursive lockdep
report. Any other approaches?

Thanx, Paul