Re: [PATCH] arm64/smp: Move rcu_cpu_starting() earlier

[Qian Cai]

On Thu, 2020-10-29 at 09:10 +0000, Will Deacon wrote:
> On Wed, Oct 28, 2020 at 02:26:14PM -0400, Qian Cai wrote:
> > The call to rcu_cpu_starting() in secondary_start_kernel() is not early
> > enough in the CPU-hotplug onlining process, which results in lockdep
> > splats as follows:
> > 
> >  WARNING: suspicious RCU usage
> >  -----------------------------
> >  kernel/locking/lockdep.c:3497 RCU-list traversed in non-reader section!!
> > 
> >  other info that might help us debug this:
> > 
> >  RCU used illegally from offline CPU!
> >  rcu_scheduler_active = 1, debug_locks = 1
> >  no locks held by swapper/1/0.
> > 
> >  Call trace:
> >   dump_backtrace+0x0/0x3c8
> >   show_stack+0x14/0x60
> >   dump_stack+0x14c/0x1c4
> >   lockdep_rcu_suspicious+0x134/0x14c
> >   __lock_acquire+0x1c30/0x2600
> >   lock_acquire+0x274/0xc48
> >   _raw_spin_lock+0xc8/0x140
> >   vprintk_emit+0x90/0x3d0
> >   vprintk_default+0x34/0x40
> >   vprintk_func+0x378/0x590
> >   printk+0xa8/0xd4
> >   __cpuinfo_store_cpu+0x71c/0x868
> >   cpuinfo_store_cpu+0x2c/0xc8
> >   secondary_start_kernel+0x244/0x318
> > 
> > This is avoided by moving the call to rcu_cpu_starting up near the
> > beginning of the secondary_start_kernel() function.
> 
> Hmm, it's not really a move though -- we'll end up calling this thing twice
> afaict. It would be better to make sure we've called notify_cpu_starting()
> early enough. Can we do that instead?

Paul mentioned that it is fine to call rcu_cpu_starting() multiple times, and
Peter mentioned that CPU bringup is complicated. Thus, I thought about doing
something safe here.

I tested a bit of patch below which seems fine, but I can't tell for sure if it
is safe. Any suggestion?

--- a/arch/arm64/kernel/smp.c
+++ b/arch/arm64/kernel/smp.c
@@ -224,6 +224,7 @@ asmlinkage notrace void secondary_start_kernel(void)
 
        preempt_disable();
        trace_hardirqs_off();
+       notify_cpu_starting(cpu);
 
        /*
         * If the system has established the capabilities, make sure
@@ -244,7 +245,6 @@ asmlinkage notrace void secondary_start_kernel(void)
        /*
         * Enable GIC and timers.
         */
-       notify_cpu_starting(cpu);
 
        ipi_setup(cpu);