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

[Qian Cai]

On Thu, 2020-11-05 at 22:22 +0000, Will Deacon wrote:
> On Fri, Oct 30, 2020 at 04:33:25PM +0000, Will Deacon wrote:
> > On Wed, 28 Oct 2020 14:26:14 -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!!
> > > 
> > > [...]
> > 
> > Applied to arm64 (for-next/fixes), thanks!
> > 
> > [1/1] arm64/smp: Move rcu_cpu_starting() earlier
> >       https://git.kernel.org/arm64/c/ce3d31ad3cac
> 
> Hmm, this patch has caused a regression in the case that we fail to
> online a CPU because it has incompatible CPU features and so we park it
> in cpu_die_early(). We now get an endless spew of RCU stalls because the
> core will never come online, but is being tracked by RCU. So I'm tempted
> to revert this and live with the lockdep warning while we figure out a
> proper fix.
> 
> What's the correct say to undo rcu_cpu_starting(), given that we cannot
> invoke the full hotplug machinery here? Is it correct to call
> rcutree_dying_cpu() on the bad CPU and then rcutree_dead_cpu() from the
> CPU doing cpu_up(), or should we do something else?
It looks to me that rcu_report_dead() does the opposite of rcu_cpu_starting(),
so lift rcu_report_dead() out of CONFIG_HOTPLUG_CPU and use it there to rewind,
Paul?