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

[Paul E. McKenney]

On Thu, Nov 05, 2020 at 06:02:49PM -0500, Qian Cai wrote:
> 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?

Yes, rcu_report_dead() should do the trick.  Presumably the earlier
online-time CPU-hotplug notifiers are also unwound?

							Thanx, Paul