Re: [PATCH v3 4/4] rcu: Unlock non-start node only after accessing its gp_seq_needed

[Joel Fernandes <joel@joelfernandes.org>]

On Mon, May 21, 2018 at 04:25:38PM -0700, Paul E. McKenney wrote:
> On Sun, May 20, 2018 at 09:42:20PM -0700, Joel Fernandes wrote:
> > We acquire gp_seq_needed locklessly. To be safe, lets do the unlocking
> > after the access.
> 
> Actually, no, we hold rnp_start's ->lock throughout.  And this CPU (or in
> the case of no-CBs CPUs, this task) is in charge of rdp->gp_seq_needed,
> so nothing else is accessing it.  Or at least that is the intent.  ;-)

I was talking about protecting the internal node's rnp->gp_seq_needed, not
the rnp_start's gp_seq_needed.

We are protecting them in the loop:

like this:
for(...)
	if (rnp != rnp_start)
		raw_spin_lock_rcu_node(rnp);
	[...]
	// access rnp->gp_seq and rnp->gp_seq_needed
	[...]
	if (rnp != rnp_start)
		raw_spin_unlock_rcu_node(rnp);

But we don't need to do such protection in unlock_out ? I'm sorry if I'm
missing something, but I'm wondering if rnp->gp_seq_needed of an internal
node can be accessed locklessly, then why can't that be done also in the
funnel locking loop - after all we are holding the rnp_start's lock through
out right?

thanks!

 - Joel