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

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Mon, May 21, 2018 at 05:28:23PM -0700, Paul E. McKenney wrote:
> On Mon, May 21, 2018 at 05:07:34PM -0700, Joel Fernandes wrote:
> > 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.
> 
> Ah, good point, I missed the "if" condition.  This can be argued to work,
> sort of, given that we still hold the leaf rcu_node structure's lock,
> so that there is a limit to how far grace periods can advance.
> 
> But the code would of course be much cleaner with your change.
> 
> > 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?
> 
> I was focused on the updates, and missed the rnp->gp_seq_req access in the
> "if" statement.  The current code does sort of work, but only assuming
> that the compiler doesn't tear the load, and so your change would help.
> Could you please resend with your other two updated patches?  It depends
> on one of the earlier patches, so does not apply cleanly as-is.  I could
> hand-apply it, but that sounds like a good way to make your updated
> series fail to apply.  ;-)
> 
> But could you also make the commit log explicitly call out the "if"
> condition as being the offending access?

Never mind, me being stupid.  I need to apply this change to the original
commit "rcu: Make rcu_nocb_wait_gp() check if GP already requested", which
I have done with this attribution:

[ paulmck: Move lock release past "if" as suggested by Joel Fernandes. ]

I have rebased my stack on top of the updated commit.

							Thanx, Paul