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SubjectRe: [PATCH v3 4/4] rcu: Unlock non-start node only after accessing its gp_seq_needed
On Mon, May 21, 2018 at 09:16:51PM -0700, Paul E. McKenney wrote:
> 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.

Cool, makes sense. I am assuming this means I don't have to resend this
patch, if I do let me know :)

Either way, once you push your updated tree to kernel.org, I'll double check
to make sure the change is in :)

thanks, good night,

- Joel

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