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