From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751311AbeEVEn3 (ORCPT ); Tue, 22 May 2018 00:43:29 -0400 Received: from mail-pf0-f193.google.com ([209.85.192.193]:33724 "EHLO mail-pf0-f193.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751205AbeEVEn2 (ORCPT ); Tue, 22 May 2018 00:43:28 -0400 X-Google-Smtp-Source: AB8JxZoDsnCrEB6RxR5wTd8E5aYhaF9osAckXAIOAvFg8ogg39UWEumWJjIkWYq889hppOYD30gDvQ== Date: Mon, 21 May 2018 21:43:27 -0700 From: Joel Fernandes To: "Paul E. McKenney" Cc: Joel Fernandes , linux-kernel@vger.kernel.org, Josh Triplett , Steven Rostedt , Mathieu Desnoyers , Lai Jiangshan , byungchul.park@lge.com, kernel-team@android.com Subject: Re: [PATCH v3 4/4] rcu: Unlock non-start node only after accessing its gp_seq_needed Message-ID: <20180522044327.GF40541@joelaf.mtv.corp.google.com> References: <20180521044220.123933-1-joel@joelfernandes.org> <20180521044220.123933-5-joel@joelfernandes.org> <20180521232537.GJ3803@linux.vnet.ibm.com> <20180522000734.GD40541@joelaf.mtv.corp.google.com> <20180522002823.GP3803@linux.vnet.ibm.com> <20180522041651.GA18405@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180522041651.GA18405@linux.vnet.ibm.com> User-Agent: Mutt/1.9.2 (2017-12-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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