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McKenney" Cc: linux-kernel@vger.kernel.org, Neeraj Upadhyay , Davidlohr Bueso , Jonathan Corbet , Josh Triplett , Lai Jiangshan , linux-doc@vger.kernel.org, Mathieu Desnoyers , Mauro Carvalho Chehab , peterz@infradead.org, Randy Dunlap , rcu@vger.kernel.org, Steven Rostedt , tglx@linutronix.de, vineethrp@gmail.com Subject: Re: [PATCH v4 1/5] rcu/tree: Add a warning if CPU being onlined did not report QS already Message-ID: <20200810192554.GD2865655@google.com> References: <20200807170722.2897328-1-joel@joelfernandes.org> <20200807170722.2897328-2-joel@joelfernandes.org> <20200810154654.GJ4295@paulmck-ThinkPad-P72> <20200810173931.GB2253395@google.com> <20200810175717.GM4295@paulmck-ThinkPad-P72> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200810175717.GM4295@paulmck-ThinkPad-P72> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Aug 10, 2020 at 10:57:17AM -0700, Paul E. McKenney wrote: > On Mon, Aug 10, 2020 at 01:39:31PM -0400, Joel Fernandes wrote: > > On Mon, Aug 10, 2020 at 08:46:54AM -0700, Paul E. McKenney wrote: > > > On Fri, Aug 07, 2020 at 01:07:18PM -0400, Joel Fernandes (Google) wrote: > > > > Currently, rcu_cpu_starting() checks to see if the RCU core expects a > > > > quiescent state from the incoming CPU. However, the current interaction > > > > between RCU quiescent-state reporting and CPU-hotplug operations should > > > > mean that the incoming CPU never needs to report a quiescent state. > > > > First, the outgoing CPU reports a quiescent state if needed. Second, > > > > the race where the CPU is leaving just as RCU is initializing a new > > > > grace period is handled by an explicit check for this condition. Third, > > > > the CPU's leaf rcu_node structure's ->lock serializes these checks. > > > > > > > > This means that if rcu_cpu_starting() ever feels the need to report > > > > a quiescent state, then there is a bug somewhere in the CPU hotplug > > > > code or the RCU grace-period handling code. This commit therefore > > > > adds a WARN_ON_ONCE() to bring that bug to everyone's attention. > > > > > > > > Cc: Paul E. McKenney > > > > Cc: Neeraj Upadhyay > > > > Suggested-by: Paul E. McKenney > > > > Signed-off-by: Joel Fernandes (Google) > > > > --- > > > > kernel/rcu/tree.c | 9 ++++++++- > > > > 1 file changed, 8 insertions(+), 1 deletion(-) > > > > > > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > > > > index 65e1b5e92319..a49fa3b60faa 100644 > > > > --- a/kernel/rcu/tree.c > > > > +++ b/kernel/rcu/tree.c > > > > @@ -3996,7 +3996,14 @@ void rcu_cpu_starting(unsigned int cpu) > > > > rcu_gpnum_ovf(rnp, rdp); /* Offline-induced counter wrap? */ > > > > rdp->rcu_onl_gp_seq = READ_ONCE(rcu_state.gp_seq); > > > > rdp->rcu_onl_gp_flags = READ_ONCE(rcu_state.gp_flags); > > > > - if (rnp->qsmask & mask) { /* RCU waiting on incoming CPU? */ > > > > + > > > > + /* > > > > + * XXX: The following rcu_report_qs_rnp() is redundant. If the below > > > > + * warning does not fire, consider replacing it with the "else" block, > > > > + * by June 2021 or so (while keeping the warning). Refer to RCU's > > > > + * Requirements documentation for the rationale. > > > > > > Let's suppose that this change is made, and further that in a year or > > > two the "if" statement below is replaced with its "else" block. > > > > > > Now let's suppose that (some years after that) a hard-to-trigger bug > > > makes its way into RCU's CPU-hotplug code that would have resulted in > > > the WARN_ON_ONCE() triggering, but that this bug turns out to be not so > > > hard to trigger in certain large production environments. > > > > > > Let's suppose further that you have moved on to where you are responsible > > > for one of these large production environments. How would this > > > hypothetical RCU/CPU-hotplug bug manifest? > > > > It could manifest as an RCU stall (after the warning triggers) since RCU > > would wait forever. > > > > Were you thinking it is not worth doing this? I thought we wanted to remove > > the reundant rcu_report_qs_rnp here to solidify everyone's understanding of > > the code and fail early if there's something misunderstood (since such > > misunderstanding could mean there are other hidden bugs somewhere). The > > counter-argument to that being, making the code robust is more important for > > the large production failure scenario where failures are costly. > > The benefits of removing code that is in theory redundant was my thought > at one point, but sleeping on this several times since has made me much > less favorable to this change. And perhaps my experiences with my new > employer have affected my views on this as well. You never know! ;-) Can we just keep the warning then, and delete the comments to revisit? IMHO a comment saying this rcu_report_qs_rnp() is not necessary here but is done anyway, would be quite useful to a code reader, (with appropriate comments to point to RCU requirements section and the added warning) :-) thanks, - Joel > > Thanx, Paul > > > thanks, > > > > - Joel > > > > > > > Thanx, Paul > > > > > > > + */ > > > > + if (WARN_ON_ONCE(rnp->qsmask & mask)) { /* RCU waiting on incoming CPU? */ > > > > rcu_disable_urgency_upon_qs(rdp); > > > > /* Report QS -after- changing ->qsmaskinitnext! */ > > > > rcu_report_qs_rnp(mask, rnp, rnp->gp_seq, flags); > > > > -- > > > > 2.28.0.236.gb10cc79966-goog > > > >