From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754156Ab2HaQVl (ORCPT ); Fri, 31 Aug 2012 12:21:41 -0400 Received: from relay3-d.mail.gandi.net ([217.70.183.195]:42871 "EHLO relay3-d.mail.gandi.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753275Ab2HaQVk (ORCPT ); Fri, 31 Aug 2012 12:21:40 -0400 X-Originating-IP: 217.70.178.143 X-Originating-IP: 50.43.46.74 Date: Fri, 31 Aug 2012 09:21:30 -0700 From: Josh Triplett To: "Paul E. McKenney" Cc: linux-kernel@vger.kernel.org, mingo@elte.hu, laijs@cn.fujitsu.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@polymtl.ca, niv@us.ibm.com, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, Valdis.Kletnieks@vt.edu, dhowells@redhat.com, eric.dumazet@gmail.com, darren@dvhart.com, fweisbec@gmail.com, sbw@mit.edu, patches@linaro.org, "Paul E. McKenney" Subject: Re: [PATCH tip/core/rcu 2/3] rcu: Disallow callback registry on offline CPUs Message-ID: <20120831162130.GB21673@leaf> References: <20120830190245.GA32625@linux.vnet.ibm.com> <1346353383-350-1-git-send-email-paulmck@linux.vnet.ibm.com> <1346353383-350-2-git-send-email-paulmck@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1346353383-350-2-git-send-email-paulmck@linux.vnet.ibm.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Aug 30, 2012 at 12:03:02PM -0700, Paul E. McKenney wrote: > From: "Paul E. McKenney" > > Posting a callback after the CPU_DEAD notifier effectively leaks > that callback unless/until that CPU comes back online. Silence is > unhelpful when attempting to track down such leaks, so this commit emits > a WARN_ON_ONCE() and unconditionally leaks the callback when an offline > CPU attempts to register a callback. The rdp->nxttail[RCU_NEXT_TAIL] is > set to NULL in the CPU_DEAD notifier and restored in the CPU_UP_PREPARE > notifier, allowing _call_rcu() to determine exactly when posting callbacks > is illegal. > > Signed-off-by: Paul E. McKenney > Signed-off-by: Paul E. McKenney One suggestion below; with or without that change: Reviewed-by: Josh Triplett > --- > kernel/rcutree.c | 10 ++++++++++ > 1 files changed, 10 insertions(+), 0 deletions(-) > > diff --git a/kernel/rcutree.c b/kernel/rcutree.c > index 9854a00..5f8c4dd 100644 > --- a/kernel/rcutree.c > +++ b/kernel/rcutree.c > @@ -1503,6 +1503,9 @@ static void rcu_cleanup_dead_cpu(int cpu, struct rcu_state *rsp) > WARN_ONCE(rdp->qlen != 0 || rdp->nxtlist != NULL, > "rcu_cleanup_dead_cpu: Callbacks on offline CPU %d: qlen=%lu, nxtlist=%p\n", > cpu, rdp->qlen, rdp->nxtlist); > + init_callback_list(rdp); > + /* Disallow further callbacks on this CPU. */ > + rdp->nxttail[RCU_NEXT_TAIL] = NULL; > } > > #else /* #ifdef CONFIG_HOTPLUG_CPU */ > @@ -1925,6 +1928,12 @@ __call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu), > rdp = this_cpu_ptr(rsp->rda); > > /* Add the callback to our list. */ > + if (unlikely(rdp->nxttail[RCU_NEXT_TAIL] == NULL)) { > + /* _call_rcu() is illegal on offline CPU; leak the callback. */ > + WARN_ON_ONCE(1); You can write this as: if (WARN_ON_ONCE(rdp->nxttail[RCU_NEXT_TAIL] == NULL)) WARN_ON_ONCE also has a built-in unlikely() already. > + local_irq_restore(flags); > + return; > + } > ACCESS_ONCE(rdp->qlen)++; > if (lazy) > rdp->qlen_lazy++; > @@ -2462,6 +2471,7 @@ rcu_init_percpu_data(int cpu, struct rcu_state *rsp, int preemptible) > rdp->qlen_last_fqs_check = 0; > rdp->n_force_qs_snap = rsp->n_force_qs; > rdp->blimit = blimit; > + init_callback_list(rdp); /* Re-enable callbacks on this CPU. */ > rdp->dynticks->dynticks_nesting = DYNTICK_TASK_EXIT_IDLE; > atomic_set(&rdp->dynticks->dynticks, > (atomic_read(&rdp->dynticks->dynticks) & ~0x1) + 1); > -- > 1.7.8 >