From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-8.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 02B4CC4BA15 for ; Wed, 26 Feb 2020 15:02:21 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id C707624688 for ; Wed, 26 Feb 2020 15:02:20 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1582729340; bh=YdavwjKW764gaMMCY/6DItdn27P+LhGI44b50YrPEik=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=D1IrsK01bMZ8qfklUD0/ltgqPNOfWFaJBcbqMzssYyfCifHBMqqR2ZJlD8rB/GLJ0 ++BM6bV+qLYHqLXyu4XcLHNs/MjXuUBbDIXCPnu8BLWr58THDIgiuaB2ONyTTIfdtf 35Vr35tzoXZ2WBZoXRZl4At2qnAMQ8MQA3wNsqxk= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727426AbgBZPCT (ORCPT ); Wed, 26 Feb 2020 10:02:19 -0500 Received: from mail.kernel.org ([198.145.29.99]:48978 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726148AbgBZPCT (ORCPT ); Wed, 26 Feb 2020 10:02:19 -0500 Received: from paulmck-ThinkPad-P72.home (199-192-87-166.static.wiline.com [199.192.87.166]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 85CEF24685; Wed, 26 Feb 2020 15:02:18 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1582729338; bh=YdavwjKW764gaMMCY/6DItdn27P+LhGI44b50YrPEik=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=2sbgL7e94ntYhGUYANr6/gsCMc4+cpMtj2zUZzW86ef6QQ/sINUHOGKCC+zaZtY22 9tF1d/YcOZy8tHHY3SWS+aE5sOqTjoWXdcu3Sou4jrMnhLZSCl4YQ5Tg5MKs47Vgvs fLmHXpRZ5ZEtdywYOBJJv+82msglW71Xvhdb8PUY= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id 14E833521EAF; Wed, 26 Feb 2020 07:02:18 -0800 (PST) Date: Wed, 26 Feb 2020 07:02:18 -0800 From: "Paul E. McKenney" To: Boqun Feng Cc: rcu@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-team@fb.com, mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com, oleg@redhat.com, joel@joelfernandes.org, "# 5 . 5 . x" Subject: Re: [PATCH tip/core/rcu 30/30] rcu: Make rcu_barrier() account for offline no-CBs CPUs Message-ID: <20200226150218.GA2935@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <20200214235536.GA13364@paulmck-ThinkPad-P72> <20200214235607.13749-30-paulmck@kernel.org> <20200225102436.GF110915@debian-boqun.qqnc3lrjykvubdpftowmye0fmh.lx.internal.cloudapp.net> <20200226031455.GZ2935@paulmck-ThinkPad-P72> <20200226061430.GG110915@debian-boqun.qqnc3lrjykvubdpftowmye0fmh.lx.internal.cloudapp.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200226061430.GG110915@debian-boqun.qqnc3lrjykvubdpftowmye0fmh.lx.internal.cloudapp.net> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Feb 26, 2020 at 02:14:30PM +0800, Boqun Feng wrote: > On Tue, Feb 25, 2020 at 07:14:55PM -0800, Paul E. McKenney wrote: > > On Tue, Feb 25, 2020 at 06:24:36PM +0800, Boqun Feng wrote: > > > Hi Paul, > > > > > > On Fri, Feb 14, 2020 at 03:56:07PM -0800, paulmck@kernel.org wrote: > > > > From: "Paul E. McKenney" > > > > > > > > Currently, rcu_barrier() ignores offline CPUs, However, it is possible > > > > for an offline no-CBs CPU to have callbacks queued, and rcu_barrier() > > > > must wait for those callbacks. This commit therefore makes rcu_barrier() > > > > directly invoke the rcu_barrier_func() with interrupts disabled for such > > > > CPUs. This requires passing the CPU number into this function so that > > > > it can entrain the rcu_barrier() callback onto the correct CPU's callback > > > > list, given that the code must instead execute on the current CPU. > > > > > > > > While in the area, this commit fixes a bug where the first CPU's callback > > > > might have been invoked before rcu_segcblist_entrain() returned, which > > > > would also result in an early wakeup. > > > > > > > > Fixes: 5d6742b37727 ("rcu/nocb: Use rcu_segcblist for no-CBs CPUs") > > > > Signed-off-by: Paul E. McKenney > > > > Cc: # 5.5.x > > > > --- > > > > include/trace/events/rcu.h | 1 + > > > > kernel/rcu/tree.c | 32 ++++++++++++++++++++------------ > > > > 2 files changed, 21 insertions(+), 12 deletions(-) > > > > > > > > diff --git a/include/trace/events/rcu.h b/include/trace/events/rcu.h > > > > index 5e49b06..d56d54c 100644 > > > > --- a/include/trace/events/rcu.h > > > > +++ b/include/trace/events/rcu.h > > > > @@ -712,6 +712,7 @@ TRACE_EVENT_RCU(rcu_torture_read, > > > > * "Begin": rcu_barrier() started. > > > > * "EarlyExit": rcu_barrier() piggybacked, thus early exit. > > > > * "Inc1": rcu_barrier() piggyback check counter incremented. > > > > + * "OfflineNoCBQ": rcu_barrier() found offline no-CBs CPU with callbacks. > > > > * "OnlineQ": rcu_barrier() found online CPU with callbacks. > > > > * "OnlineNQ": rcu_barrier() found online CPU, no callbacks. > > > > * "IRQ": An rcu_barrier_callback() callback posted on remote CPU. > > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > > > > index d15041f..160643e 100644 > > > > --- a/kernel/rcu/tree.c > > > > +++ b/kernel/rcu/tree.c > > > > @@ -3098,9 +3098,10 @@ static void rcu_barrier_callback(struct rcu_head *rhp) > > > > /* > > > > * Called with preemption disabled, and from cross-cpu IRQ context. > > > > */ > > > > -static void rcu_barrier_func(void *unused) > > > > +static void rcu_barrier_func(void *cpu_in) > > > > { > > > > - struct rcu_data *rdp = raw_cpu_ptr(&rcu_data); > > > > + uintptr_t cpu = (uintptr_t)cpu_in; > > > > + struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu); > > > > > > > > rcu_barrier_trace(TPS("IRQ"), -1, rcu_state.barrier_sequence); > > > > rdp->barrier_head.func = rcu_barrier_callback; > > > > @@ -3127,7 +3128,7 @@ static void rcu_barrier_func(void *unused) > > > > */ > > > > void rcu_barrier(void) > > > > { > > > > - int cpu; > > > > + uintptr_t cpu; > > > > struct rcu_data *rdp; > > > > unsigned long s = rcu_seq_snap(&rcu_state.barrier_sequence); > > > > > > > > @@ -3150,13 +3151,14 @@ void rcu_barrier(void) > > > > rcu_barrier_trace(TPS("Inc1"), -1, rcu_state.barrier_sequence); > > > > > > > > /* > > > > - * Initialize the count to one rather than to zero in order to > > > > - * avoid a too-soon return to zero in case of a short grace period > > > > - * (or preemption of this task). Exclude CPU-hotplug operations > > > > - * to ensure that no offline CPU has callbacks queued. > > > > + * Initialize the count to two rather than to zero in order > > > > + * to avoid a too-soon return to zero in case of an immediate > > > > + * invocation of the just-enqueued callback (or preemption of > > > > + * this task). Exclude CPU-hotplug operations to ensure that no > > > > + * offline non-offloaded CPU has callbacks queued. > > > > */ > > > > init_completion(&rcu_state.barrier_completion); > > > > - atomic_set(&rcu_state.barrier_cpu_count, 1); > > > > + atomic_set(&rcu_state.barrier_cpu_count, 2); > > > > get_online_cpus(); > > > > > > > > /* > > > > @@ -3166,13 +3168,19 @@ void rcu_barrier(void) > > > > */ > > > > for_each_possible_cpu(cpu) { > > > > rdp = per_cpu_ptr(&rcu_data, cpu); > > > > - if (!cpu_online(cpu) && > > > > + if (cpu_is_offline(cpu) && > > > > !rcu_segcblist_is_offloaded(&rdp->cblist)) > > > > continue; > > > > - if (rcu_segcblist_n_cbs(&rdp->cblist)) { > > > > + if (rcu_segcblist_n_cbs(&rdp->cblist) && cpu_online(cpu)) { > > > > rcu_barrier_trace(TPS("OnlineQ"), cpu, > > > > rcu_state.barrier_sequence); > > > > - smp_call_function_single(cpu, rcu_barrier_func, NULL, 1); > > > > + smp_call_function_single(cpu, rcu_barrier_func, (void *)cpu, 1); > > > > + } else if (cpu_is_offline(cpu)) { > > > > > > I wonder whether this should be: > > > > > > else if (rcu_segcblist_n_cbs(&rdp->cblist) && cpu_is_offline(cpu)) > > > > > > ? Because I think we only want to queue the barrier call back if there > > > are callbacks for a particular CPU. Am I missing something subtle? > > > > I don't believe that you are missing anything at all! > > > > Thank you very much -- this bug would not have shown up in any validation > > setup that I am aware of. ;-) > > > > Thanx, Paul > > > > > Regards, > > > Boqun > > > > > > > + rcu_barrier_trace(TPS("OfflineNoCBQ"), cpu, > > > > + rcu_state.barrier_sequence); > > > > + local_irq_disable(); > > > > + rcu_barrier_func((void *)cpu); > > > > + local_irq_enable(); > > Another (interesting) thing I found here is that we actually don't need > the irq-off section to call rcu_barrier_func() in this branch. Because > the target CPU is offlined, so only the cblist is only accessed at two > places, IIUC, one is the rcuo kthread and one is here (in > rcu_barrier()), and both places are in the process context rather than > irq context, so irq-off is not required to prevent the deadlock. > > But yes, I know, if we drop the local_irq_disable/enable() pair here, > it will make lockdep very unhappy ;-) And acquiring ->nocb_lock with interrupts enabled would be rather scary. And probably would be an accident waiting to happen. So I am happy to disable interrupts on this path, given that it should be infrequent, only being executed for a short time after a no-CBs CPU goes offline. Much nicer to let lockdep do its thing than to have to second-guess it on every change that involves acquiring ->nocb_lock in an interrupt handler! ;-) Thanx, Paul > Regards, > Boqun > > > > > } else { > > > > rcu_barrier_trace(TPS("OnlineNQ"), cpu, > > > > rcu_state.barrier_sequence); > > > > @@ -3184,7 +3192,7 @@ void rcu_barrier(void) > > > > * Now that we have an rcu_barrier_callback() callback on each > > > > * CPU, and thus each counted, remove the initial count. > > > > */ > > > > - if (atomic_dec_and_test(&rcu_state.barrier_cpu_count)) > > > > + if (atomic_sub_and_test(2, &rcu_state.barrier_cpu_count)) > > > > complete(&rcu_state.barrier_completion); > > > > > > > > /* Wait for all rcu_barrier_callback() callbacks to be invoked. */ > > > > -- > > > > 2.9.5 > > > >