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=-6.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS autolearn=ham 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 CEC51C3A5A1 for ; Fri, 23 Aug 2019 03:23:29 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id AA6B22341C for ; Fri, 23 Aug 2019 03:23:29 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2391506AbfHWDX2 (ORCPT ); Thu, 22 Aug 2019 23:23:28 -0400 Received: from mx1.redhat.com ([209.132.183.28]:38744 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1732939AbfHWDX2 (ORCPT ); Thu, 22 Aug 2019 23:23:28 -0400 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.phx2.redhat.com [10.5.11.15]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id AA05D83F3C; Fri, 23 Aug 2019 03:23:27 +0000 (UTC) Received: from ovpn-117-150.phx2.redhat.com (ovpn-117-150.phx2.redhat.com [10.3.117.150]) by smtp.corp.redhat.com (Postfix) with ESMTP id 76FD45D784; Fri, 23 Aug 2019 03:23:24 +0000 (UTC) Message-ID: Subject: Re: [PATCH RT v2 1/3] rcu: Acquire RCU lock when disabling BHs From: Scott Wood To: Joel Fernandes , "Paul E. McKenney" Cc: Sebastian Andrzej Siewior , linux-rt-users@vger.kernel.org, linux-kernel@vger.kernel.org, Thomas Gleixner , Peter Zijlstra , Juri Lelli , Clark Williams Date: Thu, 22 Aug 2019 22:23:23 -0500 In-Reply-To: <20190822133955.GA29841@google.com> References: <20190821231906.4224-1-swood@redhat.com> <20190821231906.4224-2-swood@redhat.com> <20190821233358.GU28441@linux.ibm.com> <20190822133955.GA29841@google.com> Organization: Red Hat Content-Type: text/plain; charset="UTF-8" User-Agent: Evolution 3.30.5 (3.30.5-1.fc29) MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.27]); Fri, 23 Aug 2019 03:23:27 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 2019-08-22 at 09:39 -0400, Joel Fernandes wrote: > On Wed, Aug 21, 2019 at 04:33:58PM -0700, Paul E. McKenney wrote: > > On Wed, Aug 21, 2019 at 06:19:04PM -0500, Scott Wood wrote: > > > Signed-off-by: Scott Wood > > > --- > > > Another question is whether non-raw spinlocks are intended to create > > > an > > > RCU read-side critical section due to implicit preempt disable. > > > > Hmmm... Did non-raw spinlocks act like rcu_read_lock_sched() > > and rcu_read_unlock_sched() pairs in -rt prior to the RCU flavor > > consolidation? If not, I don't see why they should do so after that > > consolidation in -rt. > > May be I am missing something, but I didn't see the connection between > consolidation and this patch. AFAICS, this patch is so that > rcu_read_lock_bh_held() works at all on -rt. Did I badly miss something? Before consolidation, RT mapped rcu_read_lock_bh_held() to rcu_read_lock_bh() and called rcu_read_lock() from rcu_read_lock_bh(). This somehow got lost when rebasing on top of 5.0. > > > include/linux/rcupdate.h | 4 ++++ > > > kernel/rcu/update.c | 4 ++++ > > > kernel/softirq.c | 12 +++++++++--- > > > 3 files changed, 17 insertions(+), 3 deletions(-) > > > > > > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h > > > index 388ace315f32..d6e357378732 100644 > > > --- a/include/linux/rcupdate.h > > > +++ b/include/linux/rcupdate.h > > > @@ -615,10 +615,12 @@ static inline void rcu_read_unlock(void) > > > static inline void rcu_read_lock_bh(void) > > > { > > > local_bh_disable(); > > > +#ifndef CONFIG_PREEMPT_RT_FULL > > > __acquire(RCU_BH); > > > rcu_lock_acquire(&rcu_bh_lock_map); > > > RCU_LOCKDEP_WARN(!rcu_is_watching(), > > > "rcu_read_lock_bh() used illegally while idle"); > > > +#endif > > > > Any chance of this using "if (!IS_ENABLED(CONFIG_PREEMPT_RT_FULL))"? > > We should be OK providing a do-nothing __maybe_unused rcu_bh_lock_map > > for lockdep-enabled -rt kernels, right? > > Since this function is small, I prefer if -rt defines their own > rcu_read_lock_bh() which just does the local_bh_disable(). That would be > way > cleaner IMO. IIRC, -rt does similar things for spinlocks, but it has been > sometime since I look at the -rt patchset. I'll do it whichever way you all decide, though I'm not sure I agree about it being cleaner (especially while RT is still out-of-tree and a change to the non-RT version that fails to trigger a merge conflict is a concern). What about moving everything but the local_bh_disable into a separate function called from rcu_read_lock_bh, and making that a no-op on RT? > > > > > > diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c > > > index 016c66a98292..a9cdf3d562bc 100644 > > > --- a/kernel/rcu/update.c > > > +++ b/kernel/rcu/update.c > > > @@ -296,7 +296,11 @@ int rcu_read_lock_bh_held(void) > > > return 0; > > > if (!rcu_lockdep_current_cpu_online()) > > > return 0; > > > +#ifdef CONFIG_PREEMPT_RT_FULL > > > + return lock_is_held(&rcu_lock_map) || irqs_disabled(); > > > +#else > > > return in_softirq() || irqs_disabled(); > > > +#endif > > > > And globally. > > And could be untangled a bit as well: > > if (irqs_disabled()) > return 1; > > if (IS_ENABLED(CONFIG_PREEMPT_RT_FULL)) > return lock_is_held(&rcu_lock_map); > > return in_softirq(); OK. -Scott