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=-16.2 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED 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 9EEA9C48BDF for ; Tue, 22 Jun 2021 23:46:54 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 85F8B6100B for ; Tue, 22 Jun 2021 23:46:54 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229900AbhFVXtJ (ORCPT ); Tue, 22 Jun 2021 19:49:09 -0400 Received: from mail.kernel.org ([198.145.29.99]:56224 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229746AbhFVXtJ (ORCPT ); Tue, 22 Jun 2021 19:49:09 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id 8E2216113D; Tue, 22 Jun 2021 23:46:52 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1624405612; bh=eDidqUCi0B/ZziFJA1v2poKwgZ1oRVVh6CdrHkKZChc=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=mmuEhYkGZD+TESbC9GNiFDT9o4AEHYB8GnXflItHA5GOWNmrLmzphbWa8K8/yAVvc vy+P09SfDkv6TjrS0oP/7ExKWvUFn48DW59KI6pmewxqbO29EJ25eQQLWOl8YMeKOE pn+fQm3c7uT+I1D0ksiU1gkDyxl5DEbqzqn8FLkcDilTK5NbIlEN+RFKwPVRf/gK+y n070D0OmmbsEFELBom7hMxa4Q8LJ/JOnc4EaDDSZqUCuhtbz8uUZb5UQCiLEEjQtxx 6jr1hbSYPKVBxqJ44Iwyj+YcvCZX7+IYZMD/Lj/F0XO9T6A6VoOH2v71Pl9IImhy91 zs0/PvsuUuu4A== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id 5049C5C0168; Tue, 22 Jun 2021 16:46:52 -0700 (PDT) Date: Tue, 22 Jun 2021 16:46:52 -0700 From: "Paul E. McKenney" To: Neeraj Upadhyay Cc: josh@joshtriplett.org, rostedt@goodmis.org, mathieu.desnoyers@efficios.com, jiangshanlai@gmail.com, joel@joelfernandes.org, rcu@vger.kernel.org, linux-kernel@vger.kernel.org, urezki@gmail.com, frederic@kernel.org Subject: Re: [PATCH] rcu: update: Check rcu_bh_lock_map state in rcu_read_lock_bh_held Message-ID: <20210622234652.GL4397@paulmck-ThinkPad-P17-Gen-1> Reply-To: paulmck@kernel.org References: <1624363521-19702-1-git-send-email-neeraju@codeaurora.org> <20210622175855.GE4397@paulmck-ThinkPad-P17-Gen-1> <61bed875-5ebf-03d8-58ea-e9263c534201@codeaurora.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <61bed875-5ebf-03d8-58ea-e9263c534201@codeaurora.org> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jun 23, 2021 at 12:38:09AM +0530, Neeraj Upadhyay wrote: > > > On 6/22/2021 11:28 PM, Paul E. McKenney wrote: > > On Tue, Jun 22, 2021 at 05:35:21PM +0530, Neeraj Upadhyay wrote: > > > In addition to irq and softirq state, check rcu_bh_lock_map > > > state, to decide whether RCU bh lock is held. > > > > > > Signed-off-by: Neeraj Upadhyay > > > > My initial reaction was that "in_softirq() || irqs_disabled()" covers > > it because rcu_read_lock_bh() disables BH. But you are right that it > > does seem a bit silly to ignore lockdep. > > > > So would it also make sense to have a WARN_ON_ONCE() if lockdep claims > > we are under rcu_read_lock_bh() protection, but "in_softirq() || > > irqs_disabled()" think otherwise? > > After thinking more on this, looks like one intention of not > having lockdep check here was to catch scenarios where some code enables bh > after doing rcu_read_lock_bh(), as is mentioned in the comment above > rcu_read_lock_bh_held(): > > Note that if someone uses > rcu_read_lock_bh(), but then later enables BH, lockdep (if enabled) > will show the situation. This is useful for debug checks in functions > that require that they be called within an RCU read-side critical > section. > > Client users seem to be doing lockdep checks on returned value: > drivers/net/wireguard/peer.c > RCU_LOCKDEP_WARN(!rcu_read_lock_bh_held(), > > Similarly, any rcu_dereference_check(..., rcu_read_lock_bh_held()) usage > also triggers warning, if bh is enabled, inside rcu_read_lock_bh() > section. > > So, using 'in_softirq() || irqs_disabled()' condition looks to be sufficient > condition, to mark all read lock bh regions and adding '|| > lock_is_held(&rcu_bh_lock_map)' to this condition does not seem to fit > well with the RCU_LOCKDEP_WARN(!rcu_read_lock_bh_held()) and > rcu_dereference_check(..., rcu_read_lock_bh_held()) calls, if we hit > the scenario, where bh lockmap state (shows bh lock acquired) conflicts with > the softirq/irq state . That makes sense to me! But should there be checks somewhere for something like "lock_is_held(&rcu_bh_lock_map) && !in_softirq() && !irqs_disabled()"? Thanx, Paul > Thanks > Neeraj > > > > --- > > > kernel/rcu/update.c | 2 +- > > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > > > diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c > > > index c21b38c..d416f1c 100644 > > > --- a/kernel/rcu/update.c > > > +++ b/kernel/rcu/update.c > > > @@ -333,7 +333,7 @@ int rcu_read_lock_bh_held(void) > > > if (rcu_read_lock_held_common(&ret)) > > > return ret; > > > - return in_softirq() || irqs_disabled(); > > > + return lock_is_held(&rcu_bh_lock_map) || in_softirq() || irqs_disabled(); > > > } > > > EXPORT_SYMBOL_GPL(rcu_read_lock_bh_held); > > > -- > > > QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum, > > > hosted by The Linux Foundation > > > > > -- > QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a member of > the Code Aurora Forum, hosted by The Linux Foundation