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=-11.0 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,INCLUDES_PATCH,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS 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 DB264C1B0D8 for ; Tue, 8 Dec 2020 15:55:44 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id B21E923AA9 for ; Tue, 8 Dec 2020 15:55:44 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730194AbgLHPzl (ORCPT ); Tue, 8 Dec 2020 10:55:41 -0500 Received: from mail.kernel.org ([198.145.29.99]:48886 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729936AbgLHPzk (ORCPT ); Tue, 8 Dec 2020 10:55:40 -0500 Date: Tue, 8 Dec 2020 16:54:57 +0100 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1607442900; bh=oeMJmaYdkYBRZuie6pH9o0N2Zu33NAcbpL9OlNBHGvM=; h=From:To:Cc:Subject:References:In-Reply-To:From; b=hPvERiQONzGEcL0ZxgpBFY0CnWmBAWZwhEtPY670+YmKEbh8op9bSqfmjCzOczaub UrDZnwmnifKIXlOwvrsBdeE0O3EeY8gA3PnXd9eHK/tXN3C8/duDOPTYqhpkT9myIP 1YmeeN+ArKGnAcQeI7Qb/KelcG6z3IW8jBJhzLZ5gAH8Mz0PAEo65PwkVTGWMHvSXo ZM+Lk+ICQpuQYTQsWxvMw105+g9oFHpPdmO4z/AnFzN/ONT64Ge2qfqUxm5zjvO8Py fyK7VZ3yq0DoqS6Q8dNdtct7G7Gnyh3ceLoeSeaeCTV+po/Fss+NODgVth73VaswX6 xj4qgPZ6jnQ3g== From: Frederic Weisbecker To: "Paul E. McKenney" Cc: boqun.feng@gmail.com, rcu@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: One potential issue with concurrent execution of RCU callbacks... Message-ID: <20201208155457.GA3916@lothringen> References: <20201208145810.GA4875@paulmck-ThinkPad-P72> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201208145810.GA4875@paulmck-ThinkPad-P72> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Dec 08, 2020 at 06:58:10AM -0800, Paul E. McKenney wrote: > Hello, Frederic, > > Boqun just asked if RCU callbacks ran in BH-disabled context to avoid > concurrent execution of the same callback. Of course, this raises the > question of whether a self-posting callback can have two instances of > itself running concurrently while a CPU is in the process of transitioning > between softirq and rcuo invocation of callbacks. > > I believe that the answer is "no" because BH-disabled context is > an implicit RCU read-side critical section. Therefore, the initial > invocation of the RCU callback must complete in order for a new grace > period to complete, and a new grace period must complete before the > second invocation of that same callback to start. > > Does that make sense, or am I missing something? Sounds like a good explanation. But then why are we actually calling the entire rcu_do_batch() under BH-disabled context? Was it to quieten lockdep against rcu_callback_map ? Wouldn't rcu_read_lock() around callbacks invocation be enough? Or is there another reason for the BH-disabled context that I'm missing? Untested below: diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index bd04b09b84b3..207eff8a4e1a 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -2468,6 +2468,7 @@ static void rcu_do_batch(struct rcu_data *rdp) debug_rcu_head_unqueue(rhp); + rcu_read_lock(); rcu_lock_acquire(&rcu_callback_map); trace_rcu_invoke_callback(rcu_state.name, rhp); @@ -2476,6 +2477,7 @@ static void rcu_do_batch(struct rcu_data *rdp) f(rhp); rcu_lock_release(&rcu_callback_map); + rcu_read_unlock(); /* * Stop only if limit reached and CPU has something to do. @@ -2494,11 +2496,9 @@ static void rcu_do_batch(struct rcu_data *rdp) } if (offloaded) { WARN_ON_ONCE(in_serving_softirq()); - local_bh_enable(); lockdep_assert_irqs_enabled(); cond_resched_tasks_rcu_qs(); lockdep_assert_irqs_enabled(); - local_bh_disable(); } } diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h index fd8a52e9a887..2a3d3c59d650 100644 --- a/kernel/rcu/tree_plugin.h +++ b/kernel/rcu/tree_plugin.h @@ -2095,9 +2095,7 @@ static void nocb_cb_wait(struct rcu_data *rdp) local_irq_save(flags); rcu_momentary_dyntick_idle(); local_irq_restore(flags); - local_bh_disable(); rcu_do_batch(rdp); - local_bh_enable(); lockdep_assert_irqs_enabled(); rcu_nocb_lock_irqsave(rdp, flags); if (rcu_segcblist_nextgp(&rdp->cblist, &cur_gp_seq) &&