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=-7.5 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 autolearn=no 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 ACD5AC4361B for ; Tue, 8 Dec 2020 18:25:06 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 8725323B46 for ; Tue, 8 Dec 2020 18:25:06 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730493AbgLHSYv (ORCPT ); Tue, 8 Dec 2020 13:24:51 -0500 Received: from mail.kernel.org ([198.145.29.99]:51550 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730424AbgLHSYv (ORCPT ); Tue, 8 Dec 2020 13:24:51 -0500 Date: Tue, 8 Dec 2020 10:24:09 -0800 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1607451850; bh=KkXJZf6suN8ZSAFkTq+0D7NxW5P4HswxVh8yAgzaLEY=; h=From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=uuy90LTvbsX4ESDngPRdEZXvJeP0EU94kktMByziFDYKHrhtUQxMYZO8iL6gWw0fQ fyOxjcWuY2D6pbFcdTQTjPdg6YVLc7QXyag/SL6QDiqZpi/B6/5Fl0uddw0ZwqPC0t wqkLa0YSmtxDTv6pO4dXO1tfAcIJrLGSbBISxRekkuOi57XkxUEjQMEBg+VL9PD8LQ jH1cZ1z0aGmF/5jHO2zESnYizBkHOKvBFF8u/H3+nu5GR350JrFg7P+U41Ru4wYQXT dErOKGruUsZSdeOKBZlmk89YuHTh4zgXH5EypE0sA0ejsAozqU1Uy4vk8EnlZ6h3EO cW09+O9FaPUiQ== From: "Paul E. McKenney" To: Frederic Weisbecker 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: <20201208182409.GT2657@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <20201208145810.GA4875@paulmck-ThinkPad-P72> <20201208155457.GA3916@lothringen> <20201208171927.GS2657@paulmck-ThinkPad-P72> <20201208175230.GB3916@lothringen> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201208175230.GB3916@lothringen> User-Agent: Mutt/1.9.4 (2018-02-28) Precedence: bulk List-ID: X-Mailing-List: rcu@vger.kernel.org On Tue, Dec 08, 2020 at 06:52:30PM +0100, Frederic Weisbecker wrote: > On Tue, Dec 08, 2020 at 09:19:27AM -0800, Paul E. McKenney wrote: > > On Tue, Dec 08, 2020 at 04:54:57PM +0100, Frederic Weisbecker wrote: > > > 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 ? > > > > Inertia and lack of complaints about it. ;-) > > > > Plus when called from softirq, neither local_bh_disable() nor > > rcu_read_lock() is necessary, and so represents pointless overhead. > > > > > Wouldn't rcu_read_lock() around callbacks invocation be enough? Or is > > > there another reason for the BH-disabled context that I'm missing? > > > > There are likely to be callback functions that use spin_lock() instead > > of spin_lock_bh() because they know that they are invoked in BH-disabled > > context. > > Ah right. So perhaps we can keep local_bh_disable() instead. > > > But what does this change help? > > It reduces the code scope running with BH disabled. > Also narrowing down helps to understand what it actually protects. I thought that you would call out unnecessarily delaying other softirq handlers. ;-) But if such delays are a problem (and they might well be), then to avoid them on non-rcu_nocb CPUs would instead/also require changing the early-exit checks to check for other pending softirqs to the existing checks involving time, need_resched, and idle. At which point, entering and exiting BH-disabled again doesn't help, other than your point about the difference in BH-disabled scopes on rcu_nocb and non-rcu_nocb CPUs. Would it make sense to exit rcu_do_batch() if more than some amount of time had elapsed and there was some non-RCU softirq pending? My guess is that the current tlimit checks in rcu_do_batch() make this unnecessary. Thoughts? Thanx, Paul