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=-1.1 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 B4197C43603 for ; Tue, 17 Dec 2019 14:59:29 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 840482465E for ; Tue, 17 Dec 2019 14:59:29 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1576594769; bh=X1PiVxcDVotj0DhdvGj1bIfqqxEYbzQbmZ7rzefwzlI=; h=Date:From:To:Cc:Subject:In-Reply-To:References:List-ID:From; b=nUsCYf0meCk0+ezCqnoLA5SC3mWZrqUGCNBROnJnczuIJmxR6vRqrFI0bz3amLyeO ben3ONXIvrsHMp3H/t9Eo4Cym/fHVjEfAS9bsyW476veeaaUPMelubos8SD3gqOzTP mhSctZt6wcTXEomiSezvxWiWjAwZfdUpYNsStVFE= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727909AbfLQO72 (ORCPT ); Tue, 17 Dec 2019 09:59:28 -0500 Received: from mail.kernel.org ([198.145.29.99]:45918 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726560AbfLQO72 (ORCPT ); Tue, 17 Dec 2019 09:59:28 -0500 Received: from devnote2 (NE2965lan1.rev.em-net.ne.jp [210.141.244.193]) (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 7C1432146E; Tue, 17 Dec 2019 14:59:24 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1576594766; bh=X1PiVxcDVotj0DhdvGj1bIfqqxEYbzQbmZ7rzefwzlI=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=nr0K06QNQmm1n9qx+9jVB2d+Ydzi4sALusD8JzCqB3ntD3yvFbLJNuPHAptIVLsF+ PDs806kXbdNEAiy/H7HLANWQHb4WI5T2N345xg8/syBy4SMsutm/n6ctLJ9sEI3da3 HgtLDZWf/zydqfQ+j3AHA3IguZqFxvBe8fuWClG4= Date: Tue, 17 Dec 2019 23:59:21 +0900 From: Masami Hiramatsu To: paulmck@kernel.org Cc: Joel Fernandes , Ingo Molnar , Peter Zijlstra , Masami Hiramatsu , Anders Roxell , "Naveen N . Rao" , Anil S Keshavamurthy , David Miller , Linux Kernel Mailing List Subject: Re: [PATCH -tip] kprobes: Lock rcu_read_lock() while searching kprobe Message-Id: <20191217235921.01cecb379e5e58493a0815af@kernel.org> In-Reply-To: <20191209033910.GD2889@paulmck-ThinkPad-P72> References: <157527193358.11113.14859628506665612104.stgit@devnote2> <20191202210854.GD17234@google.com> <20191203071329.GC115767@gmail.com> <20191203175712.GI2889@paulmck-ThinkPad-P72> <20191204100549.GB114697@gmail.com> <20191204161239.GL2889@paulmck-ThinkPad-P72> <20191206011137.GB142442@google.com> <20191206031151.GY2889@paulmck-ThinkPad-P72> <20191208000842.GA62607@google.com> <20191209033910.GD2889@paulmck-ThinkPad-P72> X-Mailer: Sylpheed 3.5.1 (GTK+ 2.24.32; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi, On Sun, 8 Dec 2019 19:39:11 -0800 "Paul E. McKenney" wrote: > On Sat, Dec 07, 2019 at 07:08:42PM -0500, Joel Fernandes wrote: > > On Thu, Dec 05, 2019 at 07:11:51PM -0800, Paul E. McKenney wrote: > > > On Thu, Dec 05, 2019 at 08:11:37PM -0500, Joel Fernandes wrote: > > > > On Wed, Dec 04, 2019 at 08:12:39AM -0800, Paul E. McKenney wrote: > > > > > On Wed, Dec 04, 2019 at 11:05:50AM +0100, Ingo Molnar wrote: > > > > > > > > > > > > * Paul E. McKenney wrote: > > > > > > > > > > > > > > * This list-traversal primitive may safely run concurrently with > > > > > > > > * the _rcu list-mutation primitives such as hlist_add_head_rcu() > > > > > > > > * as long as the traversal is guarded by rcu_read_lock(). > > > > > > > > */ > > > > > > > > #define hlist_for_each_entry_rcu(pos, head, member, cond...) \ > > > > > > > > > > > > > > > > is actively harmful. Why is it there? > > > > > > > > > > > > > > For cases where common code might be invoked both from the reader > > > > > > > (with RCU protection) and from the updater (protected by some > > > > > > > lock). This common code can then use the optional argument to > > > > > > > hlist_for_each_entry_rcu() to truthfully tell lockdep that it might be > > > > > > > called with either form of protection in place. > > > > > > > > > > > > > > This also combines with the __rcu tag used to mark RCU-protected > > > > > > > pointers, in which case sparse complains when a non-RCU API is applied > > > > > > > to these pointers, to get back to your earlier question about use of > > > > > > > hlist_for_each_entry_rcu() within the update-side lock. > > > > > > > > > > > > > > But what are you seeing as actively harmful about all of this? > > > > > > > What should we be doing instead? > > > > > > > > > > > > Yeah, so basically in the write-locked path hlist_for_each_entry() > > > > > > generates (slightly) more efficient code than hlist_for_each_entry_rcu(), > > > > > > correct? > > > > > > > > > > Potentially yes, if the READ_ONCE() constrains the compiler. Or not, > > > > > depending of course on the compiler and the surrounding code. > > > > > > > > > > > Also, the principle of passing warning flags around is problematic - but > > > > > > I can see the point in this specific case. > > > > > > > > > > Would it help to add an hlist_for_each_entry_protected() that expected > > > > > RCU-protected pointers and write-side protection, analogous to > > > > > rcu_dereference_protected()? Or would that expansion of the RCU API > > > > > outweigh any benefits? > > > > > > > > Personally, I like keeping the same API and using the optional argument like > > > > we did thus preventing too many APIs / new APIs. > > > > > > Would you be willing to put together a prototype patch so that people > > > can see exactly how it would look? > > > > Hi Paul, > > > > I was referring to the same API we have at the moment (that is > > hlist_for_each_entry_rcu() with the additional cond parameter). I was saying > > let us keep that and not add a hlist_for_each_entry_protected() instead, so > > as to not proliferate the number of APIs. > > > > Or did I miss the point? > > This would work for me. The only concern would be inefficiency, but we > have heard from people saying that the unnecessary inefficiency is only > on code paths that they do not care about, so we should be good. So, what will be the conclusion here, Ingo? I faced other warnings in tracing subsystem, so I need to add more lockdep_is_held()s there to suppress warnings. Thank you, -- Masami Hiramatsu