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=-2.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,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 3838FC433DF for ; Thu, 14 May 2020 18:13:08 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 103A220671 for ; Thu, 14 May 2020 18:13:08 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1589479988; bh=K6NKA083JKcvBkEFVEjRKQHwmmMVK7aiJCAoZ7eOmPc=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=SqZIVydpFXXa+mnciSn2rwtOxs1cffN9gIu+2uNjtvgJHTBFP72Rg6IYuULrlZEpz ml1/Sj1xF+r6IQHzaZZxkzvsAh9cPey31rKj5YT0NcnNBnSCaWxwQid6dtQoGjke6Z D58WeLvIW8TwZfOmZ4m1AaEwiSXfRavdaFYI0g4U= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726161AbgENSNH (ORCPT ); Thu, 14 May 2020 14:13:07 -0400 Received: from mail.kernel.org ([198.145.29.99]:59236 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726075AbgENSNH (ORCPT ); Thu, 14 May 2020 14:13:07 -0400 Received: from paulmck-ThinkPad-P72.home (unknown [50.39.105.78]) (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 852782065D; Thu, 14 May 2020 18:13:06 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1589479986; bh=K6NKA083JKcvBkEFVEjRKQHwmmMVK7aiJCAoZ7eOmPc=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=d1aPOCTNwyHeaHG+iWHrF7E5IEnHDZ9yqNqPDhmaywtwCiK6dUsAeOL88r96MJ8Kf QjTMsb6UqmRbcmSQpStcJhOkJ2UY9WXErwxnMEnW27qbq1+xF64ZrvkP6V12VM07f5 E/26dgoExPmjEz/8fAYOseLEnfQ8QuUWfEg1O5Bw= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id A9E2835229C8; Thu, 14 May 2020 11:13:05 -0700 (PDT) Date: Thu, 14 May 2020 11:13:05 -0700 From: "Paul E. McKenney" To: Qian Cai Cc: Stephen Rothwell , Linux Next Mailing List , Linux Kernel Mailing List , Madhuparna Bhowmik , Amol Grover , Dmitry Vyukov Subject: Re: Default enable RCU list lockdep debugging with PROVE_RCU Message-ID: <20200514181305.GT2869@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <20200514222535.259cb69e@canb.auug.org.au> <20200514133328.GG2869@paulmck-ThinkPad-P72> <20200514135402.GI2869@paulmck-ThinkPad-P72> <20200514153400.GJ2869@paulmck-ThinkPad-P72> <6089C62B-52AA-47CB-BCA0-9096B3482509@lca.pw> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <6089C62B-52AA-47CB-BCA0-9096B3482509@lca.pw> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-next-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-next@vger.kernel.org On Thu, May 14, 2020 at 11:46:23AM -0400, Qian Cai wrote: > > > > On May 14, 2020, at 11:34 AM, Paul E. McKenney wrote: > > > > On Thu, May 14, 2020 at 10:03:21AM -0400, Qian Cai wrote: > >> > >> > >>> On May 14, 2020, at 9:54 AM, Paul E. McKenney wrote: > >>> > >>> On Thu, May 14, 2020 at 09:44:28AM -0400, Qian Cai wrote: > >>>> > >>>> > >>>>> On May 14, 2020, at 9:33 AM, Paul E. McKenney wrote: > >>>>> > >>>>> On Thu, May 14, 2020 at 08:31:13AM -0400, Qian Cai wrote: > >>>>>> > >>>>>> > >>>>>>> On May 14, 2020, at 8:25 AM, Stephen Rothwell wrote: > >>>>>>> > >>>>>>> Hi Paul, > >>>>>>> > >>>>>>> This patch in the rcu tree > >>>>>>> > >>>>>>> d13fee049fa8 ("Default enable RCU list lockdep debugging with PROVE_RCU") > >>>>>>> > >>>>>>> is causing whack-a-mole in the syzbot testing of linux-next. Because > >>>>>>> they always do a debug build of linux-next, no testing is getting done. :-( > >>>>>>> > >>>>>>> Can we find another way to find all the bugs that are being discovered > >>>>>>> (very slowly)? > >>>>>> > >>>>>> Alternatively, could syzbot to use PROVE_RCU=n temporarily because it can’t keep up with it? I personally found PROVE_RCU_LIST=y is still useful for my linux-next testing, and don’t want to lose that coverage overnight. > >>>>> > >>>>> The problem is that PROVE_RCU is exactly PROVE_LOCKING, and asking people > >>>>> to test without PROVE_LOCKING is a no-go in my opinion. But of course > >>>>> on the other hand if there is no testing of RCU list lockdep debugging, > >>>>> those issues will never be found, let alone fixed. > >>>>> > >>>>> One approach would be to do as Stephen asks (either remove d13fee049fa8 > >>>>> or pull it out of -next) and have testers force-enable the RCU list > >>>>> lockdep debugging. > >>>>> > >>>>> Would that work for you? > >>>> > >>>> Alternatively, how about having > >>>> > >>>> PROVE_RCU_LIST=n if DEBUG_AID_FOR_SYZBOT > >>>> > >>>> since it is only syzbot can’t keep up with it? > >>> > >>> Sound good to me, assuming that this works for the syzkaller guys. > >>> Or could there be a "select PROVE_RCU_LIST" for the people who would > >>> like to test it. > >>> > >>> Alternatively, if we revert d13fee049fa8 from -next, I could provide > >>> you a script that updates your .config to set both RCU_EXPERT and > >>> PROVE_RCU_LIST. > >>> > >>> There are a lot of ways to appraoch this. > >>> > >>> So what would work best for everyone? > >> > >> > >> If PROVE_RCU_LIST=n if DEBUG_AID_FOR_SYZBOT works for syzbot guys, that would be great, so other testing agents could still report/fix those RCU-list bugs and then pave a way for syzbot to return back once all those false positives had been sorted out. > > > > On that, I must defer to the syzbot guys. > > > >> Otherwise, “select PROVE_RCU_LIST” *might* be better than buried into RCU_EXPERT where we will probably never saw those false positives been addressed since my configs does not cover a wide range of subsystems and probably not many other bots would enable RCU_EXPERT. > > > > Yet another option would be to edit your local kernel/rcu/Kconfig.debug > > and change the code to the following: > > > > config PROVE_RCU_LIST > > def_bool y > > help > > Enable RCU lockdep checking for list usages. It is default > > enabled with CONFIG_PROVE_RCU. > > > > Removing the RCU_EXPERT dependency would not go over at all well with > > some people whose opinions are difficult to ignore. ;-) > > I am trying to not getting into a game of carrying any custom patch myself. > > Let’s see what syzbot guys will say, and then I’ll enable RCU_EXPERT myself if needed, but again we probably never see PROVE_RCU_LIST to be used again in syzbot for this path. I surely have no cycles to expand the testing coverage for more subsystems at the moment. Fair enough! And yes, the Linux kernel is quite large, so I certainly am not asking you to test the whole thing yourself. Thanx, Paul