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=-6.0 required=3.0 tests=BAYES_00,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 3199AC433ED for ; Wed, 14 Apr 2021 21:25:04 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 06003600D1 for ; Wed, 14 Apr 2021 21:25:03 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232723AbhDNVZZ (ORCPT ); Wed, 14 Apr 2021 17:25:25 -0400 Received: from mail.kernel.org ([198.145.29.99]:35552 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232559AbhDNVZY (ORCPT ); Wed, 14 Apr 2021 17:25:24 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id C13AD61155; Wed, 14 Apr 2021 21:25:02 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1618435502; bh=jZIX9LrM0id9K+Xt+PCBTE3R8mFcr0lcJUfT2FcH2CQ=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=TsS+XDDpLesMAlkm153mt4faQ1RD1g/J/Ee2UKX9nqbG1dE6RdKECrhY5LU1sDJzt Qjr9ehiPC0jdqMXxY7i76yzlRUZ99pIOp5LbvETz6Ss8oTrQUeg04bUq+jJsJyQzuN c49WGJW4Nw/eC8NSkxpCAXGAOps+7gOXUHMNpTgH4ZYd/VqxVKuHYYqqRbNB4iNvKc mq3M7DdxNVk/yMWSzmtti1q6jWZEbtF59kgCKVQki/tnuNK7xUMU+HD5CEy085yPBW IOvdKJtDvpMkzigo5NvONM/sp+L5CxK0aqXawKC4EvlwjcjAAKIg3KwGl8xcDed3Bj dbGVctpfr6T4w== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id 93FB85C26C5; Wed, 14 Apr 2021 14:25:02 -0700 (PDT) Date: Wed, 14 Apr 2021 14:25:02 -0700 From: "Paul E. McKenney" To: Toke =?iso-8859-1?Q?H=F8iland-J=F8rgensen?= Cc: Alexei Starovoitov , Andrii Nakryiko , bpf Subject: Re: Selftest failures related to kern_sync_rcu() Message-ID: <20210414212502.GX4510@paulmck-ThinkPad-P17-Gen-1> Reply-To: paulmck@kernel.org References: <87im4qo9ey.fsf@toke.dk> <20210414175245.GT4510@paulmck-ThinkPad-P17-Gen-1> <20210414181934.GV4510@paulmck-ThinkPad-P17-Gen-1> <87czuwlnhz.fsf@toke.dk> <20210414184133.GW4510@paulmck-ThinkPad-P17-Gen-1> <87a6q0llou.fsf@toke.dk> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <87a6q0llou.fsf@toke.dk> Precedence: bulk List-ID: X-Mailing-List: bpf@vger.kernel.org On Wed, Apr 14, 2021 at 09:18:09PM +0200, Toke Høiland-Jørgensen wrote: > "Paul E. McKenney" writes: > > > On Wed, Apr 14, 2021 at 08:39:04PM +0200, Toke Høiland-Jørgensen wrote: > >> "Paul E. McKenney" writes: > >> > >> > On Wed, Apr 14, 2021 at 10:59:23AM -0700, Alexei Starovoitov wrote: > >> >> On Wed, Apr 14, 2021 at 10:52 AM Paul E. McKenney wrote: > >> >> > > >> >> > > > > > if (num_online_cpus() > 1) > >> >> > > > > > synchronize_rcu(); > >> >> > > >> >> > In CONFIG_PREEMPT_NONE=y and CONFIG_PREEMPT_VOLUNTARY=y kernels, this > >> >> > synchronize_rcu() will be a no-op anyway due to there only being the > >> >> > one CPU. Or are these failures all happening in CONFIG_PREEMPT=y kernels, > >> >> > and in tests where preemption could result in the observed failures? > >> >> > > >> >> > Could you please send your .config file, or at least the relevant portions > >> >> > of it? > >> >> > >> >> That's my understanding as well. I assumed Toke has preempt=y. > >> >> Otherwise the whole thing needs to be root caused properly. > >> > > >> > Given that there is only a single CPU, I am still confused about what > >> > the tests are expecting the membarrier() system call to do for them. > >> > >> It's basically a proxy for waiting until the objects are freed on the > >> kernel side, as far as I understand... > > > > There are in-kernel objects that are freed via call_rcu(), and the idea > > is to wait until these objects really are freed? Or am I still missing > > out on what is going on? > > Something like that? Although I'm not actually sure these are using > call_rcu()? One of them needs __put_task_struct() to run, and the other > waits for map freeing, with this comment: > > > /* we need to either wait for or force synchronize_rcu(), before > * checking for "still exists" condition, otherwise map could still be > * resolvable by ID, causing false positives. > * > * Older kernels (5.8 and earlier) freed map only after two > * synchronize_rcu()s, so trigger two, to be entirely sure. > */ > CHECK(kern_sync_rcu(), "sync_rcu", "failed\n"); > CHECK(kern_sync_rcu(), "sync_rcu", "failed\n"); OK, so the issue is that the membarrier() system call is designed to force ordering only within a user process, and you need it in the kernel. Give or take my being puzzled as to why the membarrier() system call doesn't do it for you on a CONFIG_PREEMPT_NONE=y system, this brings us back to the question Alexei asked me in the first place, what is the best way to invoke an in-kernel synchronize_rcu() from userspace? You guys gave some reasonable examples. Here are a few others: o Bring a CPU online, then force it offline, or vice versa. But in this case, sys_membarrier() would do what you need given more than one CPU. o Use the membarrier() system call, but require that the tests run on systems with at least two CPUs. o Create a kernel module whose init function does a synchronize_rcu() and then returns failure. This will avoid the overhead of removing that kernel module. o Create a sysfs or debugfs interface that does a synchronize_rcu(). But I am still concerned that you are needing more than synchronize_rcu() can do. Otherwise, the membarrier() system call would work just fine on a single CPU on your CONFIG_PREEMPT_VOLUNTARY=y kernel. Thanx, Paul