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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 78EE1C001DD for ; Thu, 13 Jul 2023 04:41:26 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233842AbjGMElY (ORCPT ); Thu, 13 Jul 2023 00:41:24 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:42594 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232133AbjGMElV (ORCPT ); Thu, 13 Jul 2023 00:41:21 -0400 Received: from out30-97.freemail.mail.aliyun.com (out30-97.freemail.mail.aliyun.com [115.124.30.97]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 84A39211E; Wed, 12 Jul 2023 21:41:16 -0700 (PDT) X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R711e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=ay29a033018045192;MF=hsiangkao@linux.alibaba.com;NM=1;PH=DS;RN=21;SR=0;TI=SMTPD_---0VnFWGm4_1689223270; Received: from 30.97.48.217(mailfrom:hsiangkao@linux.alibaba.com fp:SMTPD_---0VnFWGm4_1689223270) by smtp.aliyun-inc.com; Thu, 13 Jul 2023 12:41:11 +0800 Message-ID: Date: Thu, 13 Jul 2023 12:41:09 +0800 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:102.0) Gecko/20100101 Thunderbird/102.13.0 Subject: Re: [PATCH v1] rcu: Fix and improve RCU read lock checks when !CONFIG_DEBUG_LOCK_ALLOC To: paulmck@kernel.org Cc: Joel Fernandes , Sandeep Dhavale , Frederic Weisbecker , Neeraj Upadhyay , Josh Triplett , Boqun Feng , Steven Rostedt , Mathieu Desnoyers , Lai Jiangshan , Zqiang , Matthias Brugger , AngeloGioacchino Del Regno , linux-erofs@lists.ozlabs.org, xiang@kernel.org, Will Shiu , kernel-team@android.com, rcu@vger.kernel.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-mediatek@lists.infradead.org References: <20230711233816.2187577-1-dhavale@google.com> <20230713003201.GA469376@google.com> <161f1615-3d85-cf47-d2d5-695adf1ca7d4@linux.alibaba.com> <0d9e7b4d-6477-47a6-b3d2-2c9d9b64903d@paulmck-laptop> From: Gao Xiang In-Reply-To: <0d9e7b4d-6477-47a6-b3d2-2c9d9b64903d@paulmck-laptop> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2023/7/13 12:27, Paul E. McKenney wrote: > On Thu, Jul 13, 2023 at 10:02:17AM +0800, Gao Xiang wrote: >> >> >> On 2023/7/13 08:32, Joel Fernandes wrote: >>> On Wed, Jul 12, 2023 at 02:20:56PM -0700, Sandeep Dhavale wrote: >>> [..] >>>>> As such this patch looks correct to me, one thing I noticed is that >>>>> you can check rcu_is_watching() like the lockdep-enabled code does. >>>>> That will tell you also if a reader-section is possible because in >>>>> extended-quiescent-states, RCU readers should be non-existent or >>>>> that's a bug. >>>>> >>>> Please correct me if I am wrong, reading from the comment in >>>> kernel/rcu/update.c rcu_read_lock_held_common() >>>> .. >>>> * The reason for this is that RCU ignores CPUs that are >>>> * in such a section, considering these as in extended quiescent state, >>>> * so such a CPU is effectively never in an RCU read-side critical section >>>> * regardless of what RCU primitives it invokes. >>>> >>>> It seems rcu will treat this as lock not held rather than a fact that >>>> lock is not held. Is my understanding correct? >>> >>> If RCU treats it as a lock not held, that is a fact for RCU ;-). Maybe you >>> mean it is not a fact for erofs? >> >> I'm not sure if I get what you mean, EROFS doesn't take any RCU read lock >> here: > > The key point is that we need lockdep to report errors when > rcu_read_lock(), rcu_dereference(), and friends are used when RCU is > not watching. We also need lockdep to report an error when someone > uses rcu_dereference() when RCU is not watching, but also forgets the > rcu_read_lock(). > > And this is the job of rcu_read_lock_held(), which is one reason why > that rcu_is_watching() is needed. > >> z_erofs_decompressqueue_endio() is actually a "bio->bi_end_io", previously >> which can be called under two scenarios: >> >> 1) under softirq context, which is actually part of device I/O compleltion; >> >> 2) under threaded context, like what dm-verity or likewise calls. >> >> But EROFS needs to decompress in a threaded context anyway, so we trigger >> a workqueue to resolve the case 1). >> >> Recently, someone reported there could be some case 3) [I think it was >> introduced recently but I have no time to dig into it]: >> >> case 3: under RCU read lock context, which is shown by this: >> https://lore.kernel.org/r/4a8254eb-ac39-1e19-3d82-417d3a7b9f94@linux.alibaba.com/T/#u >> >> and such RCU read lock is taken in __blk_mq_run_dispatch_ops(). >> >> But as the commit shown, we only need to trigger a workqueue for case 1) >> and 3) due to performance reasons. > > Just out of curiosity, exactly how much is it costing to trigger the > workqueue? There are lots of performance issues here and even a plumber topic last year to show that, see: [1] https://lore.kernel.org/r/20230519001709.2563-1-tj@kernel.org [2] https://lore.kernel.org/r/CAHk-=wgE9kORADrDJ4nEsHHLirqPCZ1tGaEPAZejHdZ03qCOGg@mail.gmail.com [3] https://lore.kernel.org/r/CAB=BE-SBtO6vcoyLNA9F-9VaN5R0t3o_Zn+FW8GbO6wyUqFneQ@mail.gmail.com [4] https://lpc.events/event/16/contributions/1338/ and more. I'm not sure if it's necessary to look info all of that, andSandeep knows more than I am (the scheduling issue becomes vital on some aarch64 platform.) Thanks, Gao Xiang