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 AD044C001B0 for ; Thu, 13 Jul 2023 02:10:42 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233298AbjGMCKl (ORCPT ); Wed, 12 Jul 2023 22:10:41 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:41352 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231859AbjGMCKi (ORCPT ); Wed, 12 Jul 2023 22:10:38 -0400 Received: from out30-113.freemail.mail.aliyun.com (out30-113.freemail.mail.aliyun.com [115.124.30.113]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 5334512D; Wed, 12 Jul 2023 19:10:37 -0700 (PDT) X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R941e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=ay29a033018046049;MF=hsiangkao@linux.alibaba.com;NM=1;PH=DS;RN=21;SR=0;TI=SMTPD_---0VnEugHV_1689214232; Received: from 30.97.48.217(mailfrom:hsiangkao@linux.alibaba.com fp:SMTPD_---0VnEugHV_1689214232) by smtp.aliyun-inc.com; Thu, 13 Jul 2023 10:10:33 +0800 Message-ID: Date: Thu, 13 Jul 2023 10:10:31 +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 From: Gao Xiang To: Joel Fernandes , Sandeep Dhavale Cc: "Paul E. McKenney" , 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> In-Reply-To: <161f1615-3d85-cf47-d2d5-695adf1ca7d4@linux.alibaba.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2023/7/13 10:02, 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: > > 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 Sorry about the incorrect link (I really don't know who initally reported this and on which device): https://lore.kernel.org/linux-erofs/161f1615-3d85-cf47-d2d5-695adf1ca7d4@linux.alibaba.com/T/#t > >  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. > > Hopefully I show it more clear. > > Thanks, > Gao Xiang