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 19F0BC001B0 for ; Thu, 13 Jul 2023 02:02:28 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233442AbjGMCC0 (ORCPT ); Wed, 12 Jul 2023 22:02:26 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:38910 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233421AbjGMCCZ (ORCPT ); Wed, 12 Jul 2023 22:02:25 -0400 Received: from out30-110.freemail.mail.aliyun.com (out30-110.freemail.mail.aliyun.com [115.124.30.110]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 721F819B4; Wed, 12 Jul 2023 19:02:23 -0700 (PDT) X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R121e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=ay29a033018046050;MF=hsiangkao@linux.alibaba.com;NM=1;PH=DS;RN=21;SR=0;TI=SMTPD_---0VnEzPEY_1689213737; Received: from 30.97.48.217(mailfrom:hsiangkao@linux.alibaba.com fp:SMTPD_---0VnEzPEY_1689213737) by smtp.aliyun-inc.com; Thu, 13 Jul 2023 10:02:19 +0800 Message-ID: <161f1615-3d85-cf47-d2d5-695adf1ca7d4@linux.alibaba.com> Date: Thu, 13 Jul 2023 10:02:17 +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: 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> From: Gao Xiang In-Reply-To: <20230713003201.GA469376@google.com> 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 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 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