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=-10.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=ham 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 9CA2FC43611 for ; Thu, 20 May 2021 11:50:08 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 782A3611AB for ; Thu, 20 May 2021 11:50:08 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S243481AbhETLt7 (ORCPT ); Thu, 20 May 2021 07:49:59 -0400 Received: from mx2.suse.de ([195.135.220.15]:34568 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S241613AbhETL1o (ORCPT ); Thu, 20 May 2021 07:27:44 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1621509975; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=jqNfPIxIDmqFlBeVEmMp4wkx0fm6Zo+X3532NE3LEc4=; b=SU6XlUHSjsUKY9kB25dfVXQOqOY1IQ2sWs77YhL2HANF3qiNJNgUC3QRsbQnosBqxLWV9K 1l9MF2P059V77UXwJQQcZ8eMUUxj+R4WqJ66BK+qb2aD650bsM9h+g8E91PTecH4hXcxw5 Oe40taPMTPJOn/LjXWhG7cNnyxXrdek= Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 7879AAC85; Thu, 20 May 2021 11:26:15 +0000 (UTC) Date: Thu, 20 May 2021 13:26:15 +0200 From: Petr Mladek To: Martin Liu , Andrew Morton Cc: Nathan Chancellor , Nick Desaulniers , Tejun Heo , minchan@google.com, www@google.com, davidchao@google.com, jenhaochen@google.com, linux-kernel@vger.kernel.org, clang-built-linux@googlegroups.com Subject: Re: [PATCH] kthread: Fix kthread_mod_delayed_work vs kthread_cancel_delayed_work_sync race Message-ID: References: <20210513065458.941403-1-liumartin@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20210513065458.941403-1-liumartin@google.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu 2021-05-13 14:54:57, Martin Liu wrote: > We encountered a system hang issue while doing the tests. The callstack > is as following > > schedule+0x80/0x100 > schedule_timeout+0x48/0x138 > wait_for_common+0xa4/0x134 > wait_for_completion+0x1c/0x2c > kthread_flush_work+0x114/0x1cc > kthread_cancel_work_sync.llvm.16514401384283632983+0xe8/0x144 > kthread_cancel_delayed_work_sync+0x18/0x2c > xxxx_pm_notify+0xb0/0xd8 > blocking_notifier_call_chain_robust+0x80/0x194 > pm_notifier_call_chain_robust+0x28/0x4c > suspend_prepare+0x40/0x260 > enter_state+0x80/0x3f4 > pm_suspend+0x60/0xdc > state_store+0x108/0x144 > kobj_attr_store+0x38/0x88 > sysfs_kf_write+0x64/0xc0 > kernfs_fop_write_iter+0x108/0x1d0 > vfs_write+0x2f4/0x368 > ksys_write+0x7c/0xec > > When we started investigating, we found race between > kthread_mod_delayed_work vs kthread_cancel_delayed_work_sync. The race's > result could be simply reproduced as a kthread_mod_delayed_work with > a following kthread_flush_work call. > > Thing is we release kthread_mod_delayed_work kspin_lock in > __kthread_cancel_work so it opens a race window for > kthread_cancel_delayed_work_sync to change the canceling count used to > prevent dwork from being requeued before calling kthread_flush_work. > However, we don't check the canceling count after returning from > __kthread_cancel_work and then insert the dwork to the worker. It > results the following kthread_flush_work inserts flush work to dwork's > tail which is at worker's dealyed_work_list. Therefore, flush work will > never get moved to the worker's work_list to be executed. Finally, > kthread_cancel_delayed_work_sync will NOT be able to get completed and > wait forever. The code sequence diagram is as following > > Thread A Thread B > kthread_mod_delayed_work > spin_lock > __kthread_cancel_work > canceling = 1 > spin_unlock > kthread_cancel_delayed_work_sync > spin_lock > kthread_cancel_work > canceling = 2 > spin_unlock > del_timer_sync > spin_lock > canceling = 1 // canceling count gets update in ThreadB before > queue_delayed_work // dwork is put into the woker’s dealyed_work_list > without checking the canceling count > spin_unlock > kthread_flush_work > spin_lock > Insert flush work // at the tail of the > dwork which is at > the worker’s > dealyed_work_list > spin_unlock > wait_for_completion // Thread B stuck here as > flush work will never > get executed > > The canceling count could change in __kthread_cancel_work as the spinlock > get released and regained in between, let's check the count again before > we queue the delayed work to avoid the race. > > Fixes: 37be45d49dec2 ("kthread: allow to cancel kthread work") > Tested-by: David Chao > Signed-off-by: Martin Liu Great catch! The patch makes perfect sense. Reviewed-by: Petr Mladek Andrew, could you please queue it via -mm tree? Best Regards, Petr PS: I am sorry for the late review. I was somehow busy last week it it has fallen too low in the mailbox :-(