From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752311AbcEINxL (ORCPT ); Mon, 9 May 2016 09:53:11 -0400 Received: from mail.pqgruber.com ([178.189.19.235]:42877 "EHLO mail.pqgruber.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751282AbcEINxJ (ORCPT ); Mon, 9 May 2016 09:53:09 -0400 Date: Mon, 9 May 2016 15:53:03 +0200 From: Clemens Gruber To: Guenter Roeck , Wim Van Sebroeck Cc: linux-watchdog@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] watchdog: core: Fix circular locking dependency Message-ID: <20160509135303.GA1474@archie.localdomain> References: <1461249494-24924-1-git-send-email-linux@roeck-us.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1461249494-24924-1-git-send-email-linux@roeck-us.net> User-Agent: Mutt/1.6.1 (2016-04-27) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Apr 21, 2016 at 07:38:14AM -0700, Guenter Roeck wrote: > lockdep reports the following circular locking dependency. > > ====================================================== > INFO: possible circular locking dependency detected ] > 4.6.0-rc3-00191-gfabf418 #162 Not tainted > ------------------------------------------------------- > systemd/1 is trying to acquire lock: > ((&(&wd_data->work)->work)){+.+...}, at: [<80141650>] flush_work+0x0/0x280 > > but task is already holding lock: > > (&wd_data->lock){+.+...}, at: [<804acfa8>] watchdog_release+0x18/0x190 > > which lock already depends on the new lock. > the existing dependency chain (in reverse order) is: > > -> #1 (&wd_data->lock){+.+...}: > [<80662310>] mutex_lock_nested+0x64/0x4a8 > [<804aca4c>] watchdog_ping_work+0x18/0x4c > [<80143128>] process_one_work+0x1ac/0x500 > [<801434b4>] worker_thread+0x38/0x554 > [<80149510>] kthread+0xf4/0x108 > [<80107c10>] ret_from_fork+0x14/0x24 > > -> #0 ((&(&wd_data->work)->work)){+.+...}: > [<8017c4e8>] lock_acquire+0x70/0x90 > [<8014169c>] flush_work+0x4c/0x280 > [<801440f8>] __cancel_work_timer+0x9c/0x1e0 > [<804acfcc>] watchdog_release+0x3c/0x190 > [<8022c5e8>] __fput+0x80/0x1c8 > [<80147b28>] task_work_run+0x94/0xc8 > [<8010b998>] do_work_pending+0x8c/0xb4 > [<80107ba8>] slow_work_pending+0xc/0x20 > > other info that might help us debug this: > Possible unsafe locking scenario: > > CPU0 CPU1 > ---- ---- > lock(&wd_data->lock); > lock((&(&wd_data->work)->work)); > lock(&wd_data->lock); > lock((&(&wd_data->work)->work)); > > *** DEADLOCK *** > > 1 lock held by systemd/1: > > stack backtrace: > CPU: 2 PID: 1 Comm: systemd Not tainted 4.6.0-rc3-00191-gfabf418 #162 > Hardware name: Freescale i.MX6 Quad/DualLite (Device Tree) > [<8010f5e4>] (unwind_backtrace) from [<8010c038>] (show_stack+0x10/0x14) > [<8010c038>] (show_stack) from [<8039d7fc>] (dump_stack+0xa8/0xd4) > [<8039d7fc>] (dump_stack) from [<80177ee0>] (print_circular_bug+0x214/0x334) > [<80177ee0>] (print_circular_bug) from [<80179230>] (check_prevs_add+0x4dc/0x8e8) > [<80179230>] (check_prevs_add) from [<8017b3d8>] (__lock_acquire+0xc6c/0x14ec) > [<8017b3d8>] (__lock_acquire) from [<8017c4e8>] (lock_acquire+0x70/0x90) > [<8017c4e8>] (lock_acquire) from [<8014169c>] (flush_work+0x4c/0x280) > [<8014169c>] (flush_work) from [<801440f8>] (__cancel_work_timer+0x9c/0x1e0) > [<801440f8>] (__cancel_work_timer) from [<804acfcc>] (watchdog_release+0x3c/0x190) > [<804acfcc>] (watchdog_release) from [<8022c5e8>] (__fput+0x80/0x1c8) > [<8022c5e8>] (__fput) from [<80147b28>] (task_work_run+0x94/0xc8) > [<80147b28>] (task_work_run) from [<8010b998>] (do_work_pending+0x8c/0xb4) > [<8010b998>] (do_work_pending) from [<80107ba8>] (slow_work_pending+0xc/0x20) > > Turns out the call to cancel_delayed_work_sync() in watchdog_release() > is not necessary and can be dropped. If the worker is no longer necessary, > the subsequent call to watchdog_update_worker() will cancel it. If it is > already running, it won't do anything, since the worker function checks > if it needs to ping the watchdog or not. > > Reported-by: Clemens Gruber > Tested-by: Clemens Gruber > Fixes: 11d7aba9ceb7 ("watchdog: imx2: Convert to use infrastructure triggered keepalives") > Signed-off-by: Guenter Roeck > --- > drivers/watchdog/watchdog_dev.c | 1 - > 1 file changed, 1 deletion(-) > > diff --git a/drivers/watchdog/watchdog_dev.c b/drivers/watchdog/watchdog_dev.c > index e2c5abbb45ff..3595cffa24ea 100644 > --- a/drivers/watchdog/watchdog_dev.c > +++ b/drivers/watchdog/watchdog_dev.c > @@ -736,7 +736,6 @@ static int watchdog_release(struct inode *inode, struct file *file) > watchdog_ping(wdd); > } > > - cancel_delayed_work_sync(&wd_data->work); > watchdog_update_worker(wdd); > > /* make sure that /dev/watchdog can be re-opened */ > -- > 2.5.0 > Hi, I don't see this patch in the torvalds/linux tree. Will this get in before the 4.6 release? Thanks, Clemens