Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753444Ab3HAUGU (ORCPT ); Thu, 1 Aug 2013 16:06:20 -0400 Received: from mailout39.mail01.mtsvc.net ([216.70.64.83]:56886 "EHLO n12.mail01.mtsvc.net" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751723Ab3HAUGS (ORCPT ); Thu, 1 Aug 2013 16:06:18 -0400 Message-ID: <51FABFB7.9070409@hurleysoftware.com> Date: Thu, 01 Aug 2013 16:06:15 -0400 From: Peter Hurley User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130623 Thunderbird/17.0.7 MIME-Version: 1.0 To: Michel Lespinasse , Artem Savkov CC: gregkh@linuxfoundation.org, jslaby@suse.cz, linux-kernel@vger.kernel.org, Ingo Molnar Subject: Re: [PATCH] n_tty: release atomic_read_lock before calling schedule_timeout() References: <1375198500-17414-1-git-send-email-artem.savkov@gmail.com> <51F7EC5A.2030609@hurleysoftware.com> <20130731114726.GA11570@cpv436-motbuntu.spb.ea.mot-mobility.com> In-Reply-To: <20130731114726.GA11570@cpv436-motbuntu.spb.ea.mot-mobility.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Authenticated-User: 990527 peter@hurleysoftware.com X-MT-INTERNAL-ID: 8fa290c2a27252aacf65dbc4a42f3ce3735fb2a4 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4456 Lines: 91 On 07/31/2013 07:47 AM, Artem Savkov wrote: > On Tue, Jul 30, 2013 at 12:39:54PM -0400, Peter Hurley wrote: >> On 07/30/2013 11:35 AM, Artem Savkov wrote: >>> ldata->atomic_read_lock should be released before scheduling as well as >>> tty->termios_rwsem, otherwise there is a potential deadlock detected by lockdep >> >> False positive. >> >>> Introduced in "n_tty: Access termios values safely" >>> (9356b535fcb71db494fc434acceb79f56d15bda2 in linux-next.git) >>> >>> [ 16.822058] ====================================================== >>> [ 16.822058] [ INFO: possible circular locking dependency detected ] >>> [ 16.822058] 3.11.0-rc3-next-20130730+ #140 Tainted: G W >>> [ 16.822058] ------------------------------------------------------- >>> [ 16.822058] bash/1198 is trying to acquire lock: >>> [ 16.822058] (&tty->termios_rwsem){++++..}, at: [] n_tty_read+0x49b/0x660 >>> [ 16.822058] >>> [ 16.822058] but task is already holding lock: >>> [ 16.822058] (&ldata->atomic_read_lock){+.+...}, at: [] n_tty_read+0x1d0/0x660 >>> [ 16.822058] >>> [ 16.822058] which lock already depends on the new lock. >>> [ 16.822058] >>> [ 16.822058] >>> [ 16.822058] the existing dependency chain (in reverse order) is: >>> [ 16.822058] >>> -> #1 (&ldata->atomic_read_lock){+.+...}: >>> [ 16.822058] [] validate_chain+0x73c/0x850 >>> [ 16.822058] [] __lock_acquire+0x500/0x5d0 >>> [ 16.822058] [] lock_acquire+0x179/0x1d0 >>> [ 16.822058] [] mutex_lock_interruptible_nested+0x7c/0x540 >>> [ 16.822058] [] n_tty_read+0x1d0/0x660 >>> [ 16.822058] [] tty_read+0x86/0xf0 >>> [ 16.822058] [] vfs_read+0xc3/0x130 >>> [ 16.822058] [] SyS_read+0x62/0xa0 >>> [ 16.822058] [] system_call_fastpath+0x16/0x1b >>> [ 16.822058] >>> -> #0 (&tty->termios_rwsem){++++..}: >>> [ 16.822058] [] check_prev_add+0x14f/0x590 >>> [ 16.822058] [] validate_chain+0x73c/0x850 >>> [ 16.822058] [] __lock_acquire+0x500/0x5d0 >>> [ 16.822058] [] lock_acquire+0x179/0x1d0 >>> [ 16.822058] [] down_read+0x51/0xa0 >>> [ 16.822058] [] n_tty_read+0x49b/0x660 >>> [ 16.822058] [] tty_read+0x86/0xf0 >>> [ 16.822058] [] vfs_read+0xc3/0x130 >>> [ 16.822058] [] SyS_read+0x62/0xa0 >>> [ 16.822058] [] system_call_fastpath+0x16/0x1b >>> [ 16.822058] >>> [ 16.822058] other info that might help us debug this: >>> [ 16.822058] >>> [ 16.822058] Possible unsafe locking scenario: >>> [ 16.822058] >>> [ 16.822058] CPU0 CPU1 >>> [ 16.822058] ---- ---- >>> [ 16.822058] lock(&ldata->atomic_read_lock); >>> [ 16.822058] lock(&tty->termios_rwsem); >>> [ 16.822058] lock(&ldata->atomic_read_lock); >>> [ 16.822058] lock(&tty->termios_rwsem); >>> [ 16.822058] >>> [ 16.822058] *** DEADLOCK *** >> >> This situation is not possible since termios_rwsem is a read/write semaphore; >> CPU1 cannot prevent CPU0 from obtaining a read lock on termios_rwsem. > Oops, yes, sorry. > >> This looks like a regression caused by: >> >> commit a51805efae5dda0da66f79268ffcf0715f9dbea4 >> Author: Michel Lespinasse >> Date: Mon Jul 8 14:23:49 2013 -0700 >> >> lockdep: Introduce lock_acquire_exclusive()/shared() helper macros > Doesn't seem to be this commit. I see nothing wrong here and just to be > sure I've checked the kernel with this commit reverted. The issue is > still there. Yes, you're right. Apologies to Michel for the too-hasty blame. Thanks for the report anyway. I'll track down the lockdep regression as soon as I fix a real deadlock in the nouveau driver that disables lockdep. Regards, Peter Hurley -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/