Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755737Ab3HLKvs (ORCPT ); Mon, 12 Aug 2013 06:51:48 -0400 Received: from mail-ee0-f45.google.com ([74.125.83.45]:63753 "EHLO mail-ee0-f45.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753030Ab3HLKvn (ORCPT ); Mon, 12 Aug 2013 06:51:43 -0400 Date: Mon, 12 Aug 2013 13:50:41 +0300 From: Sergey Senozhatsky To: Peter Hurley , Greg Kroah-Hartman , linux-next@vger.kernel.org, linux-kernel@vger.kernel.org, Jiri Slaby , Sergey Senozhatsky , Belisko Marek Subject: Re: [PATCH tty-next] n_tty: Fix termios_rwsem lockdep false positive Message-ID: <20130812105041.GA2268@swordfish> References: <20130731114726.GA11570@cpv436-motbuntu.spb.ea.mot-mobility.com> <1376222663-5666-1-git-send-email-peter@hurleysoftware.com> <20130812092810.GA26400@cpv436-motbuntu.spb.ea.mot-mobility.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20130812092810.GA26400@cpv436-motbuntu.spb.ea.mot-mobility.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 6730 Lines: 175 On (08/12/13 13:28), Artem Savkov wrote: > Hi Peter, > > On Sun, Aug 11, 2013 at 08:04:23AM -0400, Peter Hurley wrote: > > Lockdep reports a circular lock dependency between > > atomic_read_lock and termios_rwsem [1]. However, a lock > > order deadlock is not possible since CPU1 only holds a > > read lock which cannot prevent CPU0 from also acquiring > > a read lock on the same r/w semaphore. > > > > Unfortunately, lockdep cannot currently distinguish whether > > the locks are read or write for any particular lock graph, > > merely that the locks _were_ previously read and/or write. > > > > Until lockdep is fixed, re-order atomic_read_lock so > > termios_rwsem can be dropped and reacquired without > > triggering lockdep. > > Works fine, thanks. > > Reported-and-tested-by: Artem Savkov > > > Reported-by: Artem Savkov > > Reported-by: Sergey Senozhatsky > > Signed-off-by: Peter Hurley > > > > [1] Initial lockdep report from Artem Savkov > > > > ====================================================== > > [ INFO: possible circular locking dependency detected ] > > 3.11.0-rc3-next-20130730+ #140 Tainted: G W > > ------------------------------------------------------- > > bash/1198 is trying to acquire lock: > > (&tty->termios_rwsem){++++..}, at: [] n_tty_read+0x49b/0x660 > > > > but task is already holding lock: > > (&ldata->atomic_read_lock){+.+...}, at: [] n_tty_read+0x1d0/0x660 > > > > which lock already depends on the new lock. > > > > the existing dependency chain (in reverse order) is: > > > > -> #1 (&ldata->atomic_read_lock){+.+...}: > > [] validate_chain+0x73c/0x850 > > [] __lock_acquire+0x500/0x5d0 > > [] lock_acquire+0x179/0x1d0 > > [] mutex_lock_interruptible_nested+0x7c/0x540 > > [] n_tty_read+0x1d0/0x660 > > [] tty_read+0x86/0xf0 > > [] vfs_read+0xc3/0x130 > > [] SyS_read+0x62/0xa0 > > [] system_call_fastpath+0x16/0x1b > > > > -> #0 (&tty->termios_rwsem){++++..}: > > [] check_prev_add+0x14f/0x590 > > [] validate_chain+0x73c/0x850 > > [] __lock_acquire+0x500/0x5d0 > > [] lock_acquire+0x179/0x1d0 > > [] down_read+0x51/0xa0 > > [] n_tty_read+0x49b/0x660 > > [] tty_read+0x86/0xf0 > > [] vfs_read+0xc3/0x130 > > [] SyS_read+0x62/0xa0 > > [] system_call_fastpath+0x16/0x1b > > > > other info that might help us debug this: > > > > Possible unsafe locking scenario: > > > > CPU0 CPU1 > > ---- ---- > > lock(&ldata->atomic_read_lock); > > lock(&tty->termios_rwsem); > > lock(&ldata->atomic_read_lock); > > lock(&tty->termios_rwsem); > > > > *** DEADLOCK *** > > > > 2 locks held by bash/1198: > > #0: (&tty->ldisc_sem){.+.+.+}, at: [] tty_ldisc_ref_wait+0x24/0x60 > > #1: (&ldata->atomic_read_lock){+.+...}, at: [] n_tty_read+0x1d0/0x660 > > > > stack backtrace: > > CPU: 1 PID: 1198 Comm: bash Tainted: G W 3.11.0-rc3-next-20130730+ #140 > > Hardware name: Bochs Bochs, BIOS Bochs 01/01/2007 > > 0000000000000000 ffff880019acdb28 ffffffff81d34074 0000000000000002 > > 0000000000000000 ffff880019acdb78 ffffffff8110ed75 ffff880019acdb98 > > ffff880019fd0000 ffff880019acdb78 ffff880019fd0638 ffff880019fd0670 > > Call Trace: > > [] dump_stack+0x59/0x7d > > [] print_circular_bug+0x105/0x120 > > [] check_prev_add+0x14f/0x590 > > [] ? _raw_spin_unlock_irq+0x4f/0x70 > > [] validate_chain+0x73c/0x850 > > [] ? trace_hardirqs_off_caller+0x1f/0x190 > > [] __lock_acquire+0x500/0x5d0 > > [] lock_acquire+0x179/0x1d0 > > [] ? n_tty_read+0x49b/0x660 > > [] down_read+0x51/0xa0 > > [] ? n_tty_read+0x49b/0x660 > > [] n_tty_read+0x49b/0x660 > > [] ? try_to_wake_up+0x210/0x210 > > [] tty_read+0x86/0xf0 > > [] vfs_read+0xc3/0x130 > > [] SyS_read+0x62/0xa0 > > [] ? trace_hardirqs_on_thunk+0x3a/0x3f > > [] system_call_fastpath+0x16/0x1b > > --- > > drivers/tty/n_tty.c | 25 +++++++++++-------------- > > 1 file changed, 11 insertions(+), 14 deletions(-) > > I hate to do this, but isn't it actually my patch posted here https://lkml.org/lkml/2013/8/1/510 which was tagged as `wrong'? -ss > > diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c > > index dd8ae0c..c9a9ddd 100644 > > --- a/drivers/tty/n_tty.c > > +++ b/drivers/tty/n_tty.c > > @@ -2122,6 +2122,17 @@ static ssize_t n_tty_read(struct tty_struct *tty, struct file *file, > > if (c < 0) > > return c; > > > > + /* > > + * Internal serialization of reads. > > + */ > > + if (file->f_flags & O_NONBLOCK) { > > + if (!mutex_trylock(&ldata->atomic_read_lock)) > > + return -EAGAIN; > > + } else { > > + if (mutex_lock_interruptible(&ldata->atomic_read_lock)) > > + return -ERESTARTSYS; > > + } > > + > > down_read(&tty->termios_rwsem); > > > > minimum = time = 0; > > @@ -2141,20 +2152,6 @@ static ssize_t n_tty_read(struct tty_struct *tty, struct file *file, > > } > > } > > > > - /* > > - * Internal serialization of reads. > > - */ > > - if (file->f_flags & O_NONBLOCK) { > > - if (!mutex_trylock(&ldata->atomic_read_lock)) { > > - up_read(&tty->termios_rwsem); > > - return -EAGAIN; > > - } > > - } else { > > - if (mutex_lock_interruptible(&ldata->atomic_read_lock)) { > > - up_read(&tty->termios_rwsem); > > - return -ERESTARTSYS; > > - } > > - } > > packet = tty->packet; > > > > add_wait_queue(&tty->read_wait, &wait); > > -- > > 1.8.1.2 > > > > -- > Regards, > Artem > -- 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/