From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1753017AbbKZSRw (ORCPT <rfc822;w@1wt.eu>);
	Thu, 26 Nov 2015 13:17:52 -0500
Received: from mail-wm0-f51.google.com ([74.125.82.51]:33679 "EHLO
	mail-wm0-f51.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1752442AbbKZSRt (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Thu, 26 Nov 2015 13:17:49 -0500
Subject: Re: Deadlock in n_hdlc_buf_put
To: Dmitry Vyukov <dvyukov@google.com>, paulkf@microgate.com,
        LKML <linux-kernel@vger.kernel.org>
References: <CACT4Y+bRrmT8PepDU0WZsYwksULJrdJKSTsMWRDXojFsf=GwBg@mail.gmail.com>
Cc: Eric Dumazet <edumazet@google.com>,
        Alexander Potapenko <glider@google.com>,
        Kostya Serebryany <kcc@google.com>,
        syzkaller <syzkaller@googlegroups.com>,
        Sasha Levin <sasha.levin@oracle.com>
From: Jiri Slaby <jslaby@suse.cz>
Message-ID: <56574CCA.8010807@suse.cz>
Date: Thu, 26 Nov 2015 19:17:46 +0100
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101
 Thunderbird/38.4.0
MIME-Version: 1.0
In-Reply-To: <CACT4Y+bRrmT8PepDU0WZsYwksULJrdJKSTsMWRDXojFsf=GwBg@mail.gmail.com>
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On 11/26/2015, 01:37 PM, Dmitry Vyukov wrote:
> [  341.376188] =============================================
> [  341.376607] [ INFO: possible recursive locking detected ]
> [  341.376607] 4.4.0-rc1+ #117 Not tainted
> [  341.376607] ---------------------------------------------
> [  341.376607] syzkaller_execu/14066 is trying to acquire lock:
> [  341.376607]  (&(&list->spinlock)->rlock){......}, at:
> [<ffffffff82a9f548>] n_hdlc_buf_put+0x28/0x170
> [  341.376607]
> [  341.376607] but task is already holding lock:
> [  341.376607]  (&(&list->spinlock)->rlock){......}, at:
> [<ffffffff82aa1368>] n_hdlc_tty_ioctl+0x2b8/0x3f0
> [  341.376607]
> [  341.376607] other info that might help us debug this:
> [  341.376607]  Possible unsafe locking scenario:
> [  341.376607]
> [  341.376607]        CPU0
> [  341.376607]        ----
> [  341.376607]   lock(&(&list->spinlock)->rlock);
> [  341.376607]   lock(&(&list->spinlock)->rlock);

Hi,

this is a lockdep false positive. The first one is tx_buf_list.spinlock,
the latter tx_free_buf_list.spinlock, both in flush_tx_queue. So we need
a lockdep annotation here.

thanks,
-- 
js
suse labs