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Date: Wed, 20 Jan 2016 14:02:01 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Dmitry Vyukov <dvyukov@google.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Jiri Slaby <jslaby@suse.com>, LKML <linux-kernel@vger.kernel.org>,
        J Freyensee <james_p_freyensee@linux.intel.com>,
        syzkaller <syzkaller@googlegroups.com>,
        Kostya Serebryany <kcc@google.com>,
        Alexander Potapenko <glider@google.com>,
        Sasha Levin <sasha.levin@oracle.com>,
        Eric Dumazet <edumazet@google.com>, gnomes@lxorguk.ukuu.org.uk
Subject: Re: tty: deadlock between n_tracerouter_receivebuf and flush_to_ldisc
Message-ID: <20160120130201.GA1027@worktop>
References: <CACT4Y+Z3or-Tb=COdS0iMhKSy_+ECs7QGnreLCPdDttUmnc-Cw@mail.gmail.com>
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On Wed, Dec 30, 2015 at 11:44:01AM +0100, Dmitry Vyukov wrote:
> -> #3 (&buf->lock){+.+...}:
>        [<ffffffff813f0acf>] lock_acquire+0x19f/0x3c0 kernel/locking/lockdep.c:3585
>        [<     inline     >] __raw_spin_lock_irqsave include/linux/spinlock_api_smp.h:112
>        [<ffffffff85c8e790>] _raw_spin_lock_irqsave+0x50/0x70 kernel/locking/spinlock.c:159
>        [<ffffffff82b8c050>] tty_get_pgrp+0x20/0x80 drivers/tty/tty_io.c:2502

So in any recent code that I look at this function tries to acquire
tty->ctrl_lock, not buf->lock. Am I missing something ?!

>        [<ffffffff82b9a09a>] __isig+0x1a/0x50 drivers/tty/n_tty.c:1112
>        [<ffffffff82b9c16e>] isig+0xae/0x2c0 drivers/tty/n_tty.c:1131
>        [<ffffffff82b9ef02>] n_tty_receive_signal_char+0x22/0xf0 drivers/tty/n_tty.c:1243
>        [<ffffffff82ba4958>] n_tty_receive_char_special+0x1278/0x2bf0 drivers/tty/n_tty.c:1289
>        [<     inline     >] n_tty_receive_buf_fast drivers/tty/n_tty.c:1613
>        [<     inline     >] __receive_buf drivers/tty/n_tty.c:1647
>        [<ffffffff82ba7ca6>] n_tty_receive_buf_common+0x19d6/0x2450 drivers/tty/n_tty.c:1745
>        [<ffffffff82ba8753>] n_tty_receive_buf2+0x33/0x40 drivers/tty/n_tty.c:1780
>        [<     inline     >] receive_buf drivers/tty/tty_buffer.c:450
>        [<ffffffff82bafa6f>] flush_to_ldisc+0x3bf/0x7f0 drivers/tty/tty_buffer.c:517
>        [<ffffffff8133833c>] process_one_work+0x76c/0x13e0 kernel/workqueue.c:2030
>        [<ffffffff81339093>] worker_thread+0xe3/0xe90 kernel/workqueue.c:2162
>        [<ffffffff8134b63f>] kthread+0x23f/0x2d0 drivers/block/aoe/aoecmd.c:1303
>        [<ffffffff85c8eeef>] ret_from_fork+0x3f/0x70 arch/x86/entry/entry_64.S:468