[linux-next / tty] possible circular locking dependency detected

[Sergey Senozhatsky]

Hello,

[ 1274.378287] ======================================================
[ 1274.378289] WARNING: possible circular locking dependency detected
[ 1274.378290] 4.12.0-rc1-next-20170522-dbg-00007-gc09b2ab28b74-dirty #1317 Not tainted
[ 1274.378291] ------------------------------------------------------
[ 1274.378293] kworker/u8:5/111 is trying to acquire lock:
[ 1274.378294]  (&buf->lock){+.+...}, at: [<ffffffff812f2831>] tty_buffer_flush+0x34/0x88
[ 1274.378300] 
               but task is already holding lock:
[ 1274.378301]  (&o_tty->termios_rwsem/1){++++..}, at: [<ffffffff812ee5c7>] isig+0x47/0xd2
[ 1274.378307] 
               which lock already depends on the new lock.

[ 1274.378309] 
               the existing dependency chain (in reverse order) is:
[ 1274.378310] 
               -> #2 (&o_tty->termios_rwsem/1){++++..}:
[ 1274.378316]        lock_acquire+0x183/0x1ae
[ 1274.378319]        down_read+0x3e/0x62
[ 1274.378321]        n_tty_write+0x6c/0x3d6
[ 1274.378322]        tty_write+0x1cc/0x25f
[ 1274.378325]        __vfs_write+0x26/0xec
[ 1274.378327]        vfs_write+0xe1/0x16a
[ 1274.378329]        SyS_write+0x51/0x8e
[ 1274.378330]        entry_SYSCALL_64_fastpath+0x18/0xad
[ 1274.378331] 
               -> #1 (&tty->atomic_write_lock){+.+.+.}:
[ 1274.378335]        lock_acquire+0x183/0x1ae
[ 1274.378337]        __mutex_lock+0x95/0x7ba
[ 1274.378339]        mutex_lock_nested+0x1b/0x1d
[ 1274.378340]        tty_port_default_receive_buf+0x4e/0x81
[ 1274.378342]        flush_to_ldisc+0x87/0xa1
[ 1274.378345]        process_one_work+0x2be/0x52b
[ 1274.378346]        worker_thread+0x1f3/0x2c5
[ 1274.378349]        kthread+0x131/0x139
[ 1274.378350]        ret_from_fork+0x2e/0x40
[ 1274.378351] 
               -> #0 (&buf->lock){+.+...}:
[ 1274.378355]        __lock_acquire+0xec4/0x1444
[ 1274.378357]        lock_acquire+0x183/0x1ae
[ 1274.378358]        __mutex_lock+0x95/0x7ba
[ 1274.378360]        mutex_lock_nested+0x1b/0x1d
[ 1274.378362]        tty_buffer_flush+0x34/0x88
[ 1274.378364]        pty_flush_buffer+0x27/0x70
[ 1274.378366]        tty_driver_flush_buffer+0x1b/0x1e
[ 1274.378367]        isig+0x9b/0xd2
[ 1274.378369]        n_tty_receive_signal_char+0x1c/0x59
[ 1274.378371]        n_tty_receive_char_special+0xa4/0x740
[ 1274.378373]        n_tty_receive_buf_common+0x452/0x810
[ 1274.378374]        n_tty_receive_buf2+0x14/0x16
[ 1274.378376]        tty_ldisc_receive_buf+0x1f/0x4a
[ 1274.378377]        tty_port_default_receive_buf+0x5f/0x81
[ 1274.378379]        flush_to_ldisc+0x87/0xa1
[ 1274.378380]        process_one_work+0x2be/0x52b
[ 1274.378382]        worker_thread+0x1f3/0x2c5
[ 1274.378383]        kthread+0x131/0x139
[ 1274.378385]        ret_from_fork+0x2e/0x40
[ 1274.378386] 
               other info that might help us debug this:

[ 1274.378387] Chain exists of:
                 &buf->lock --> &tty->atomic_write_lock --> &o_tty->termios_rwsem/1

[ 1274.378392]  Possible unsafe locking scenario:

[ 1274.378393]        CPU0                    CPU1
[ 1274.378394]        ----                    ----
[ 1274.378394]   lock(&o_tty->termios_rwsem/1);
[ 1274.378397]                                lock(&tty->atomic_write_lock);
[ 1274.378399]                                lock(&o_tty->termios_rwsem/1);
[ 1274.378402]   lock(&buf->lock);
[ 1274.378403] 
                *** DEADLOCK ***

[ 1274.378405] 6 locks held by kworker/u8:5/111:
[ 1274.378406]  #0:  ("events_unbound"){.+.+.+}, at: [<ffffffff81058c02>] process_one_work+0x163/0x52b
[ 1274.378410]  #1:  ((&buf->work)){+.+...}, at: [<ffffffff81058c02>] process_one_work+0x163/0x52b
[ 1274.378414]  #2:  (&port->buf.lock/1){+.+...}, at: [<ffffffff812f2550>] flush_to_ldisc+0x25/0xa1
[ 1274.378419]  #3:  (&tty->ldisc_sem){++++.+}, at: [<ffffffff812f1e98>] tty_ldisc_ref+0x1f/0x41
[ 1274.378423]  #4:  (&tty->atomic_write_lock){+.+.+.}, at: [<ffffffff812f2c7e>] tty_port_default_receive_buf+0x4e/0x81
[ 1274.378427]  #5:  (&o_tty->termios_rwsem/1){++++..}, at: [<ffffffff812ee5c7>] isig+0x47/0xd2
[ 1274.378431] 
               stack backtrace:
[ 1274.378434] CPU: 1 PID: 111 Comm: kworker/u8:5 Not tainted 4.12.0-rc1-next-20170522-dbg-00007-gc09b2ab28b74-dirty #1317
[ 1274.378437] Workqueue: events_unbound flush_to_ldisc
[ 1274.378439] Call Trace:
[ 1274.378443]  dump_stack+0x70/0x9a
[ 1274.378445]  print_circular_bug+0x272/0x280
[ 1274.378447]  __lock_acquire+0xec4/0x1444
[ 1274.378450]  lock_acquire+0x183/0x1ae
[ 1274.378452]  ? lock_acquire+0x183/0x1ae
[ 1274.378453]  ? tty_buffer_flush+0x34/0x88
[ 1274.378455]  __mutex_lock+0x95/0x7ba
[ 1274.378457]  ? tty_buffer_flush+0x34/0x88
[ 1274.378459]  ? isig+0x64/0xd2
[ 1274.378460]  ? tty_buffer_flush+0x34/0x88
[ 1274.378462]  ? find_held_lock+0x31/0x77
[ 1274.378464]  mutex_lock_nested+0x1b/0x1d
[ 1274.378466]  ? mutex_lock_nested+0x1b/0x1d
[ 1274.378468]  tty_buffer_flush+0x34/0x88
[ 1274.378470]  pty_flush_buffer+0x27/0x70
[ 1274.378472]  tty_driver_flush_buffer+0x1b/0x1e
[ 1274.378473]  isig+0x9b/0xd2
[ 1274.378475]  n_tty_receive_signal_char+0x1c/0x59
[ 1274.378477]  n_tty_receive_char_special+0xa4/0x740
[ 1274.378479]  n_tty_receive_buf_common+0x452/0x810
[ 1274.378481]  ? lock_acquire+0x183/0x1ae
[ 1274.378484]  n_tty_receive_buf2+0x14/0x16
[ 1274.378485]  tty_ldisc_receive_buf+0x1f/0x4a
[ 1274.378487]  tty_port_default_receive_buf+0x5f/0x81
[ 1274.378489]  flush_to_ldisc+0x87/0xa1
[ 1274.378491]  process_one_work+0x2be/0x52b
[ 1274.378493]  worker_thread+0x1f3/0x2c5
[ 1274.378495]  ? rescuer_thread+0x2ca/0x2ca
[ 1274.378497]  kthread+0x131/0x139
[ 1274.378498]  ? kthread_create_on_node+0x3f/0x3f
[ 1274.378500]  ret_from_fork+0x2e/0x40

	-ss