Re: tty: fix a possible hang on tty device

[Greg KH <gregkh@linuxfoundation.org>]

On Sat, May 07, 2022 at 05:11:35PM +0800, cael wrote:
> We have met a hang on pty device, the reader was blocking at epoll on
> master side, the writer was sleeping at wait_woken inside n_tty_write
> on slave side , and the write buffer on tty_port was full, we found
> that the reader and writer would never be woken again and block
> forever.
> 
> We thought the problem was caused as a race between reader and kworker
> as follows:
> n_tty_read(reader):                            |
>   n_tty_receive_buf_common(kworker):
>                                                            |
>               room = N_TTY_BUF_SIZE - (ldata->read_head - tail);
>                                                            |
>               room <= 0
> copy_from_read_buf(tty, &b, &nr);     |
> n_tty_kick_worker(tty);                       |
>                                                            |
>               ldata->no_room = true
> 
> After writing to slave device, writer wakes up kworker to flush data
> on tty_port to reader, and the kworker finds that reader has no room
> to store data so room <= 0 is met. At this moment, reader consumes all
> the data on reader buffer and call n_tty_kick_worker to check
> ldata->no_room and finds that there is no need to call
> tty_buffer_restart_work to flush data to reader and reader quits
> reading. Then kworker sets  ldata->no_room = true and quits too.
> 
> If write buffer is not full, writer will wake kworker to flush data
> again after following writes, but if writer buffer is full and writer
> goes to sleep, kworker will never be woken again and tty device is
> blocked.
> 
> We think this problem can be solved with a check for read buffer
> inside function n_tty_receive_buf_common, if read buffer is empty and
> ldata->no_room is true, this means that kworker has more data to flush
> to read buffer, so a call to n_tty_kick_worker is necessary.
> 
> diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
> index f9c584244..4e65e2422 100644
> --- a/drivers/tty/n_tty.c
> +++ b/drivers/tty/n_tty.c
> @@ -1760,6 +1760,8 @@ n_tty_receive_buf_common(struct tty_struct *tty,
> const unsigned char *cp,
>         } else
>                 n_tty_check_throttle(tty);
> 
> +       if (!chars_in_buffer(tty))
> +               n_tty_kick_worker(tty);
>         up_read(&tty->termios_rwsem);
> 
>         return rcvd;


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