On Wed, Jun 15, 2022 at 11:45:10AM +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 blocked forever.
>
> The problem was caused by a race between reader and kworker:
> 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()|
> n_tty_kick_worker() |
> |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 calls
> n_tty_kick_worker to check ldata->no_room which is false 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 write buffer is full and writer
> goes to sleep, kworker will never be woken again and tty device is
> blocked.
>
> This problem can be solved with a check for read buffer size inside
> n_tty_receive_buf_common, if read buffer is empty and ldata->no_room
> is true, a call to n_tty_kick_worker is necessary to keep flushing
> data to reader.
>
> Signed-off-by: cael <juanfengpy@xxxxxxxxx>
> ---
> Patch changelogs between v1 and v2:
> -add barrier inside n_tty_read and n_tty_receive_buf_common;
> -comment why barrier is needed;
> -access to ldata->no_room is changed with READ_ONCE and WRITE_ONCE;
> Patch changelogs between v2 and v3:
> -in function n_tty_receive_buf_common, add unlikely to check
> ldata->no_room, eg: if (unlikely(ldata->no_room)), and READ_ONCE
> is removed here to get locality;
> -change comment for barrier to show the race condition to make
> comment easier to understand;
> Patch changelogs between v3 and v4:
> -change subject from 'tty: fix a possible hang on tty device' to
> 'tty: fix hang on tty device with no_room set' to make subject
> more obvious.
>
> drivers/tty/n_tty.c | 41 +++++++++++++++++++++++++++++++++++++----
> 1 file changed, 37 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
> index efc72104c840..544f782b9a11 100644
> --- a/drivers/tty/n_tty.c
> +++ b/drivers/tty/n_tty.c
> @@ -201,8 +201,8 @@ static void n_tty_kick_worker(struct tty_struct *tty)
> struct n_tty_data *ldata = tty->disc_data;
>
> /* Did the input worker stop? Restart it */
> - if (unlikely(ldata->no_room)) {
> - ldata->no_room = 0;
> + if (unlikely(READ_ONCE(ldata->no_room))) {
> + WRITE_ONCE(ldata->no_room, 0);
>
> WARN_RATELIMIT(tty->port->itty == NULL,
> "scheduling with invalid itty\n");
> @@ -1632,7 +1632,7 @@ n_tty_receive_buf_common(struct tty_struct *tty, const unsigned char *cp,
> if (overflow && room < 0)
> ldata->read_head--;
> room = overflow;
> - ldata->no_room = flow && !room;
> + WRITE_ONCE(ldata->no_room, flow && !room);
> } else
> overflow = 0;
>
> @@ -1663,6 +1663,24 @@ n_tty_receive_buf_common(struct tty_struct *tty, const unsigned char *cp,
> } else
> n_tty_check_throttle(tty);
>
> + if (unlikely(ldata->no_room)) {
> + /*
> + * Barrier here is to ensure to read the latest read_tail in
> + * chars_in_buffer() and to make sure that read_tail is not loaded
> + * before ldata->no_room is set, otherwise, following race may occur:
> + * n_tty_receive_buf_common() |n_tty_read()
> + * chars_in_buffer() > 0 |
> + * |copy_from_read_buf()->chars_in_buffer()==0
> + * |if (ldata->no_room)
> + * ldata->no_room = 1 |
> + * Then both kworker and reader will fail to kick n_tty_kick_worker(),
> + * smp_mb is paired with smp_mb() in n_tty_read().
> + */
> + smp_mb();
> + if (!chars_in_buffer(tty))
> + n_tty_kick_worker(tty);
> + }
> +
> up_read(&tty->termios_rwsem);
>
> return rcvd;
> @@ -2180,8 +2198,23 @@ static ssize_t n_tty_read(struct tty_struct *tty, struct file *file,
> if (time)
> timeout = time;
> }
> - if (tail != ldata->read_tail)
> + if (tail != ldata->read_tail) {
> + /*
> + * Make sure no_room is not read before setting read_tail,
> + * otherwise, following race may occur:
> + * n_tty_read() |n_tty_receive_buf_common()
> + * if(ldata->no_room)->false |
> + * |ldata->no_room = 1
> + * |char_in_buffer() > 0
> + * ldata->read_tail = ldata->commit_head|
> + * Then copy_from_read_buf() in reader consumes all the data
> + * in read buffer, both reader and kworker will fail to kick
> + * tty_buffer_restart_work().
> + * smp_mb is paired with smp_mb() in n_tty_receive_buf_common().
> + */
> + smp_mb();
> n_tty_kick_worker(tty);
> + }
> up_read(&tty->termios_rwsem);
>
> remove_wait_queue(&tty->read_wait, &wait);
> --
> 2.27.0
>
Hi,
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