Re: [PATCH v4] tty: fix hang on tty device with no_room set

[Greg KH <gregkh@linuxfoundation.org>]

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@gmail.com>
> ---
> 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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