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SubjectRe: [PATCH][V5] tty: fix race between flush_to_ldisc and tty_open
On Thu, Jan 31, 2019 at 05:43:16PM +0800, Li RongQing wrote:
> There still is a race window after the commit b027e2298bd588
> ("tty: fix data race between tty_init_dev and flush of buf"),
> and we encountered this crash issue if receive_buf call comes
> before tty initialization completes in tty_open and
> tty->driver_data may be NULL.
>
> CPU0 CPU1
> ---- ----
> tty_open
> tty_init_dev
> tty_ldisc_unlock
> schedule
> flush_to_ldisc
> receive_buf
> tty_port_default_receive_buf
> tty_ldisc_receive_buf
> n_tty_receive_buf_common
> __receive_buf
> uart_flush_chars
> uart_start
> /*tty->driver_data is NULL*/
> tty->ops->open
> /*init tty->driver_data*/
>
> it can be fixed by extending ldisc semaphore lock in tty_init_dev
> to driver_data initialized completely after tty->ops->open(), but
> this will lead to get lock on one function and unlock in some other
> function, and hard to maintain, so fix this race only by checking
> tty->driver_data when receiving, and return if tty->driver_data
> is NULL, and n_tty_receive_buf_common maybe calls uart_unthrottle,
> so add the same check
>
> Signed-off-by: Wang Li <wangli39@baidu.com>
> Signed-off-by: Zhang Yu <zhangyu31@baidu.com>
> Signed-off-by: Li RongQing <lirongqing@baidu.com>
> ---
> V5: move check into uart_start from n_tty_receive_buf_common
> V4: add version information
> V3: not used ldisc semaphore lock, only checking tty->driver_data with NULL
> V2: fix building error by EXPORT_SYMBOL tty_ldisc_unlock
> V1: extend ldisc lock to protect that tty->driver_data is inited
>
> drivers/tty/serial/serial_core.c | 6 ++++++
> 1 file changed, 6 insertions(+)
>
> diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
> index 5c01bb6d1c24..556f50aa1b58 100644
> --- a/drivers/tty/serial/serial_core.c
> +++ b/drivers/tty/serial/serial_core.c
> @@ -130,6 +130,9 @@ static void uart_start(struct tty_struct *tty)
> struct uart_port *port;
> unsigned long flags;
>
> + if (!state)
> + return;
> +
> port = uart_port_lock(state, flags);
> __uart_start(tty);
> uart_port_unlock(port, flags);
> @@ -727,6 +730,9 @@ static void uart_unthrottle(struct tty_struct *tty)
> upstat_t mask = UPSTAT_SYNC_FIFO;
> struct uart_port *port;
>
> + if (!state)
> + return;
> +
> port = uart_port_ref(state);
> if (!port)
> return;
> --
> 2.16.2


Hm, I wrote this patch, not you, right? So shouldn't I get the
credit/blame for it? :)

Also, this is a bug in the serial code, not necessarily the tty layer,
so the subject should change...

And you did test this, right?

thanks,

greg k-h

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