On Fri, Jan 18, 2019 at 05:27:17PM +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 n_tty_open and > tty->driver_data may be NULL. > > CPU0 CPU1 > ---- ---- > n_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 put 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 > > Signed-off-by: Wang Li <wangl...@baidu.com> > Signed-off-by: Zhang Yu <zhangy...@baidu.com> > Signed-off-by: Li RongQing <lirongq...@baidu.com> > --- > 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/tty_port.c | 3 +++ > 1 file changed, 3 insertions(+) > > diff --git a/drivers/tty/tty_port.c b/drivers/tty/tty_port.c > index 044c3cbdcfa4..86d0bec38322 100644 > --- a/drivers/tty/tty_port.c > +++ b/drivers/tty/tty_port.c > @@ -31,6 +31,9 @@ static int tty_port_default_receive_buf(struct tty_port > *port, > if (!tty) > return 0; > > + if (!tty->driver_data) > + return 0; > +
How is this working? What is setting driver_data to NULL to "stop" this race? There's no requirement that a tty driver set this field to NULL when it is "done" with the tty device, so I think you are just getting lucky in that your specific driver happens to be doing this. What driver are you testing this against? thanks, greg k-h