答复: [PATCH][V5] tty: fix race between flush_to_ldisc and tty_open

[Li,Rongqing]

> -----邮件原件-----
> 发件人: linux-kernel-owner@xxxxxxxxxxxxxxx
> [mailto:linux-kernel-owner@xxxxxxxxxxxxxxx] 代表 Greg Kroah-Hartman
> 发送时间: 2019年1月31日 18:55
> 收件人: Li,Rongqing <lirongqing@xxxxxxxxx>
> 抄送: linux-serial@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
> jslaby@xxxxxxxx; gkohli@xxxxxxxxxxxxxx
> 主题: Re: [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@xxxxxxxxx>
> > Signed-off-by: Zhang Yu <zhangyu31@xxxxxxxxx>
> > Signed-off-by: Li RongQing <lirongqing@xxxxxxxxx>
> > ---
> > 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?  :)
> 

Welcome you to add your credit/blame/signature
and I am not clear the rule, and be afraid to become fake

> 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?

I add some delay in tty_init_dev to simulate this issue.  it can fix this my issue.

Thanks

-RongQing

> 
> thanks,
> 
> greg k-h