> -----邮件原件----- > 发件人: linux-kernel-ow...@vger.kernel.org > [mailto:linux-kernel-ow...@vger.kernel.org] 代表 Greg KH > 发送时间: 2019年1月30日 18:19 > 收件人: Li,Rongqing <lirongq...@baidu.com> > 抄送: jsl...@suse.com; linux-kernel@vger.kernel.org; gko...@codeaurora.org > 主题: Re: [PATCH][v4] tty: fix race between flush_to_ldisc and tty_open > > 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? >
if tty->driver_data is NULL and return, tty_port_default_receive_buf will not step to uart_start which access tty->driver_data and trigger panic before tty_open, so it can fix the system panic > 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. > when tty_open is running, tty is allocated by kzalloc in tty_init_dev which called by tty_open_by_driver, tty is inited to 0 > What driver are you testing this against? > 8250 Thanks -RongQing > thanks, > > greg k-h
