[34-longterm 132/209] TTY: don't allow reopen when ldisc is changing

From: Paul Gortmaker
Date: Thu Apr 14 2011 - 14:16:11 EST

From: Jiri Slaby <jslaby@xxxxxxx>

| This is a commit scheduled for the next v2.6.34 longterm release. |
| If you see a problem with using this for longterm, please comment.|

commit e2efafbf139d2bfdfe96f2901f03189fecd172e4 upstream

There are many WARNINGs like the following reported nowadays:
WARNING: at drivers/tty/tty_io.c:1331 tty_open+0x2a2/0x49a()
Hardware name: Latitude E6500
Modules linked in:
Pid: 1207, comm: plymouthd Not tainted 2.6.37-rc3-mmotm1123 #3
Call Trace:
[<ffffffff8103b189>] warn_slowpath_common+0x80/0x98
[<ffffffff8103b1b6>] warn_slowpath_null+0x15/0x17
[<ffffffff8128a3ab>] tty_open+0x2a2/0x49a
[<ffffffff810fd53f>] chrdev_open+0x11d/0x146

This means tty_reopen is called without TTY_LDISC set. For further
considerations, note tty_lock is held in tty_open. TTY_LDISC is cleared in:
1) __tty_hangup from tty_ldisc_hangup to tty_ldisc_enable. During this
section tty_lock is held. However tty_lock is temporarily dropped in
the middle of the function by tty_ldisc_hangup.

2) tty_release via tty_ldisc_release till the end of tty existence. If
tty->count <= 1, tty_lock is taken, TTY_CLOSING bit set and then
tty_ldisc_release called. tty_reopen checks TTY_CLOSING before checking

3) tty_set_ldisc from tty_ldisc_halt to tty_ldisc_enable. We:
* take tty_lock, set TTY_LDISC_CHANGING, put tty_lock
* call tty_ldisc_halt (clear TTY_LDISC), tty_lock is _not_ held
* do some other work
* take tty_lock, call tty_ldisc_enable (set TTY_LDISC), put

I cannot see how 2) can be a problem, as there I see no race. OTOH, 1)
and 3) can happen without problems. This patch the case 3) by checking
TTY_LDISC_CHANGING along with TTY_CLOSING in tty_reopen. 1) will be
fixed in the following patch.

Nicely reproducible with two processes:
while (1) {
fd = open("/dev/ttyS1", O_RDWR);
if (fd < 0) {
while (1) {
fd = open("/dev/ttyS1", O_RDWR);
ld1 = 0; ld2 = 2;
while (1) {
ioctl(fd, TIOCSETD, &ld1);
ioctl(fd, TIOCSETD, &ld2);

Signed-off-by: Jiri Slaby <jslaby@xxxxxxx>
Reported-by: <Valdis.Kletnieks@xxxxxx>
Cc: Kyle McMartin <kyle@xxxxxxxxxxx>
Cc: Alan Cox <alan@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxx>
Signed-off-by: Paul Gortmaker <paul.gortmaker@xxxxxxxxxxxxx>
drivers/char/tty_io.c | 3 ++-
1 files changed, 2 insertions(+), 1 deletions(-)

diff --git a/drivers/char/tty_io.c b/drivers/char/tty_io.c
index d71f0fc..bc4f45d 100644
--- a/drivers/char/tty_io.c
+++ b/drivers/char/tty_io.c
@@ -1257,7 +1257,8 @@ static int tty_reopen(struct tty_struct *tty)
struct tty_driver *driver = tty->driver;

- if (test_bit(TTY_CLOSING, &tty->flags))
+ if (test_bit(TTY_CLOSING, &tty->flags) ||
+ test_bit(TTY_LDISC_CHANGING, &tty->flags))
return -EIO;

if (driver->type == TTY_DRIVER_TYPE_PTY &&

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