[34-longterm 080/179] TTY: ldisc, do not close until there are readers

From: Paul Gortmaker
Date: Mon May 14 2012 - 22:20:05 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 92f6fa09bd453ffe3351fa1f1377a1b7cfa911e6 upstream.

We restored tty_ldisc_wait_idle in 100eeae2c5c (TTY: restore
tty_ldisc_wait_idle). We used it in the ldisc changing path to fix the
case where there are tasks in n_tty_read waiting for data and somebody
tries to change ldisc.

Similar to the case above, there may be also tasks waiting in
n_tty_read while hangup is performed. As 65b770468e98 (tty-ldisc: turn
ldisc user count into a proper refcount) removed the wait-until-idle
from all paths, hangup path won't wait for them to disappear either
now. So add it back even to the hangup path.

There is a difference, we need uninterruptible sleep as there is
obviously HUP signal pending. So tty_ldisc_wait_idle now sleeps
without possibility to be interrupted. This is what original
tty_ldisc_wait_idle did. After the wait idle reintroduction
(100eeae2c5c), we have had interruptible sleeps for the ldisc changing
path. But as there is a 5s timeout anyway, we don't allow it to be
interrupted from now on. It's not worth the added complexity of
deciding what kind of sleep we want.

Before 65b770468e98 tty_ldisc_release was called also from
tty_ldisc_release. It is called from tty_release, so I don't think we
need to restore that one.

This is nicely reproducible after constifying the timing when
drivers/tty/n_tty.c is patched as follows ("TTY: ntty, add one more
sanity check" patch is needed to actually see it explode):
%% -1548,6 +1549,7 @@ static int n_tty_open(struct tty_struct *tty)

/* These are ugly. Currently a malloc failure here can panic */
if (!tty->read_buf) {
+ msleep(100);
tty->read_buf = kzalloc(N_TTY_BUF_SIZE, GFP_KERNEL);
if (!tty->read_buf)
return -ENOMEM;
%% -1785,6 +1788,7 @@ do_it_again:
timeout = schedule_timeout(timeout);
+ msleep(20);
===== With a process: =====
while (1) {
int fd = open(argv[1], O_RDWR);
read(fd, buf, sizeof(buf));
===== and its child: =====
while (1) {
int fd = open(tty, O_RDWR|O_NOCTTY);
ioctl(fd, TIOCSCTTY, 1);
usleep(100 * (10 + random() % 1000));
===== EOF =====

References: https://bugzilla.novell.com/show_bug.cgi?id=693374
References: https://bugzilla.novell.com/show_bug.cgi?id=694509
Signed-off-by: Jiri Slaby <jslaby@xxxxxxx>
Cc: Alan Cox <alan@xxxxxxxxxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxx>
[PG: account for char --> tty file rename post 2.6.34]
Signed-off-by: Paul Gortmaker <paul.gortmaker@xxxxxxxxxxxxx>
drivers/char/tty_ldisc.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/char/tty_ldisc.c b/drivers/char/tty_ldisc.c
index 236628f..48de74a 100644
--- a/drivers/char/tty_ldisc.c
+++ b/drivers/char/tty_ldisc.c
@@ -543,7 +543,7 @@ static int tty_ldisc_halt(struct tty_struct *tty)
static int tty_ldisc_wait_idle(struct tty_struct *tty)
int ret;
- ret = wait_event_interruptible_timeout(tty_ldisc_idle,
+ ret = wait_event_timeout(tty_ldisc_idle,
atomic_read(&tty->ldisc->users) == 1, 5 * HZ);
if (ret < 0)
return ret;
@@ -750,6 +750,8 @@ static int tty_ldisc_reinit(struct tty_struct *tty, int ldisc)
if (IS_ERR(ld))
return -1;

+ WARN_ON_ONCE(tty_ldisc_wait_idle(tty));
tty_ldisc_close(tty, tty->ldisc);
tty->ldisc = NULL;

To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/