We restored tty_ldisc_wait_idle in 100eeae2c5 (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 65b770468e (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
(100eeae2c5), 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 65b770468e 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:
break;
}
timeout = schedule_timeout(timeout);
+ msleep(20);
continue;
}
__set_current_state(TASK_RUNNING);
===== With a process: =====
while (1) {
int fd = open(argv[1], O_RDWR);
read(fd, buf, sizeof(buf));
close(fd);
}
===== and its child: =====
setsid();
while (1) {
int fd = open(tty, O_RDWR|O_NOCTTY);
ioctl(fd, TIOCSCTTY, 1);
vhangup();
close(fd);
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@suse.cz>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: stable <stable@kernel.org> [32, 33, 34, 39]
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
92f6fa09bd