[PATCH -next v2 02/26] tty: Invert tty_lock/ldisc_sem lock order

Peter Hurley
2014-11-05T17:23:49Z

Dropping the tty lock to acquire the tty->ldisc_sem allows several
race conditions (such as hangup while changing the ldisc) which requires
extra states and testing. The ldisc_sem->tty_lock lock order has
not been required since tty buffer ownership was moved to tty_port.

Reviewed-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
---
drivers/tty/tty_ldisc.c | 7 +++----
1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/drivers/tty/tty_ldisc.c b/drivers/tty/tty_ldisc.c
index 332a622..28858eb 100644
--- a/drivers/tty/tty_ldisc.c
+++ b/drivers/tty/tty_ldisc.c
@@ -523,9 +523,11 @@ int tty_set_ldisc(struct tty_struct *tty, int ldisc)
if (IS_ERR(new_ldisc))
return PTR_ERR(new_ldisc);

+ tty_lock(tty);
retval = tty_ldisc_lock_pair_timeout(tty, o_tty, 5 * HZ);
if (retval) {
tty_ldisc_put(new_ldisc);
+ tty_unlock(tty);
return retval;
}

@@ -536,11 +538,11 @@ int tty_set_ldisc(struct tty_struct *tty, int ldisc)
if (tty->ldisc->ops->num == ldisc) {
tty_ldisc_enable_pair(tty, o_tty);
tty_ldisc_put(new_ldisc);
+ tty_unlock(tty);
return 0;
}

old_ldisc = tty->ldisc;
- tty_lock(tty);

if (test_bit(TTY_HUPPING, &tty->flags) ||
test_bit(TTY_HUPPED, &tty->flags)) {
@@ -675,8 +677,6 @@ void tty_ldisc_hangup(struct tty_struct *tty)
wake_up_interruptible_poll(&tty->write_wait, POLLOUT);
wake_up_interruptible_poll(&tty->read_wait, POLLIN);

- tty_unlock(tty);
-
/*
* Shutdown the current line discipline, and reset it to
* N_TTY if need be.
@@ -684,7 +684,6 @@ void tty_ldisc_hangup(struct tty_struct *tty)
* Avoid racing set_ldisc or tty_ldisc_release
*/
tty_ldisc_lock_pair(tty, tty->link);
- tty_lock(tty);

if (tty->ldisc) {

--
2.1.3