[PATCH 30/75] tty: Fix PPP hang under load

Alan Cox
2009-01-02T13:46:03Z

From: Alan Cox <alan@redhat.com>

Signed-off-by: Alan Cox <alan@redhat.com>
---

drivers/char/tty_ldisc.c | 30 +++++++++++++++++++++---------
include/linux/tty.h | 1 +
2 files changed, 22 insertions(+), 9 deletions(-)

diff --git a/drivers/char/tty_ldisc.c b/drivers/char/tty_ldisc.c
index f307f13..7a84b40 100644
--- a/drivers/char/tty_ldisc.c
+++ b/drivers/char/tty_ldisc.c
@@ -316,8 +316,7 @@ struct tty_ldisc *tty_ldisc_ref_wait(struct tty_struct *tty)
{
/* wait_event is a macro */
wait_event(tty_ldisc_wait, tty_ldisc_try(tty));
- if (tty->ldisc.refcount == 0)
- printk(KERN_ERR "tty_ldisc_ref_wait\n");
+ WARN_ON(tty->ldisc.refcount == 0);
return &tty->ldisc;
}

@@ -376,15 +375,17 @@ EXPORT_SYMBOL_GPL(tty_ldisc_deref);
* @tty: terminal to activate ldisc on
*
* Set the TTY_LDISC flag when the line discipline can be called
- * again. Do necessary wakeups for existing sleepers.
+ * again. Do necessary wakeups for existing sleepers. Clear the LDISC
+ * changing flag to indicate any ldisc change is now over.
*
- * Note: nobody should set this bit except via this function. Clearing
- * directly is allowed.
+ * Note: nobody should set the TTY_LDISC bit except via this function.
+ * Clearing directly is allowed.
*/

void tty_ldisc_enable(struct tty_struct *tty)
{
set_bit(TTY_LDISC, &tty->flags);
+ clear_bit(TTY_LDISC_CHANGING, &tty->flags);
wake_up(&tty_ldisc_wait);
}

@@ -496,7 +497,14 @@ restart:
* reference to the line discipline. The TTY_LDISC bit
* prevents anyone taking a reference once it is clear.
* We need the lock to avoid racing reference takers.
+ *
+ * We must clear the TTY_LDISC bit here to avoid a livelock
+ * with a userspace app continually trying to use the tty in
+ * parallel to the change and re-referencing the tty.
*/
+ clear_bit(TTY_LDISC, &tty->flags);
+ if (o_tty)
+ clear_bit(TTY_LDISC, &o_tty->flags);

spin_lock_irqsave(&tty_ldisc_lock, flags);
if (tty->ldisc.refcount || (o_tty && o_tty->ldisc.refcount)) {
@@ -528,7 +536,7 @@ restart:
* If the TTY_LDISC bit is set, then we are racing against
* another ldisc change
*/
- if (!test_bit(TTY_LDISC, &tty->flags)) {
+ if (test_bit(TTY_LDISC_CHANGING, &tty->flags)) {
struct tty_ldisc *ld;
spin_unlock_irqrestore(&tty_ldisc_lock, flags);
tty_ldisc_put(new_ldisc.ops);
@@ -536,10 +544,14 @@ restart:
tty_ldisc_deref(ld);
goto restart;
}
-
- clear_bit(TTY_LDISC, &tty->flags);
+ /*
+ * This flag is used to avoid two parallel ldisc changes. Once
+ * open and close are fine grained locked this may work better
+ * as a mutex shared with the open/close/hup paths
+ */
+ set_bit(TTY_LDISC_CHANGING, &tty->flags);
if (o_tty)
- clear_bit(TTY_LDISC, &o_tty->flags);
+ set_bit(TTY_LDISC_CHANGING, &o_tty->flags);
spin_unlock_irqrestore(&tty_ldisc_lock, flags);

/*
diff --git a/include/linux/tty.h b/include/linux/tty.h
index f881697..bbbeaef 100644
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -301,6 +301,7 @@ struct tty_struct {
#define TTY_PUSH 6 /* n_tty private */
#define TTY_CLOSING 7 /* ->close() in progress */
#define TTY_LDISC 9 /* Line discipline attached */
+#define TTY_LDISC_CHANGING 10 /* Line discipline changing */
#define TTY_HW_COOK_OUT 14 /* Hardware can do output cooking */
#define TTY_HW_COOK_IN 15 /* Hardware can do input cooking */
#define TTY_PTY_LOCK 16 /* pty private */