diff options
| -rw-r--r-- | drivers/char/tty_ldisc.c | 30 | ||||
| -rw-r--r-- | 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 f307f135cbf..7a84b406a95 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 f88169787a5..bbbeaef9962 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 */ |