From 16759f6cd8c590fa23cb2956fdf32fe23a67e482 Mon Sep 17 00:00:00 2001 From: Peter Hurley Date: Mon, 11 Mar 2013 16:44:41 -0400 Subject: [PATCH] tty: Document unsafe ldisc reference acquire Merge get_ldisc() into its only call site. Note how, after merging, the unsafe acquire of an ldisc reference is obvious. CPU 0 in tty_ldisc_try() | CPU 1 in tty_ldisc_halt() | test_bit(TTY_LDISC, &tty_flags) | if (true) | clear_bit(TTY_LDISC, &tty_flags) tty->ldisc != 0? | atomic_read(&tty->ldisc->users) if (true) | ret_val == 1? atomic_inc(&tty->ldisc->users) | if (false) | wait | | | The spin lock in tty_ldisc_try() does nothing wrt synchronizing the ldisc halt since it's not acquired as part of halting. Signed-off-by: Peter Hurley Signed-off-by: Greg Kroah-Hartman --- drivers/tty/tty_ldisc.c | 14 +++++--------- 1 file changed, 5 insertions(+), 9 deletions(-) diff --git a/drivers/tty/tty_ldisc.c b/drivers/tty/tty_ldisc.c index 9362a1030c95..5ee0b2be7d72 100644 --- a/drivers/tty/tty_ldisc.c +++ b/drivers/tty/tty_ldisc.c @@ -42,13 +42,6 @@ static DECLARE_WAIT_QUEUE_HEAD(tty_ldisc_wait); /* Line disc dispatch table */ static struct tty_ldisc_ops *tty_ldiscs[NR_LDISCS]; -static inline struct tty_ldisc *get_ldisc(struct tty_ldisc *ld) -{ - if (ld) - atomic_inc(&ld->users); - return ld; -} - /** * tty_register_ldisc - install a line discipline * @disc: ldisc number @@ -269,10 +262,13 @@ static struct tty_ldisc *tty_ldisc_try(struct tty_struct *tty) unsigned long flags; struct tty_ldisc *ld; + /* FIXME: this allows reference acquire after TTY_LDISC is cleared */ raw_spin_lock_irqsave(&tty_ldisc_lock, flags); ld = NULL; - if (test_bit(TTY_LDISC, &tty->flags)) - ld = get_ldisc(tty->ldisc); + if (test_bit(TTY_LDISC, &tty->flags) && tty->ldisc) { + ld = tty->ldisc; + atomic_inc(&ld->users); + } raw_spin_unlock_irqrestore(&tty_ldisc_lock, flags); return ld; } -- 2.20.1