From 0b5759c654e74c8dc317ea2c6b3a7476160f688a Mon Sep 17 00:00:00 2001 From: Linus Torvalds Date: Sat, 3 Oct 2009 21:44:21 -0700 Subject: [PATCH] tty: Avoid dropping ldisc_mutex over hangup tty re-initialization A couple of people have hit the WARN_ON() in drivers/char/tty_io.c, tty_open() that is unhappy about seeing the tty line discipline go away during the tty hangup. See for example http://bugzilla.kernel.org/show_bug.cgi?id=14255 and the reason is that we do the tty_ldisc_halt() outside the ldisc_mutex in order to be able to flush the scheduled work without a deadlock with vhangup_work. However, it turns out that we can solve this particular case by - using "cancel_delayed_work_sync()" in tty_ldisc_halt(), which waits for just the particular work, rather than synchronizing with any random outstanding pending work. This won't deadlock, since the buf.work we synchronize with doesn't care about the ldisc_mutex, it just flushes the tty ldisc buffers. - realize that for this particular case, we don't need to wait for any hangup work, because we are inside the hangup codepaths ourselves. so as a result we can just drop the flush_scheduled_work() entirely, and then move the tty_ldisc_halt() call to inside the mutex. That way we never expose the partially torn down ldisc state to tty_open(), and hold the ldisc_mutex over the whole sequence. Reported-by: Ingo Molnar Reported-by: Heinz Diehl Cc: stable@kernel.org Signed-off-by: Linus Torvalds --- drivers/char/tty_ldisc.c | 7 ++----- 1 file changed, 2 insertions(+), 5 deletions(-) diff --git a/drivers/char/tty_ldisc.c b/drivers/char/tty_ldisc.c index aafdbae..feb5507 100644 --- a/drivers/char/tty_ldisc.c +++ b/drivers/char/tty_ldisc.c @@ -518,7 +518,7 @@ static void tty_ldisc_restore(struct tty_struct *tty, struct tty_ldisc *old) static int tty_ldisc_halt(struct tty_struct *tty) { clear_bit(TTY_LDISC, &tty->flags); - return cancel_delayed_work(&tty->buf.work); + return cancel_delayed_work_sync(&tty->buf.work); } /** @@ -756,12 +756,9 @@ void tty_ldisc_hangup(struct tty_struct *tty) * N_TTY. */ if (tty->driver->flags & TTY_DRIVER_RESET_TERMIOS) { - /* Make sure the old ldisc is quiescent */ - tty_ldisc_halt(tty); - flush_scheduled_work(); - /* Avoid racing set_ldisc or tty_ldisc_release */ mutex_lock(&tty->ldisc_mutex); + tty_ldisc_halt(tty); if (tty->ldisc) { /* Not yet closed */ /* Switch back to N_TTY */ tty_ldisc_reinit(tty); -- 2.7.4