Commit 0b5759c6 authored by Linus Torvalds's avatar Linus Torvalds

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: default avatarIngo Molnar <mingo@elte.hu>
Reported-by: default avatarHeinz Diehl <htd@fancy-poultry.org>
Cc: stable@kernel.org
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent f0a221ef
......@@ -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);
......
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