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 <mingo@elte.hu>
Reported-by: Heinz Diehl <htd@fancy-poultry.org>
Cc: stable@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 2 insertions, 5 deletions

diff --git a/drivers/char/tty_ldisc.c b/drivers/char/tty_ldisc.c
index aafdbaebc16a..feb55075819b 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);