Block when initially opening a TTY multiple times.

In the original MPSAFE TTY code, I changed the behaviour by returning
EBUSY. I thought this made more sense, because it's basically a race to
see who gets the TTY first.

It turns out this is not a good change, because it also causes EBUSY to
be returned when another process is closing the TTY. This can happen
during startup, when /etc/rc (or one of its children) is still busy
draining its data and /sbin/init is attempting to open the TTY to spawn
a getty.

Reported by:	bz
Tested by:	bz

1 files changed, 11 insertions, 5 deletions

diff --git a/sys/kern/tty.c b/sys/kern/tty.c
index 2468e88..804635e 100644
--- a/sys/kern/tty.c
+++ b/sys/kern/tty.c
@@ -206,6 +206,7 @@ ttydev_leave(struct tty *tp)
 		ttydevsw_close(tp);
 
 	tp->t_flags &= ~TF_OPENCLOSE;
+	cv_broadcast(&tp->t_dcdwait);
 	tty_rel_free(tp);
 }
 
@@ -231,13 +232,17 @@ ttydev_open(struct cdev *dev, int oflags, int devtype, struct thread *td)
 		tty_unlock(tp);
 		return (ENXIO);
 	}
+
 	/*
-	 * Prevent the TTY from being opened when being torn down or
-	 * built up by unrelated processes.
+	 * Block when other processes are currently opening or closing
+	 * the TTY.
 	 */
-	if (tp->t_flags & TF_OPENCLOSE) {
-		tty_unlock(tp);
-		return (EBUSY);
+	while (tp->t_flags & TF_OPENCLOSE) {
+		error = tty_wait(tp, &tp->t_dcdwait);
+		if (error != 0) {
+			tty_unlock(tp);
+			return (error);
+		}
 	}
 	tp->t_flags |= TF_OPENCLOSE;
 
@@ -299,6 +304,7 @@ ttydev_open(struct cdev *dev, int oflags, int devtype, struct thread *td)
 		tp->t_flags |= TF_OPENED_IN;
 
 done:	tp->t_flags &= ~TF_OPENCLOSE;
+	cv_broadcast(&tp->t_dcdwait);
 	ttydev_leave(tp);
 
 	return (error);