tty: reorder ldisc locking

We need to release the BTM in paste_selection() when
sleeping in tty_ldisc_ref_wait to avoid deadlocks
with tty_ldisc_enable.

In tty_set_ldisc, we now always grab the BTM before
taking the ldisc_mutex in order to avoid AB-BA
deadlocks between the two.

tty_ldisc_halt potentially blocks on a workqueue
function that takes the BTM, so we must release
the BTM before calling it.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/char/selection.c

@@ -319,7 +319,12 @@ int paste_selection(struct tty_struct *tty)
 	poke_blanked_console();
 	release_console_sem();
 
+	ld = tty_ldisc_ref(tty);
+	if (!ld) {
+		tty_unlock();
 		ld = tty_ldisc_ref_wait(tty);
+		tty_lock();
+	}
 
 	add_wait_queue(&vc->paste_wait, &wait);
 	while (sel_buffer && sel_buffer_lth > pasted) {

drivers/char/tty_ldisc.c

@@ -582,6 +582,7 @@ int tty_set_ldisc(struct tty_struct *tty, int ldisc)
 
 	tty_wait_until_sent(tty, 0);
 
+	tty_lock();
 	mutex_lock(&tty->ldisc_mutex);
 
 	/*
@@ -591,13 +592,13 @@ int tty_set_ldisc(struct tty_struct *tty, int ldisc)
 
 	while (test_bit(TTY_LDISC_CHANGING, &tty->flags)) {
 		mutex_unlock(&tty->ldisc_mutex);
+		tty_unlock();
 		wait_event(tty_ldisc_wait,
 			test_bit(TTY_LDISC_CHANGING, &tty->flags) == 0);
+		tty_lock();
 		mutex_lock(&tty->ldisc_mutex);
 	}
 
-	tty_lock();
-
 	set_bit(TTY_LDISC_CHANGING, &tty->flags);
 
 	/*
@@ -634,8 +635,8 @@ int tty_set_ldisc(struct tty_struct *tty, int ldisc)
 
 	flush_scheduled_work();
 
-	mutex_lock(&tty->ldisc_mutex);
 	tty_lock();
+	mutex_lock(&tty->ldisc_mutex);
 	if (test_bit(TTY_HUPPED, &tty->flags)) {
 		/* We were raced by the hangup method. It will have stomped
 		   the ldisc data and closed the ldisc down */
@@ -782,7 +783,20 @@ void tty_ldisc_hangup(struct tty_struct *tty)
 	 * Avoid racing set_ldisc or tty_ldisc_release
 	 */
 	mutex_lock(&tty->ldisc_mutex);
-	tty_ldisc_halt(tty);
+
+	/*
+	 * this is like tty_ldisc_halt, but we need to give up
+	 * the BTM before calling cancel_delayed_work_sync,
+	 * which may need to wait for another function taking the BTM
+	 */
+	clear_bit(TTY_LDISC, &tty->flags);
+	tty_unlock();
+	cancel_delayed_work_sync(&tty->buf.work);
+	mutex_unlock(&tty->ldisc_mutex);
+
+	tty_lock();
+	mutex_lock(&tty->ldisc_mutex);
+
 	/* At this point we have a closed ldisc and we want to
 	   reopen it. We could defer this to the next open but
 	   it means auditing a lot of other paths so this is
@@ -853,8 +867,10 @@ void tty_ldisc_release(struct tty_struct *tty, struct tty_struct *o_tty)
 	 * race with the set_ldisc code path.
 	 */
 
+	tty_unlock();
 	tty_ldisc_halt(tty);
 	flush_scheduled_work();
+	tty_lock();
 
 	mutex_lock(&tty->ldisc_mutex);
 	/*