tty: Wait interruptibly for tty lock on reopen

Allow a signal to interrupt the wait for a tty reopen; eg., if
the tty has starting final close and is waiting for the device to
drain.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Cc: stable <stable@vger.kernel.org> # 4.4
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/tty_io.c

@@ -2065,7 +2065,12 @@ retry_open:
 
 		if (tty) {
 			mutex_unlock(&tty_mutex);
-			tty_lock(tty);
+			retval = tty_lock_interruptible(tty);
+			if (retval) {
+				if (retval == -EINTR)
+					retval = -ERESTARTSYS;
+				goto err_unref;
+			}
 			/* safe to drop the kref from tty_driver_lookup_tty() */
 			tty_kref_put(tty);
 			retval = tty_reopen(tty);
@@ -2152,6 +2157,7 @@ retry_open:
 	return 0;
 err_unlock:
 	mutex_unlock(&tty_mutex);
+err_unref:
 	/* after locks to avoid deadlock */
 	if (!IS_ERR_OR_NULL(driver))
 		tty_driver_kref_put(driver);

drivers/tty/tty_mutex.c

@@ -19,6 +19,14 @@ void __lockfunc tty_lock(struct tty_struct *tty)
 }
 EXPORT_SYMBOL(tty_lock);
 
+int tty_lock_interruptible(struct tty_struct *tty)
+{
+	if (WARN(tty->magic != TTY_MAGIC, "L Bad %p\n", tty))
+		return -EIO;
+	tty_kref_get(tty);
+	return mutex_lock_interruptible(&tty->legacy_mutex);
+}
+
 void __lockfunc tty_unlock(struct tty_struct *tty)
 {
 	if (WARN(tty->magic != TTY_MAGIC, "U Bad %p\n", tty))

include/linux/tty.h

@@ -649,6 +649,7 @@ extern long vt_compat_ioctl(struct tty_struct *tty,
 /* tty_mutex.c */
 /* functions for preparation of BKL removal */
 extern void __lockfunc tty_lock(struct tty_struct *tty);
+extern int  tty_lock_interruptible(struct tty_struct *tty);
 extern void __lockfunc tty_unlock(struct tty_struct *tty);
 extern void __lockfunc tty_lock_slave(struct tty_struct *tty);
 extern void __lockfunc tty_unlock_slave(struct tty_struct *tty);