patch "tty: Don't hold ldisc lock in tty_reopen() if ldisc present" added to tty-linus

This is a note to let you know that I've just added the patch titled

tty: Don't hold ldisc lock in tty_reopen() if ldisc present

to my tty git tree which can be found at
git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/tty.git
in the tty-linus branch.

The patch will show up in the next release of the linux-next tree (usually sometime within the next 24 hours during the week.)

The patch will hopefully also be merged in Linus's tree for the next -rc kernel release.

If you have any questions about this process, please let me know.

From d3736d82e8169768218ee0ef68718875918091a0 Mon Sep 17 00:00:00 2001
From: Dmitry Safonov <[email protected]>
Date: Wed, 9 Jan 2019 01:17:40 +0000
Subject: tty: Don't hold ldisc lock in tty_reopen() if ldisc present

Try to get reference for ldisc during tty_reopen().
If ldisc present, we don't need to do tty_ldisc_reinit() and lock the write side for line discipline semaphore.
Effectively, it optimizes fast-path for tty_reopen(), but more importantly it won't interrupt ongoing IO on the tty as no ldisc change is needed.
Fixes user-visible issue when tty_reopen() interrupted login process for user with a long password, observed and reported by Lukas.

---
 drivers/tty/tty_io.c | 20 +++++++++++++-------
 1 file changed, 13 insertions(+), 7 deletions(-)

index bfe9ad85b362..23c6fd238422 100644
@@ -1256,7 +1256,8 @@ static void tty_driver_remove_tty(struct tty_driver *driver, struct tty_struct *
 static int tty_reopen(struct tty_struct *tty)
 {
 	struct tty_driver *driver = tty->driver;
-	int retval;
+	struct tty_ldisc *ld;
+	int retval = 0;
 
 	if (driver->type == TTY_DRIVER_TYPE_PTY &&
 	    driver->subtype == PTY_TYPE_MASTER)

@@ -1268,13 +1269,18 @@ static int tty_reopen(struct tty_struct *tty)
 	if (test_bit(TTY_EXCLUSIVE, &tty->flags) && !capable(CAP_SYS_ADMIN))
 		return -EBUSY;
 
-	retval = tty_ldisc_lock(tty, 5 * HZ);
-	if (retval)
-		return retval;
+	ld = tty_ldisc_ref_wait(tty);
+	if (ld) {
+		tty_ldisc_deref(ld);
+	} else {
+		retval = tty_ldisc_lock(tty, 5 * HZ);
+		if (retval)
+			return retval;
 
-	if (!tty->ldisc)
-		retval = tty_ldisc_reinit(tty, tty->termios.c_line);
-	tty_ldisc_unlock(tty);
+		if (!tty->ldisc)
+			retval = tty_ldisc_reinit(tty, tty->termios.c_line);
+		tty_ldisc_unlock(tty);
+	}
 
 	if (retval == 0)
 		tty->count++;

2.20.1