tty: Don't block on IO when ldisc change is pending

BugLink: https://bugs.launchpad.net/bugs/1791758

There might be situations where tty_ldisc_lock() has blocked, but there
is already IO on tty and it prevents line discipline changes.
It might theoretically turn into dead-lock.

Basically, provide more priority to pending tty_ldisc_lock() than to
servicing reads/writes over tty.

User-visible issue was reported by Mikulas where on pa-risc with
Debian 5 reboot took either 80 seconds, 3 minutes or 3:25 after proper
locking in tty_reopen().

Cc: Jiri Slaby <jslaby@suse.com>
Reported-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Dmitry Safonov <dima@arista.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(backported from c96cf923)
[gpiccoli: context adjustment]
Signed-off-by: Guilherme G. Piccoli <gpiccoli@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Kleber Souza <kleber.souza@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

drivers/tty/n_hdlc.c

@@ -613,7 +613,7 @@ static ssize_t n_hdlc_tty_read(struct tty_struct *tty, struct file *file,
 		}
 			
 		/* no data */
-		if (file->f_flags & O_NONBLOCK) {
+		if (tty_io_nonblock(tty, file)) {
 			ret = -EAGAIN;
 			break;
 		}
@@ -680,7 +680,7 @@ static ssize_t n_hdlc_tty_write(struct tty_struct *tty, struct file *file,
 		if (tbuf)
 			break;
 
-		if (file->f_flags & O_NONBLOCK) {
+		if (tty_io_nonblock(tty, file)) {
 			error = -EAGAIN;
 			break;
 		}

drivers/tty/n_r3964.c

@@ -1080,7 +1080,7 @@ static ssize_t r3964_read(struct tty_struct *tty, struct file *file,
 		pMsg = remove_msg(pInfo, pClient);
 		if (pMsg == NULL) {
 			/* no messages available. */
-			if (file->f_flags & O_NONBLOCK) {
+			if (tty_io_nonblock(tty, file)) {
 				ret = -EAGAIN;
 				goto unlock;
 			}

drivers/tty/n_tty.c

@@ -1736,7 +1736,7 @@ n_tty_receive_buf_common(struct tty_struct *tty, const unsigned char *cp,
 
 	down_read(&tty->termios_rwsem);
 
-	while (1) {
+	do {
 		/*
 		 * When PARMRK is set, each input char may take up to 3 chars
 		 * in the read buf; reduce the buffer space avail by 3x
@@ -1778,7 +1778,7 @@ n_tty_receive_buf_common(struct tty_struct *tty, const unsigned char *cp,
 			fp += n;
 		count -= n;
 		rcvd += n;
-	}
+	} while (!test_bit(TTY_LDISC_CHANGING, &tty->flags));
 
 	tty->receive_room = room;
 
@@ -2270,7 +2270,7 @@ static ssize_t n_tty_read(struct tty_struct *tty, struct file *file,
 					break;
 				if (!timeout)
 					break;
-				if (file->f_flags & O_NONBLOCK) {
+				if (tty_io_nonblock(tty, file)) {
 					retval = -EAGAIN;
 					break;
 				}
@@ -2427,7 +2427,7 @@ static ssize_t n_tty_write(struct tty_struct *tty, struct file *file,
 		}
 		if (!nr)
 			break;
-		if (file->f_flags & O_NONBLOCK) {
+		if (tty_io_nonblock(tty, file)) {
 			retval = -EAGAIN;
 			break;
 		}

drivers/tty/tty_ldisc.c

@@ -330,6 +330,11 @@ tty_ldisc_lock(struct tty_struct *tty, unsigned long timeout)
 {
 	int ret;
 
+	/* Kindly asking blocked readers to release the read side */
+	set_bit(TTY_LDISC_CHANGING, &tty->flags);
+	wake_up_interruptible_all(&tty->read_wait);
+	wake_up_interruptible_all(&tty->write_wait);
+
 	ret = __tty_ldisc_lock(tty, timeout);
 	if (!ret)
 		return -EBUSY;
@@ -340,6 +345,8 @@ tty_ldisc_lock(struct tty_struct *tty, unsigned long timeout)
 void tty_ldisc_unlock(struct tty_struct *tty)
 {
 	clear_bit(TTY_LDISC_HALTED, &tty->flags);
+	/* Can be cleared here - ldisc_unlock will wake up writers firstly */
+	clear_bit(TTY_LDISC_CHANGING, &tty->flags);
 	__tty_ldisc_unlock(tty);
 }
 

include/linux/tty.h

@@ -343,6 +343,7 @@ struct tty_file_private {
 #define TTY_NO_WRITE_SPLIT 	17	/* Preserve write boundaries to driver */
 #define TTY_HUPPED 		18	/* Post driver->hangup() */
 #define TTY_HUPPING		19	/* Hangup in progress */
+#define TTY_LDISC_CHANGING	20	/* Change pending - non-block IO */
 #define TTY_LDISC_HALTED	22	/* Line discipline is halted */
 
 #define TTY_WRITE_FLUSH(tty) tty_write_flush((tty))
@@ -362,6 +363,12 @@ static inline void tty_set_flow_change(struct tty_struct *tty, int val)
 	smp_mb();
 }
 
+static inline bool tty_io_nonblock(struct tty_struct *tty, struct file *file)
+{
+	return file->f_flags & O_NONBLOCK ||
+		test_bit(TTY_LDISC_CHANGING, &tty->flags);
+}
+
 #ifdef CONFIG_TTY
 extern void console_init(void);
 extern void tty_kref_put(struct tty_struct *tty);