[SRU B/C] [PATCH 3/4] tty: Don't block on IO when ldisc change is pending

[Guilherme G. Piccoli]

From: Dmitry Safonov <dima at arista.com>

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 at suse.com>
Reported-by: Mikulas Patocka <mpatocka at redhat.com>
Signed-off-by: Dmitry Safonov <dima at arista.com>
Signed-off-by: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
(cherry-picked from c96cf923a98d1b094df9f0cf97a83e118817e31b upstream)
Signed-off-by: Guilherme G. Piccoli <gpiccoli at canonical.com>
---
 drivers/tty/n_hdlc.c    | 4 ++--
 drivers/tty/n_r3964.c   | 2 +-
 drivers/tty/n_tty.c     | 8 ++++----
 drivers/tty/tty_ldisc.c | 7 +++++++
 include/linux/tty.h     | 7 +++++++
 5 files changed, 21 insertions(+), 7 deletions(-)

diff --git a/drivers/tty/n_hdlc.c b/drivers/tty/n_hdlc.c
index eea7b6cb3cc4..33ee831bbd4e 100644
--- a/drivers/tty/n_hdlc.c
+++ b/drivers/tty/n_hdlc.c
@@ -612,7 +612,7 @@ 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;
 		}
@@ -679,7 +679,7 @@ 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;
 		}
diff --git a/drivers/tty/n_r3964.c b/drivers/tty/n_r3964.c
index 30bb0900cd2f..30ba5491f32e 100644
--- a/drivers/tty/n_r3964.c
+++ b/drivers/tty/n_r3964.c
@@ -1078,7 +1078,7 @@ 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;
 			}
diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
index 63c593a74380..31067cfd677e 100644
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -1671,7 +1671,7 @@ 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
@@ -1713,7 +1713,7 @@ 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;
 
@@ -2188,7 +2188,7 @@ 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;
 				}
@@ -2342,7 +2342,7 @@ 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;
 		}
diff --git a/drivers/tty/tty_ldisc.c b/drivers/tty/tty_ldisc.c
index 0fd18f5fa67d..48fe8d9b6df9 100644
--- a/drivers/tty/tty_ldisc.c
+++ b/drivers/tty/tty_ldisc.c
@@ -340,6 +340,11 @@ int 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;
@@ -350,6 +355,8 @@ int 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);
 }
 
diff --git a/include/linux/tty.h b/include/linux/tty.h
index 1dd587ba6d88..0cd621d8c7f0 100644
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -365,6 +365,7 @@ 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 */
 
 /* Values for tty->flow_change */
@@ -382,6 +383,12 @@ 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);
+}
+
 static inline bool tty_io_error(struct tty_struct *tty)
 {
 	return test_bit(TTY_IO_ERROR, &tty->flags);
-- 
2.19.2