From: Dmitry Safonov <dima@arista.com>
Date: Thu, 1 Nov 2018 00:24:48 +0000
Subject: tty: Don't block on IO when ldisc change is pending
Git-commit: c96cf923a98d1b094df9f0cf97a83e118817e31b
Patch-mainline: v5.0-rc1
References: bnc#1105428
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: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jslaby@suse.com>
Reported-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Dmitry Safonov <dima@arista.com>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
---
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(-)
--- a/drivers/tty/n_hdlc.c
+++ b/drivers/tty/n_hdlc.c
@@ -613,7 +613,7 @@ static ssize_t n_hdlc_tty_read(struct tt
}
/* 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 t
if (tbuf)
break;
- if (file->f_flags & O_NONBLOCK) {
+ if (tty_io_nonblock(tty, file)) {
error = -EAGAIN;
break;
}
--- a/drivers/tty/n_r3964.c
+++ b/drivers/tty/n_r3964.c
@@ -1080,7 +1080,7 @@ static ssize_t r3964_read(struct tty_str
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;
}
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -1691,7 +1691,7 @@ n_tty_receive_buf_common(struct tty_stru
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
@@ -1733,7 +1733,7 @@ n_tty_receive_buf_common(struct tty_stru
fp += n;
count -= n;
rcvd += n;
- }
+ } while (!test_bit(TTY_LDISC_CHANGING, &tty->flags));
tty->receive_room = room;
@@ -2199,7 +2199,7 @@ static ssize_t n_tty_read(struct tty_str
break;
if (!timeout)
break;
- if (file->f_flags & O_NONBLOCK) {
+ if (tty_io_nonblock(tty, file)) {
retval = -EAGAIN;
break;
}
@@ -2353,7 +2353,7 @@ static ssize_t n_tty_write(struct tty_st
}
if (!nr)
break;
- if (file->f_flags & O_NONBLOCK) {
+ if (tty_io_nonblock(tty, file)) {
retval = -EAGAIN;
break;
}
--- a/drivers/tty/tty_ldisc.c
+++ b/drivers/tty/tty_ldisc.c
@@ -339,6 +339,11 @@ int tty_ldisc_lock(struct tty_struct *tt
{
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;
@@ -349,6 +354,8 @@ int tty_ldisc_lock(struct tty_struct *tt
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);
}
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -363,6 +363,7 @@ 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 */
@@ -380,6 +381,12 @@ static inline void tty_set_flow_change(s
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);