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Subject[PATCHv6 3/7] tty: Don't block on IO when ldisc change is pending
Date
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>
---
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 dabb391909aa..99460af61b77 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 749a608c40b0..f75696f0ee2d 100644
--- a/drivers/tty/n_r3964.c
+++ b/drivers/tty/n_r3964.c
@@ -1085,7 +1085,7 @@ 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 3ad460219fd6..5dc9686697cf 100644
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -1702,7 +1702,7 @@ 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
@@ -1744,7 +1744,7 @@ 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;

@@ -2211,7 +2211,7 @@ 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;
}
@@ -2365,7 +2365,7 @@ 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 fc4c97cae01e..9434d20cf3ca 100644
--- a/drivers/tty/tty_ldisc.c
+++ b/drivers/tty/tty_ldisc.c
@@ -327,6 +327,11 @@ 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;
@@ -337,6 +342,8 @@ 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 414db2bce715..80ae5528ef8e 100644
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -366,6 +366,7 @@ 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 */
@@ -383,6 +384,12 @@ 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);
--
2.19.1
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