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

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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>
This commit is contained in:
Dmitry Safonov 2018-11-01 00:24:48 +00:00 committed by Greg Kroah-Hartman
parent 83d817f410
commit c96cf923a9
5 changed files with 21 additions and 7 deletions
repo.diff.show_diff_stats Download patch file Download diff file Expand all files Collapse all files

4
drivers/tty/n_hdlc.c
View file

@ -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;
}

2
drivers/tty/n_r3964.c
View file

@ -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;
}

8
drivers/tty/n_tty.c
View file

@ -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;
}

7
drivers/tty/tty_ldisc.c
View file

@ -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);
}

7
include/linux/tty.h
View file

@ -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);