tty: Don't block on IO when ldisc change is pending
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>
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@ -612,7 +612,7 @@ static ssize_t n_hdlc_tty_read(struct tty_struct *tty, struct file *file,
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}
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}
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/* no data */
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/* no data */
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if (file->f_flags & O_NONBLOCK) {
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if (tty_io_nonblock(tty, file)) {
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ret = -EAGAIN;
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ret = -EAGAIN;
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break;
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break;
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}
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}
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@ -679,7 +679,7 @@ static ssize_t n_hdlc_tty_write(struct tty_struct *tty, struct file *file,
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if (tbuf)
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if (tbuf)
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break;
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break;
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if (file->f_flags & O_NONBLOCK) {
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if (tty_io_nonblock(tty, file)) {
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error = -EAGAIN;
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error = -EAGAIN;
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break;
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break;
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}
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}
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@ -1085,7 +1085,7 @@ static ssize_t r3964_read(struct tty_struct *tty, struct file *file,
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pMsg = remove_msg(pInfo, pClient);
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pMsg = remove_msg(pInfo, pClient);
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if (pMsg == NULL) {
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if (pMsg == NULL) {
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/* no messages available. */
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/* no messages available. */
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if (file->f_flags & O_NONBLOCK) {
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if (tty_io_nonblock(tty, file)) {
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ret = -EAGAIN;
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ret = -EAGAIN;
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goto unlock;
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goto unlock;
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}
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}
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@ -1702,7 +1702,7 @@ n_tty_receive_buf_common(struct tty_struct *tty, const unsigned char *cp,
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down_read(&tty->termios_rwsem);
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down_read(&tty->termios_rwsem);
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while (1) {
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do {
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/*
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/*
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* When PARMRK is set, each input char may take up to 3 chars
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* When PARMRK is set, each input char may take up to 3 chars
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* in the read buf; reduce the buffer space avail by 3x
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* in the read buf; reduce the buffer space avail by 3x
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@ -1744,7 +1744,7 @@ n_tty_receive_buf_common(struct tty_struct *tty, const unsigned char *cp,
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fp += n;
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fp += n;
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count -= n;
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count -= n;
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rcvd += n;
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rcvd += n;
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}
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} while (!test_bit(TTY_LDISC_CHANGING, &tty->flags));
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tty->receive_room = room;
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tty->receive_room = room;
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@ -2211,7 +2211,7 @@ static ssize_t n_tty_read(struct tty_struct *tty, struct file *file,
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break;
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break;
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if (!timeout)
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if (!timeout)
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break;
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break;
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if (file->f_flags & O_NONBLOCK) {
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if (tty_io_nonblock(tty, file)) {
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retval = -EAGAIN;
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retval = -EAGAIN;
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break;
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break;
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}
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}
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@ -2365,7 +2365,7 @@ static ssize_t n_tty_write(struct tty_struct *tty, struct file *file,
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}
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}
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if (!nr)
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if (!nr)
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break;
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break;
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if (file->f_flags & O_NONBLOCK) {
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if (tty_io_nonblock(tty, file)) {
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retval = -EAGAIN;
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retval = -EAGAIN;
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break;
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break;
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}
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}
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@ -327,6 +327,11 @@ int tty_ldisc_lock(struct tty_struct *tty, unsigned long timeout)
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{
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{
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int ret;
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int ret;
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/* Kindly asking blocked readers to release the read side */
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set_bit(TTY_LDISC_CHANGING, &tty->flags);
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wake_up_interruptible_all(&tty->read_wait);
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wake_up_interruptible_all(&tty->write_wait);
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ret = __tty_ldisc_lock(tty, timeout);
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ret = __tty_ldisc_lock(tty, timeout);
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if (!ret)
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if (!ret)
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return -EBUSY;
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return -EBUSY;
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@ -337,6 +342,8 @@ int tty_ldisc_lock(struct tty_struct *tty, unsigned long timeout)
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void tty_ldisc_unlock(struct tty_struct *tty)
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void tty_ldisc_unlock(struct tty_struct *tty)
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{
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{
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clear_bit(TTY_LDISC_HALTED, &tty->flags);
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clear_bit(TTY_LDISC_HALTED, &tty->flags);
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/* Can be cleared here - ldisc_unlock will wake up writers firstly */
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clear_bit(TTY_LDISC_CHANGING, &tty->flags);
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__tty_ldisc_unlock(tty);
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__tty_ldisc_unlock(tty);
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}
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}
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@ -366,6 +366,7 @@ struct tty_file_private {
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#define TTY_NO_WRITE_SPLIT 17 /* Preserve write boundaries to driver */
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#define TTY_NO_WRITE_SPLIT 17 /* Preserve write boundaries to driver */
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#define TTY_HUPPED 18 /* Post driver->hangup() */
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#define TTY_HUPPED 18 /* Post driver->hangup() */
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#define TTY_HUPPING 19 /* Hangup in progress */
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#define TTY_HUPPING 19 /* Hangup in progress */
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#define TTY_LDISC_CHANGING 20 /* Change pending - non-block IO */
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#define TTY_LDISC_HALTED 22 /* Line discipline is halted */
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#define TTY_LDISC_HALTED 22 /* Line discipline is halted */
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/* Values for tty->flow_change */
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/* Values for tty->flow_change */
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@ -383,6 +384,12 @@ static inline void tty_set_flow_change(struct tty_struct *tty, int val)
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smp_mb();
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smp_mb();
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}
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}
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static inline bool tty_io_nonblock(struct tty_struct *tty, struct file *file)
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{
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return file->f_flags & O_NONBLOCK ||
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test_bit(TTY_LDISC_CHANGING, &tty->flags);
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}
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static inline bool tty_io_error(struct tty_struct *tty)
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static inline bool tty_io_error(struct tty_struct *tty)
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{
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{
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return test_bit(TTY_IO_ERROR, &tty->flags);
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return test_bit(TTY_IO_ERROR, &tty->flags);
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