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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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