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    Subject[PATCH v3 05/23] tty: Remove unnecessary re-test of ldisc ref count
    Date
    Since the tty->ldisc is prevented from being changed by tty_set_ldisc()
    when a tty is being hung up, re-testing the ldisc user count is
    unnecessary -- ie, it cannot be a different ldisc and the user count
    cannot have increased (assuming the caller meets the precondition that
    TTY_LDISC flag is cleared)

    Removal of the 'early-out' locking optimization is necessary for
    the subsequent patch 'tty: Fix ldisc halt sequence on hangup'.

    Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
    ---
    drivers/tty/tty_ldisc.c | 38 +++++++++++++++++++-------------------
    1 file changed, 19 insertions(+), 19 deletions(-)

    diff --git a/drivers/tty/tty_ldisc.c b/drivers/tty/tty_ldisc.c
    index 70d2b86..405e972 100644
    --- a/drivers/tty/tty_ldisc.c
    +++ b/drivers/tty/tty_ldisc.c
    @@ -558,29 +558,29 @@ static int tty_ldisc_halt(struct tty_struct *tty)
    * have been halted for this to guarantee it remains idle.
    *
    * Caller must hold legacy and ->ldisc_mutex.
    + *
    + * NB: tty_set_ldisc() is prevented from changing the ldisc concurrently
    + * with this function by checking the TTY_HUPPING flag.
    */
    static bool tty_ldisc_hangup_wait_idle(struct tty_struct *tty)
    {
    - while (tty->ldisc) { /* Not yet closed */
    - if (atomic_read(&tty->ldisc->users) != 1) {
    - char cur_n[TASK_COMM_LEN], tty_n[64];
    - long timeout = 3 * HZ;
    - tty_unlock(tty);
    -
    - while (tty_ldisc_wait_idle(tty, timeout) == -EBUSY) {
    - timeout = MAX_SCHEDULE_TIMEOUT;
    - printk_ratelimited(KERN_WARNING
    - "%s: waiting (%s) for %s took too long, but we keep waiting...\n",
    - __func__, get_task_comm(cur_n, current),
    - tty_name(tty, tty_n));
    - }
    - /* must reacquire both locks and preserve lock order */
    - mutex_unlock(&tty->ldisc_mutex);
    - tty_lock(tty);
    - mutex_lock(&tty->ldisc_mutex);
    - continue;
    + char cur_n[TASK_COMM_LEN], tty_n[64];
    + long timeout = 3 * HZ;
    +
    + if (tty->ldisc) { /* Not yet closed */
    + tty_unlock(tty);
    +
    + while (tty_ldisc_wait_idle(tty, timeout) == -EBUSY) {
    + timeout = MAX_SCHEDULE_TIMEOUT;
    + printk_ratelimited(KERN_WARNING
    + "%s: waiting (%s) for %s took too long, but we keep waiting...\n",
    + __func__, get_task_comm(cur_n, current),
    + tty_name(tty, tty_n));
    }
    - break;
    + /* must reacquire both locks and preserve lock order */
    + mutex_unlock(&tty->ldisc_mutex);
    + tty_lock(tty);
    + mutex_lock(&tty->ldisc_mutex);
    }
    return !!(tty->ldisc);
    }
    --
    1.8.1.2


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