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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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