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Subject[PATCH v3 06/23] tty: Fix ldisc halt sequence on hangup
Date
Flip buffer work cannot be cancelled until all outstanding ldisc
references have been released. Convert the ldisc ref wait into
a full ldisc halt with buffer work cancellation.

Note that the legacy mutex is not held while cancelling.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
---
drivers/tty/tty_ldisc.c | 39 +++++++++++++++++++--------------------
1 file changed, 19 insertions(+), 20 deletions(-)

diff --git a/drivers/tty/tty_ldisc.c b/drivers/tty/tty_ldisc.c
index 405e972..8628265 100644
--- a/drivers/tty/tty_ldisc.c
+++ b/drivers/tty/tty_ldisc.c
@@ -551,22 +551,30 @@ static int tty_ldisc_halt(struct tty_struct *tty)
}

/**
- * tty_ldisc_hangup_wait_idle - wait for the ldisc to become idle
- * @tty: tty to wait for
- *
- * Wait for the line discipline to become idle. The discipline must
- * have been halted for this to guarantee it remains idle.
+ * tty_ldisc_hangup_halt - halt the line discipline for hangup
+ * @tty: tty being hung up
*
+ * Shut down the line discipline and work queue for the tty device
+ * being hungup. Clear the TTY_LDISC flag to ensure no further
+ * references can be obtained, wait for remaining references to be
+ * released, and cancel pending buffer work to ensure no more
+ * data is fed to this ldisc.
* 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.
+ *
+ * NB: if tty->ldisc is NULL then buffer work does not need to be
+ * cancelled because it must already have done as a precondition
+ * of closing the ldisc and setting tty->ldisc to NULL
*/
-static bool tty_ldisc_hangup_wait_idle(struct tty_struct *tty)
+static bool tty_ldisc_hangup_halt(struct tty_struct *tty)
{
char cur_n[TASK_COMM_LEN], tty_n[64];
long timeout = 3 * HZ;

+ clear_bit(TTY_LDISC, &tty->flags);
+
if (tty->ldisc) { /* Not yet closed */
tty_unlock(tty);

@@ -577,6 +585,10 @@ static bool tty_ldisc_hangup_wait_idle(struct tty_struct *tty)
__func__, get_task_comm(cur_n, current),
tty_name(tty, tty_n));
}
+
+ cancel_work_sync(&tty->port->buf.work);
+ set_bit(TTY_LDISC_HALTED, &tty->flags);
+
/* must reacquire both locks and preserve lock order */
mutex_unlock(&tty->ldisc_mutex);
tty_lock(tty);
@@ -851,24 +863,11 @@ void tty_ldisc_hangup(struct tty_struct *tty)
*/
mutex_lock(&tty->ldisc_mutex);

- /*
- * this is like tty_ldisc_halt, but we need to give up
- * the BTM before calling cancel_work_sync, which may
- * need to wait for another function taking the BTM
- */
- clear_bit(TTY_LDISC, &tty->flags);
- tty_unlock(tty);
- cancel_work_sync(&tty->port->buf.work);
- set_bit(TTY_LDISC_HALTED, &tty->flags);
- mutex_unlock(&tty->ldisc_mutex);
- tty_lock(tty);
- mutex_lock(&tty->ldisc_mutex);
-
/* At this point we have a closed ldisc and we want to
reopen it. We could defer this to the next open but
it means auditing a lot of other paths so this is
a FIXME */
- if (tty_ldisc_hangup_wait_idle(tty)) {
+ if (tty_ldisc_hangup_halt(tty)) {
if (reset == 0) {

if (!tty_ldisc_reinit(tty, tty->termios.c_line))
--
1.8.1.2


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