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Message-ID: <4DE8DC6B.4070905@gmail.com>
Date: Fri, 03 Jun 2011 15:06:51 +0200
From: Jiri Slaby <jirislaby@...il.com>
To: Jiri Slaby <jslaby@...e.cz>
CC: gregkh@...e.de, linux-kernel@...r.kernel.org,
Alan Cox <alan@...rguk.ukuu.org.uk>, stable@...nel.org,
Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 1/2] TTY: ldisc, do not close until there are readers
On 06/03/2011 03:01 PM, Jiri Slaby wrote:
> We restored tty_ldisc_wait_idle in 100eeae2c5c (TTY: restore
> tty_ldisc_wait_idle). We used it in the ldisc changing path to fix the
> case where there are tasks in n_tty_read waiting for data and somebody
> tries to change ldisc.
>
> Similar to the case above, there may be also tasks waiting in
> n_tty_read. As 65b770468e98 (tty-ldisc: turn ldisc user count into a
> proper refcount) removed the wait-until-idle from all paths, hangup
> path won't wait for them to disappear either. So add it back even to
> the hangup path.
>
> There is a difference, we need uninterruptible sleep as there is
> obviously HUP signal pending. So tty_ldisc_wait_idle now takes
> interruptible parameter to decide. I'm not sure whether it's worth the
> complexity. Instead uninterruptible waiting may fit even for the ldisc
> change code. It wouldn't be possible to interrupted the change, but
> there is 5s timeout anyway.
I've detypoed the text locally, but will wait if there are any comments
on the code before resending.
> Before 65b770468e98 tty_ldisc_release was called also from
> tty_ldisc_release. It is called from tty_release, so I don't think we
> need to restore that one.
>
> This is nicely reproducible after constify the timing as follows:
> --- a/drivers/tty/n_tty.c
> +++ b/drivers/tty/n_tty.c
> @@ -1548,6 +1549,7 @@ static int n_tty_open(struct tty_struct *tty)
>
> /* These are ugly. Currently a malloc failure here can panic */
> if (!tty->read_buf) {
> + msleep(100);
> tty->read_buf = kzalloc(N_TTY_BUF_SIZE, GFP_KERNEL);
> if (!tty->read_buf)
> return -ENOMEM;
> @@ -1741,6 +1743,7 @@ do_it_again:
>
> add_wait_queue(&tty->read_wait, &wait);
> while (nr) {
> + WARN_ON(!tty->read_buf);
> /* First test for status change. */
> if (packet && tty->link->ctrl_status) {
> unsigned char cs;
> @@ -1785,6 +1788,7 @@ do_it_again:
> break;
> }
> timeout = schedule_timeout(timeout);
> + msleep(20);
> continue;
> }
> __set_current_state(TASK_RUNNING);
>
> ==================================
>
> With a process:
> while (1) {
> int fd = open(argv[1], O_RDWR);
> read(fd, buf, sizeof(buf));
> close(fd);
> }
> ===== and its child: =====
> setsid();
> while (1) {
> int fd = open(tty, O_RDWR|O_NOCTTY);
> ioctl(fd, TIOCSCTTY, 1);
> vhangup();
> close(fd);
> usleep(100 * (10 + random() % 1000));
> }
>
> References: https://bugzilla.novell.com/show_bug.cgi?id=693374
> References: https://bugzilla.novell.com/show_bug.cgi?id=694509
> Signed-off-by: Jiri Slaby <jslaby@...e.cz>
> Cc: Alan Cox <alan@...rguk.ukuu.org.uk>
> Cc: stable@...nel.org [32, 33, 34, 39]
> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> ---
> drivers/tty/tty_ldisc.c | 34 +++++++++++++++++++++++++++-------
> 1 files changed, 27 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/tty/tty_ldisc.c b/drivers/tty/tty_ldisc.c
> index 5d01d32..3c01bf0 100644
> --- a/drivers/tty/tty_ldisc.c
> +++ b/drivers/tty/tty_ldisc.c
> @@ -552,14 +552,32 @@ static void tty_ldisc_flush_works(struct tty_struct *tty)
> * Wait for the line discipline to become idle. The discipline must
> * have been halted for this to guarantee it remains idle.
> */
> -static int tty_ldisc_wait_idle(struct tty_struct *tty)
> +static int tty_ldisc_wait_idle(struct tty_struct *tty, bool interruptible)
> {
> + DEFINE_WAIT(wait);
> + long timeout = 5 * HZ;
> + int state = interruptible ? TASK_INTERRUPTIBLE : TASK_UNINTERRUPTIBLE;
> int ret;
> - ret = wait_event_interruptible_timeout(tty_ldisc_idle,
> - atomic_read(&tty->ldisc->users) == 1, 5 * HZ);
> - if (ret < 0)
> - return ret;
> - return ret > 0 ? 0 : -EBUSY;
> +
> + for (;;) {
> + prepare_to_wait(&tty_ldisc_idle, &wait, state);
> + if (atomic_read(&tty->ldisc->users) == 1) {
> + ret = 0;
> + break;
> + }
> + if (interruptible && signal_pending(current)) {
> + ret = -ERESTARTSYS;
> + break;
> + }
> +
> + timeout = schedule_timeout(timeout);
> + if (!timeout) {
> + ret = -EBUSY;
> + break;
> + }
> + }
> + finish_wait(&tty_ldisc_idle, &wait);
> + return ret;
> }
>
> /**
> @@ -666,7 +684,7 @@ int tty_set_ldisc(struct tty_struct *tty, int ldisc)
>
> tty_ldisc_flush_works(tty);
>
> - retval = tty_ldisc_wait_idle(tty);
> + retval = tty_ldisc_wait_idle(tty, 1);
>
> tty_lock();
> mutex_lock(&tty->ldisc_mutex);
> @@ -763,6 +781,8 @@ static int tty_ldisc_reinit(struct tty_struct *tty, int ldisc)
> if (IS_ERR(ld))
> return -1;
>
> + WARN_ON_ONCE(tty_ldisc_wait_idle(tty, 0));
> +
> tty_ldisc_close(tty, tty->ldisc);
> tty_ldisc_put(tty->ldisc);
> tty->ldisc = NULL;
--
js
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