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Date: Tue, 17 Feb 2015 16:28:30 -0500
From: Peter Hurley <peter@...leysoftware.com>
To: Aristeu Rozanski <aris@...hat.com>
CC: linux-kernel@...r.kernel.org,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
Jiri Slaby <jslaby@...e.cz>
Subject: Re: [PATCH] n_tty_read: check for hanging tty while waiting for input
On 02/17/2015 04:06 PM, Aristeu Rozanski wrote:
> If the console has a canonical reader and the respective tty hangs up,
> it'll waste a wake up and will never release the last ldisc reference so
> the hangup process can finish:
This behavior is by-design; /dev/console cannot be hung-up.
> n_tty_read():
> (..)
> add_wait_queue(&tty->read_wait, &wait);
> while (nr) {
> (..)
> if (!input_available_p(tty, 0)) {
> if (test_bit(TTY_OTHER_CLOSED, &tty->flags)) {
> up_read(&tty->termios_rwsem);
> tty_flush_to_ldisc(tty);
> down_read(&tty->termios_rwsem);
> if (!input_available_p(tty, 0)) {
> retval = -EIO;
> break;
> }
> } else {
> -> if (tty_hung_up_p(file))
> break;
> this won't work because file->f_op never gets set to &hung_up_tty_fops:
> __tty_hangup():
>
> spin_lock(&tty_files_lock);
> /* This breaks for file handles being sent over AF_UNIX sockets ? */
> list_for_each_entry(priv, &tty->tty_files, list) {
> filp = priv->file;
> if (filp->f_op->write == redirected_tty_write)
> cons_filp = filp;
> -> if (filp->f_op->write != tty_write)
> -> continue;
> closecount++;
> __tty_fasync(-1, filp, 0); /* can't block */
> -> filp->f_op = &hung_up_tty_fops;
> }
> spin_unlock(&tty_files_lock);
>
> refs = tty_signal_session_leader(tty, exit_session);
> /* Account for the p->signal references we killed */
> while (refs--)
> tty_kref_put(tty);
>
> /*
> * it drops BTM and thus races with reopen
> * we protect the race by TTY_HUPPING
> */
> -> tty_ldisc_hangup(tty);
>
> So while the canonical read waits for input, it'll sleep, be awaken by
> tty_ldisc_hangup() and then immediately going back to sleep without
> dropping the reference to the ldisc gained on tty_read(). This isn't
> noticiable in a non canonical read due that it'll eventually timeout.
>
> The proposed patch checks for TTY_HUPPING flag in order to leave if
> there's no input.
>
> This is easily reproduced by opening /dev/console (my test case was a
> virtual machine with serial console), setting as canonical and waiting
> on a read(). Then, in another session, killing agetty that is running on
> ttyS0 which will issue a hangup.
What process is sleeping on /dev/console read() and what is its controlling
tty? I ask because console teardown usually happens when SIGHUP is
received by the process group.
> [ 240.439045] INFO: task (agetty):1323 blocked for more than 120 seconds.
> [ 240.439569] Not tainted 3.13.0-rc3+ #11
> [ 240.439972] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> [ 240.440596] (agetty) D ffff88007fd94440 0 1323 1 0x00000080
> [ 240.441253] ffff88007bca1c50 0000000000000086 ffff88007989b0c0 0000000000014440
> [ 240.441857] ffff88007bca1fd8 0000000000014440 ffff88007989b0c0 ffff88007989b0c0
> [ 240.442561] ffff88007ad46c30 7fffffffffffffff 0000000000000001 ffff88007ad46c28
> [ 240.443296] Call Trace:
> [ 240.443506] [<ffffffff815c8c99>] schedule+0x29/0x70
> [ 240.443883] [<ffffffff815c7f59>] schedule_timeout+0x209/0x2d0
> [ 240.444395] [<ffffffff810974b5>] ? check_preempt_curr+0x85/0xa0
> [ 240.444850] [<ffffffff810974e9>] ? ttwu_do_wakeup+0x19/0xd0
> [ 240.445343] [<ffffffff8109764d>] ? ttwu_do_activate.constprop.80+0x5d/0x70
> [ 240.445868] [<ffffffff810995eb>] ? try_to_wake_up+0xeb/0x2b0
> [ 240.446363] [<ffffffff815cbdaa>] ldsem_down_write+0xda/0x227
> [ 240.446797] [<ffffffff81099822>] ? default_wake_function+0x12/0x20
> [ 240.447359] [<ffffffff815cc43d>] tty_ldisc_lock_pair_timeout+0x7d/0x100
> [ 240.447861] [<ffffffff8136e519>] tty_ldisc_hangup+0xc9/0x220
> [ 240.448355] [<ffffffff81365463>] __tty_hangup+0x363/0x4b0
> [ 240.448768] [<ffffffff81367cc5>] tty_ioctl+0x865/0xbb0
> [ 240.449219] [<ffffffff811bb52a>] ? do_filp_open+0x3a/0x90
> [ 240.449634] [<ffffffff811bd900>] do_vfs_ioctl+0x2e0/0x4c0
> [ 240.450066] [<ffffffff8124ea76>] ? file_has_perm+0x86/0xa0
> [ 240.450543] [<ffffffff811bdb61>] SyS_ioctl+0x81/0xa0
> [ 240.450921] [<ffffffff815d4b69>] system_call_fastpath+0x16/0x1b
>
> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> Cc: Jiri Slaby <jslaby@...e.cz>
> Cc: Peter Hurley <peter@...leysoftware.com>
> Signed-off-by: Aristeu Rozanski <aris@...hat.com>
>
> diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
> index 0f74945..4fb909d 100644
> --- a/drivers/tty/n_tty.c
> +++ b/drivers/tty/n_tty.c
> @@ -2189,6 +2189,8 @@ static ssize_t n_tty_read(struct tty_struct *tty, struct file *file,
> } else {
> if (tty_hung_up_p(file))
> break;
> + if (test_bit(TTY_HUPPING, &tty->flags))
> + break;
> if (!timeout)
> break;
> if (file->f_flags & O_NONBLOCK) {
>
--
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