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SubjectRe: WARNING at: drivers/char/tty_ldisc.c


On Mon, 3 Aug 2009, OGAWA Hirofumi wrote:
>
> In this case, what is happening seems to be simple.
>
> 2735 tty1 Ss 0:0 init [S] <- session leader
> 2736 tty1 S 0:0 \_ bash
>
> Sequence is,
>
> bash tty_read()
> tty_ldisc_ref_wait() <- take refcount
> n_tty_read()
> schedule_timeout()
>
> init [S] do_exit()
> [...]
> do_tty_hangup()
> tty_ldisc_hangup()
> wake_up_interruptible_poll(read_wait)
>
> bash /* n_tty_read() can't see the hangup state of tty,
> * because anybody don't teach it to tty or ldisc */
> schedule_timeout() <- wait again

Hmm. Wouldn't it trigger on tty_hung_up_p(file)?

[ Reading further.. ]

> And another related point which I'm don't know is why we don't change
> console_fops to hung_up_tty_fops in do_tty_hangup() in the below.

Yup, you're right. Because console_fops has

.write = redirected_tty_write,

we won't actually hang up the console due to that test for "write !=
tty_write".

That's just a classic example of some of the crazy hacks we have in the
tty layers. I do wonder whether it's even necessary any more. Maybe we
could just hang things up forcefully now and get rid of that console
handling special case.

But I guess that all does explain why it only happens in single-user mode.

So exactly what _does_ happen if we get rid of that hack?

Linus

---
drivers/char/tty_io.c | 19 +------------------
1 files changed, 1 insertions(+), 18 deletions(-)

diff --git a/drivers/char/tty_io.c b/drivers/char/tty_io.c
index a3afa0c..80540ec 100644
--- a/drivers/char/tty_io.c
+++ b/drivers/char/tty_io.c
@@ -496,10 +496,8 @@ static void do_tty_hangup(struct work_struct *work)
{
struct tty_struct *tty =
container_of(work, struct tty_struct, hangup_work);
- struct file *cons_filp = NULL;
struct file *filp, *f = NULL;
struct task_struct *p;
- int closecount = 0, n;
unsigned long flags;
int refs = 0;

@@ -520,11 +518,6 @@ static void do_tty_hangup(struct work_struct *work)
file_list_lock();
/* This breaks for file handles being sent over AF_UNIX sockets ? */
list_for_each_entry(filp, &tty->tty_files, f_u.fu_list) {
- 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;
}
@@ -574,17 +567,7 @@ static void do_tty_hangup(struct work_struct *work)
while (refs--)
tty_kref_put(tty);

- /*
- * If one of the devices matches a console pointer, we
- * cannot just call hangup() because that will cause
- * tty->count and state->count to go out of sync.
- * So we just call close() the right number of times.
- */
- if (cons_filp) {
- if (tty->ops->close)
- for (n = 0; n < closecount; n++)
- tty->ops->close(tty, cons_filp);
- } else if (tty->ops->hangup)
+ if (tty->ops->hangup)
(tty->ops->hangup)(tty);
/*
* We don't want to have driver/ldisc interactions beyond

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