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SubjectRe: [PATCH 4/7] hvc_console: Fix race between hvc_close and hvc_remove

Hi,

> > Looking at the commit e74d098c66543d0731de62eb747ccd5b636a6f4c,
> > i see that for every tty_kref_get() there is a corresponding
> > tty_kref_put() except maybe for the one in the following patch snippet
> >
> > spin_lock_irqsave(&hp->lock, flags);
> > /* Check and then increment for fast path open. */
> > if (hp->count++ > 0) {
> > + tty_kref_get(tty);
> > spin_unlock_irqrestore(&hp->lock, flags);
> > hvc_kick();
> > return 0;
> >
> > I don't know this code very well but we might be missing a
> > corresponding tty_kref_put() some place ?
>
> See hvc_hangup:
>
> temp_open_count = hp->count;
> ...
> while(temp_open_count) {
> --temp_open_count;
> tty_kref_put(tty);
> kref_put(&hp->kref, destroy_hvc_struct);
> }

I don't claim to understand the tty layer, but it seems like hvc_open and
hvc_close should be balanced in their kref reference counting.

Right now we get a kref every call to hvc_open:

if (hp->count++ > 0) {
tty_kref_get(tty); <----- here
spin_unlock_irqrestore(&hp->lock, flags);
hvc_kick();
return 0;
} /* else count == 0 */
tty->driver_data = hp;

hp->tty = tty_kref_get(tty); <------ or here if hp->count was 0
But hvc_close has:

tty_kref_get(tty);

if (--hp->count == 0) {
...
/* Put the ref obtained in hvc_open() */
tty_kref_put(tty);
...
}
tty_kref_put(tty);

Since the outside kref get/put balance we only do a single kref_put when
count reaches 0.

The patch below changes things to call tty_kref_put once for every
hvc_close call, and with that my machine boots fine.

Signed-off-by: Anton Blanchard <anton@samba.org>
---
diff --git a/drivers/char/hvc_console.c b/drivers/char/hvc_console.c
index d3890e8..35cca4c 100644
--- a/drivers/char/hvc_console.c
+++ b/drivers/char/hvc_console.c
@@ -368,16 +368,12 @@ static void hvc_close(struct tty_struct *tty, struct file * filp)
hp = tty->driver_data;

spin_lock_irqsave(&hp->lock, flags);
- tty_kref_get(tty);

if (--hp->count == 0) {
/* We are done with the tty pointer now. */
hp->tty = NULL;
spin_unlock_irqrestore(&hp->lock, flags);

- /* Put the ref obtained in hvc_open() */
- tty_kref_put(tty);
-
if (hp->ops->notifier_del)
hp->ops->notifier_del(hp, hp->data);


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