2010-04-06 11:44:17

by Anton Blanchard

[permalink] [raw]
Subject: Re: [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 <[email protected]>
---

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);


2010-04-06 12:11:49

by Amit Shah

[permalink] [raw]
Subject: Re: [PATCH 4/7] hvc_console: Fix race between hvc_close and hvc_remove

On (Tue) Apr 06 2010 [21:42:38], Anton Blanchard wrote:
>
> 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.

OK, makes sense and since it works for you,

Acked-by: Amit Shah <[email protected]>

> 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 <[email protected]>

2010-04-06 12:27:53

by Sachin Sant

[permalink] [raw]
Subject: Re: [PATCH 4/7] hvc_console: Fix race between hvc_close and hvc_remove

Anton Blanchard wrote:
> 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 <[email protected]>
> ---
>
Works for me. Thanks Anton.

Regards
-Sachin

> 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);
>
>
>


--

---------------------------------
Sachin Sant
IBM Linux Technology Center
India Systems and Technology Labs
Bangalore, India
---------------------------------

2010-04-06 12:30:07

by Alan

[permalink] [raw]
Subject: Re: [PATCH 4/7] hvc_console: Fix race between hvc_close and hvc_remove

> 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.

They should yes.

2010-04-08 00:26:41

by Rusty Russell

[permalink] [raw]
Subject: Re: [PATCH 4/7] hvc_console: Fix race between hvc_close and hvc_remove

On Tue, 6 Apr 2010 09:12:38 pm Anton Blanchard wrote:
> The patch below changes things to call tty_kref_put once for every
> hvc_close call, and with that my machine boots fine.

Thanks, applied.

Cheers,
Rusty.