linux-kernel - Re: [PATCH 4/7] hvc_console: Fix race between hvc_close and hvc

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Date:	Tue, 6 Apr 2010 21:42:38 +1000
From:	Anton Blanchard <anton@...ba.org>
To:	Amit Shah <amit.shah@...hat.com>
Cc:	Sachin Sant <sachinp@...ibm.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Greg Kroah-Hartman <gregkh@...e.de>, a.p.zijlstra@...llo.nl,
	Rusty Russell <rusty@...tcorp.com.au>,
	linux-kernel@...r.kernel.org, linuxppc-dev@...abs.org,
	tglx@...utronix.de, Linus Torvalds <torvalds@...ux-foundation.org>,
	mingo@...e.hu, Alan Cox <alan@...rguk.ukuu.org.uk>
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 <anton@...ba.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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