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

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Date:	Tue, 6 Apr 2010 17:39:05 +0530
From:	Amit Shah <amit.shah@...hat.com>
To:	Anton Blanchard <anton@...ba.org>
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

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 <amit.shah@...hat.com>

> 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>
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