Re: [patch 1/3] tty: resolve tty contention between kernel and user space

From: Andy Shevchenko <andy.shevchenko@gmail.com>
To: Okash Khawaja <okash.khawaja@gmail.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Jiri Slaby <jslaby@suse.com>,
	Samuel Thibault <samuel.thibault@ens-lyon.org>,
	Alan Cox <gnomes@lxorguk.ukuu.org.uk>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	William Hubbs <w.d.hubbs@gmail.com>,
	Chris Brannon <chris@the-brannons.com>,
	Kirk Reiser <kirk@reisers.ca>,
	speakup@linux-speakup.org, devel@driverdev.osuosl.org
Subject: Re: [patch 1/3] tty: resolve tty contention between kernel and user space
Date: Sun, 9 Jul 2017 18:04:17 +0300	[thread overview]
Message-ID: <CAHp75VdwWNj8PHRAMLqcQKhtT=sS5-n9T3n8GY1Rte=MKOQV-w@mail.gmail.com> (raw)
In-Reply-To: <20170709114426.618570903@gmail.com>

On Sun, Jul 9, 2017 at 2:41 PM, Okash Khawaja <okash.khawaja@gmail.com> wrote:

> +struct tty_struct *tty_kopen(dev_t device)
> +{
> +       struct tty_struct *tty;
> +       struct tty_driver *driver = NULL;
> +       int index = -1;
> +
> +       mutex_lock(&tty_mutex);
> +       driver = tty_lookup_driver(device, NULL, &index);
> +       if (IS_ERR(driver)) {

> +               mutex_unlock(&tty_mutex);
> +               return ERR_CAST(driver);

Hmm... perhaps

tty = ERR_CAST(driver);
goto out_unlock;

See below for further details.

> +       }
> +
> +       /* check whether we're reopening an existing tty */
> +       tty = tty_driver_lookup_tty(driver, NULL, index);
> +       if (IS_ERR(tty))
> +               goto out;
> +
> +       if (tty) {
> +               /* drop kref from tty_driver_lookup_tty() */
> +               tty_kref_put(tty);
> +               tty = ERR_PTR(-EBUSY);
> +       } else { /* tty_init_dev returns tty with the tty_lock held */
> +               tty = tty_init_dev(driver, index);
> +               set_bit(TTY_KOPENED, &tty->flags);
> +       }

> +out:

out_unlock: ?

> +       mutex_unlock(&tty_mutex);
> +       tty_driver_kref_put(driver);

I'm not sure I understand locking model here:

Above
1. take mutex
2. take reference

Here:
1. give mutex back
2. give reference back

I think we usually see symmetrical  calls, i.e.

1. give reference back
2. give mutex back

So, what did I miss?

> +       return tty;
> +}

-- 
With Best Regards,
Andy Shevchenko