linux-kernel - Re: [PATCH] vt: avoid BUG_ON in con_shutdown when con

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Date:	Thu, 31 Mar 2011 10:37:26 -0300
From:	Herton Ronaldo Krzesinski <herton.krzesinski@...onical.com>
To:	Alan Cox <alan@...ux.intel.com>
Cc:	linux-kernel@...r.kernel.org, Greg Kroah-Hartman <gregkh@...e.de>
Subject: Re: [PATCH] vt: avoid BUG_ON in con_shutdown when con_open returns
 with error

On Thu, Mar 31, 2011 at 10:58:11AM +0100, Alan Cox wrote:
> O
> > +static inline void con_ops_set_shutdown(void);
> >  static int con_open(struct tty_struct *, struct file *);
> >  static void vc_init(struct vc_data *vc, unsigned int rows,
> >  		    unsigned int cols, int do_clear);
> > @@ -2806,6 +2807,14 @@ static int con_open(struct tty_struct *tty,
> > struct file *filp) tty->driver_data = vc;
> >  			vc->port.tty = tty;
> >  
> > +			/* We must set shutdown only here, otherwise
> > +			 * we returned from con_open with error,
> > which
> > +			 * will make tty core call tty_release, that
> > +			 * in its call path makes con_shutdown being
> > +			 * called without tty->driver_data being set,
> > +			 * triggering the BUG_ON there */
> > +			con_ops_set_shutdown();
> 
> No we cannot go around patching the tty_operations - they are not
> locked for one.

Indeed, I feel in those "wearing a brown paper bag" moments now...

> 
> Probably this is one case where making con_shutdown() check is the
> right answer.
> 

I still wonder though if really the BUG_ON is ok, or I still didn't got
that code right. Even if it returns -ERESTARTSYS we will hit that
BUG_ON, if vc->port.tty != tty no? Or that is what the BUG_ON is trying
to catch?

While looking I noted a minor thing, in tty_open we don't need to check
retval again in one place, as !retval is always true, because we return
earlier (in retval = tty_add_file ...):

diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index 936a4ea..349fa67 100644
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -1902,12 +1902,10 @@ got_driver:
 #ifdef TTY_DEBUG_HANGUP
 	printk(KERN_DEBUG "opening %s...", tty->name);
 #endif
-	if (!retval) {
-		if (tty->ops->open)
-			retval = tty->ops->open(tty, filp);
-		else
-			retval = -ENODEV;
-	}
+	if (tty->ops->open)
+		retval = tty->ops->open(tty, filp);
+	else
+		retval = -ENODEV;
 	filp->f_flags = saved_flags;
 
 	if (!retval && test_bit(TTY_EXCLUSIVE, &tty->flags) &&

-- 
[]'s
Herton
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