Re: [PATCH 3/5] tty: handle VT specific compat ioctls in vt driver

Frederic Weisbecker
2009-08-07T08:04:37Z

On Fri, Aug 07, 2009 at 09:04:11AM +0200, Arnd Bergmann wrote:
> On Friday 07 August 2009, Frederic Weisbecker wrote:
>
> > On Thu, Aug 06, 2009 at 03:09:28PM +0200, Arnd Bergmann wrote:
> > > The VT specific compat_ioctl handlers are the only ones
> > > in common code that require the BKL. Moving them into
> > > the vt driver lets us remove the BKL from the other handlers
> > > and cleans up the code.
> >
> >
> > Why does it require the bkl?
> >
>
> All the VT ioctls are currently called under the BKL. I did not try
> to find out why that is but simply kept that state. All other
> compat ioctl do not interact with device driver state at all,
> so they obviously do not need the BKL.

Ah ok.

> > > +
> > > +long vt_compat_ioctl(struct tty_struct *tty, struct file * file,
> > > + unsigned int cmd, unsigned long arg)
> > > +{
> > > + struct vc_data *vc = tty->driver_data;
> > > + struct console_font_op op; /* used in multiple places here */
> > > + struct kbd_struct *kbd;
> > > + unsigned int console;
> > > + void __user *up = (void __user *)arg;
> > > + int perm;
> > > + int ret = 0;
> > > +
> > > + console = vc->vc_num;
> > > +
> > > + lock_kernel();
> >
> >
> >
> > It would be really nice to add a comment here that explain what it
> > is protecting.
> > I would like to work on removing the bkl from tty, and such nude lock_kernel()
> > don't help much to work in this area.
> > This is more than ever a FUD lock, nothing about its role in tty in the Lockronomicon,
> > and even not in the comments :-)
>
> This function is a straight copy from vt_ioctl, with the data structures replaced,
> and calling it vt_ioctl where they are identical.
>
> You are right that this needs more code comments to make that obvious.
>
> Arnd <><

Ok. This looks like a nice series. A bkl pushdown that only goes down
in one site among several others enlightens the understanding of what it
is protecting (beside the nice fact it also burned three bkl callsites :-)

Thanks!