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

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Date:	Fri, 7 Aug 2009 10:04:29 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	linux-kernel@...r.kernel.org, Christoph Hellwig <hch@....de>,
	Andi Kleen <andi@...stfloor.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Greg Kroah-Hartman <gregkh@...e.de>
Subject: Re: [PATCH 3/5] tty: handle VT specific compat ioctls in vt driver

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!

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