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From: Christoph Hellwig on 13 Apr 2010 14:10
On Mon, Apr 12, 2010 at 07:34:17PM +0200, Arnd Bergmann wrote:
> > - make sure every file operation either has a ->llseek instead or
> > calls nonseekable_open from ->open
>
> I still think it would be better to always set llseek if we do that,
> even if nonseekable_open is already there. I can come up with scripts
> that check that case, but checking that the open function always
> calls nonseekable_open when it returns success is beyond my grep
> skills ;-)

Yes, it's not quite easily greppable. Making no seek allowed the
implicit default will fortunately allow us to get rid of that oddness.

> > - walk through the instances now using default_llseek and chose
> > a better implementation for this particular instance. Often
> > this will be just removing the the lssek method as not allowing
> > seeks is the right thing to do for character drivers, even if it
> > is a behaviour change from the current version which usually
> > is the result of sloppy coding.
>
> This part is really hard. While in many cases, the driver maintainer
> might know what user space is potentially opening some character
> device, it's really hard to tell for outsiders whether the behaviour
> should be no_llseek (then the default) or noop_llseek to work around
> broken user space.

That's why it's last on the list.

> I think the rule set for the conversion needs to be one that can
> be done purely based on the code. How about this:
>
> For each file operation {
> if (uses f_pos) {
> if (same module uses BKL)
> -> default_llseek
> else
> -> generic_file_llseek
> } else {
> if (driver maintained)
> -> no_llseek (with maintainer ACK)
> else
> -> noop_llseek
> }
> }
>
> Once that is done, we can turn the default into nonseekable
> behavior and start removing instances of explicit no_llseek
> and nonseekable_open.

That plan sounds good to me.

> Should we also rename default_llseek to deprecated_llseek in the
> process, to go along with the approach for ioctl?

I wouldn't bother. If you can actually work on your plan default_llseek
should be gone soon enough.

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From: Frederic Weisbecker on 13 Apr 2010 16:50
On Tue, Apr 13, 2010 at 11:26:27AM +0200, Arnd Bergmann wrote:
> On Monday 12 April 2010, Frederic Weisbecker wrote:
> > On Mon, Apr 12, 2010 at 07:34:17PM +0200, Arnd Bergmann wrote:
> > >
> > > I think the rule set for the conversion needs to be one that can
> > > be done purely based on the code. How about this:
> > >
> > > For each file operation {
> > > if (uses f_pos) {
> > > if (same module uses BKL)
> > > -> default_llseek
> > > else
> > > -> generic_file_llseek
> > > } else {
> > > if (driver maintained)
> > > -> no_llseek (with maintainer ACK)
> > > else
> > > -> noop_llseek
> > > }
> > > }
> >
> > It is also hard to determine a given driver really doesn't use
> > the bkl. A sole lock_kernel() grep in its files is not sufficient.
> > But a manual second pass should do the trick.
>
> Why not? In my 2.6.33 based series, I have removed all implicit
> uses of the BKL, so we can be sure that it doesn't use the BKL
> unless the module is part of that series. The only two cases
> I can think of are:
>
> - ioctl callback, which we should do in the same change, like I
> originally did. If a driver defines ->ioctl(), make it use
> deprecated_ioctl() and default_llseek()/deprecated_llseek.
>
> - Any of the file systems from Jan's series.
>
> Arnd


Ok looks like a good plan then.

Thanks.

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