Re: [PATCH 2/5] vt: Fix up unregistration of vt drivers

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Fri, Jun 06, 2014 at 11:40:50AM +0200, Daniel Vetter wrote:
> On Fri, Jun 06, 2014 at 10:47:25AM +0200, David Herrmann wrote:
> > Hi
> > 
> > On Fri, Jun 6, 2014 at 9:56 AM, Daniel Vetter <daniel@ffwll.ch> wrote:
> > > On Fri, Jun 06, 2014 at 09:24:35AM +0200, David Herrmann wrote:
> > >> Hi
> > >>
> > >> On Thu, Jun 5, 2014 at 4:58 PM, Daniel Vetter <daniel.vetter@ffwll.ch> wrote:
> > >> > A bunch of issues:
> > >> > - We should not kick out the default console (which is tracked in
> > >> >   conswitchp), so check for that.
> > >> > - Add better error codes so callers can differentiate between "something
> > >> >   went wrong" and "your driver isn't registered already". i915 needs
> > >> >   that so it doesn't fall over when reloading the driver and hence
> > >> >   vga_con is already unregistered.
> > >> > - There's a mess with the driver flags: What we need to check for is
> > >> >   that the driver isn't used any more, i.e. unbound completely (FLAG_INIT).
> > >> >   And not whether it's the boot console or not (which is the only one
> > >> >   which doesn't have FLAG_MODULE). Otherwise there's no way to kick
> > >> >   out the boot console, which i915 wants to do to prevent havoc with
> > >> >   vga_con interferring (which tends to hang machines).
> > >> >
> > >> > Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> > >> > Cc: Jiri Slaby <jslaby@suse.cz>
> > >> > Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
> > >> > ---
> > >> >  drivers/tty/vt/vt.c | 16 +++++++++-------
> > >> >  1 file changed, 9 insertions(+), 7 deletions(-)
> > >> >
> > >> > diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
> > >> > index ea600f482eeb..5077fe87324d 100644
> > >> > --- a/drivers/tty/vt/vt.c
> > >> > +++ b/drivers/tty/vt/vt.c
> > >> > @@ -3573,17 +3573,20 @@ err:
> > >> >   */
> > >> >  int do_unregister_con_driver(const struct consw *csw)
> > >> >  {
> > >> > -       int i, retval = -ENODEV;
> > >> > +       int i;
> > >> >
> > >> >         /* cannot unregister a bound driver */
> > >> >         if (con_is_bound(csw))
> > >> > -               goto err;
> > >> > +               return -EBUSY;
> > >> > +
> > >> > +       if (csw == conswitchp)
> > >> > +               return -EINVAL;
> > >>
> > >> Ugh, that fix is correct, but I'd rather like to see
> > >> do_unbind_con_driver() do the right thing. It currently resets
> > >> conswitchp _only_ if the new fallback is unbound. Why not _always_ set
> > >> conswitchp to defcsw _iff_ conswitchp == csw there?
> > >
> > > Ha, that's what I've thought, too. But do_unbind doesn't actually change
> > > conswitchp, it only restores it because apparently the
> > > vga_con->con_startup function is a real con and changes it behind
> > > everyones back for no good reason. Or at least that's what the comment
> > > claims. Note how defconsw != defcsw ...
> > >
> > > I've tried to follow around how conswitchp is actually used, but besides
> > > that it's used to select the boot console I'm not sure at all what's going
> > > hence.
> > 
> > I missed that line:
> >   const struct consw *defconsw = conswitchp;
> > 
> > Now that I looked at it again, this looks _really_ wrong. If
> > conswitchp is vgacon and startup fails, we keep it set to vgacon..
> > ugh, weird. But ok, that's up to the people who wrote that.
> > 
> > Btw., conswitchp is used for visual_init(), that is, every new VT
> > allocation gets assigned to conswitchp. Therefore, it must be a valid
> > driver. (VTs are allocated and deallocated by agetty/systemd after a
> > session is started/ended).
> 
> Ah, I missed where conswitchp was used in the maze, was getting a bit hazy
> ;-)
> 
> Rules for changing/updateing conswitchp are indeed a bit strange, which is
> why I really don't want to touch this too much.
> 
> > >> This way, you _know_ here that if !con_is_bound(csw), then csw != conswitchp.
> > >
> > > Hence why I dropped this approach again (I've done it originally) and
> > > opted for the straightforward but albeit crude direct check.
> > >
> > >> >
> > >> >         for (i = 0; i < MAX_NR_CON_DRIVER; i++) {
> > >> >                 struct con_driver *con_driver = &registered_con_driver[i];
> > >> >
> > >> >                 if (con_driver->con == csw &&
> > >> > -                   con_driver->flag & CON_DRIVER_FLAG_MODULE) {
> > >> > +                   con_driver->flag & CON_DRIVER_FLAG_INIT) {
> > >>
> > >> That makes FLAG_MODULE almost a no-op except for ->unbind(). I wonder
> > >> why FLAG_MODULE exists, anyway.
> > >
> > > I've dug around in git history and it's less than useful. It was renamed
> > > from FLAG_BIND (which makes somewhat sense, since it roughly tracks
> > > whether a console is bound). But there's never been a justification for
> > > it, neither in the original patch nor in the one that renamed it.
> > >
> > > So I decided to not tempt fate and went with the small change here that
> > > I've understood somewhat (I've tried other, more invasive changes and
> > > failed).
> > >
> > >> Otherwise looks good.
> > >
> > > I'm really reluctant to do the right thing here since the code overall has
> > > very unclear semantics with conswitchp and FLAG_MODULE. Can I convince
> > > yout that the more direct approach here is the right one?
> > 
> > Yeah, patch looks good.
> > 
> > Reviewed-by: David Herrmann <dh.herrmann@gmail.com>
> 
> Thanks for the review.
> 
> Greg, are you ok with these 3 vt patches and with me merging them through
> drm-intel trees? We need them to properly kick out vgacon, which tends to
> result in machine hangs (mostly across module reload, but also on driver
> load on really unlucky systems) and other havoc.

Yes, no objection from me at all, please feel free to add:

Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

to them.

greg k-h