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


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