Re: [PATCH] kgdboc: Be a bit more robust about handling earlycon leaving

[Doug Anderson]

Hi,

On Fri, May 1, 2020 at 6:32 AM Daniel Thompson
<daniel.thompson@xxxxxxxxxx> wrote:
>
> On Fri, May 01, 2020 at 12:49:43PM +0100, Daniel Thompson wrote:
> > On Thu, Apr 30, 2020 at 09:59:09AM -0700, Douglas Anderson wrote:
> > > The original implementation of kgdboc_earlycon included a comment
> > > about how it was impossible to get notified about the boot console
> > > going away without making changes to the Linux core.  Since folks
> > > often don't want to change the Linux core for kgdb's purposes, the
> > > kgdboc_earlycon implementation did a bit of polling to figure out when
> > > the boot console went away.
> > >
> > > It turns out, though, that it is possible to get notified about the
> > > boot console going away.  The solution is either clever or a hack
> > > depending on your viewpoint.  ...or, perhaps, a clever hack.  All we
> > > need to do is head-patch the "exit" routine of the boot console.  We
> > > know that "struct console" must be writable because it has a "next"
> > > pointer in it, so we can just put our own exit routine in, do our
> > > stuff, and then call back to the original.
> >
> > I think I'm in the hack camp on this one!
> >
> >
> > > This works great to get notified about the boot console going away.
> > > The (slight) problem is that in the context of the boot console's exit
> > > routine we can't call tty_find_polling_driver().
> >
> > I presume this is something to do with the tty_mutex?
> > > We solve this by
> > > kicking off some work on the system_wq when we get notified and this
> > > works pretty well.
> >
> > There are some problems with the workqueue approach.
>
> ... so did a couple of experiments to avoid the workqueue.
>
> It occured to me that, since we have interfered with deinit() then the
> console hasn't actually been uninitialized meaning we could still use it.
> This does exposes us to risks similar to keep_bootcon but in exchange
> there is no window where kgdb is broken (and no need to panic).
>
> My prototype is minimal but I did wonder about ripping out all the
> code to defend against removal of the earlycon and simply keep the
> earlycon around until a new kgdbio handler is installed.

It took me a little while, but I finally see what you're saying.
You're saying that we keep calling into the boot console even though
it's no longer in the list of consoles.  Then we temporarily disable
the boot console's exit routine until kgdb_earlycon() is done.  (side
note: the exit routine was recently added and probably most consoles
don't use it).

OK, that doesn't seem totally insane.  It actually works OK for you?

It's probably at least worth a warning in the log if we detect that
we're using the boot console and it's not in the console list anymore.
Then if kgdb starts misbehaving someone might have a clue.

If your solution is OK we might also want to remove the call to
cleanup_earlycon_if_invalid() in configure_kgdboc() too.

I think you might win the "hackiest solution" prize, but your solution
definitely does seem better because I can't think of any other good
way to handle people whose consoles register a long time before their
tty.  ;-)


-Doug