Re: [PATCH v5 4/4] kdb: Switch to use safer dbg_io_ops over console APIs

[Sumit Garg]

On Wed, 3 Jun 2020 at 17:12, Daniel Thompson <daniel.thompson@xxxxxxxxxx> wrote:
>
> On Wed, Jun 03, 2020 at 03:02:02PM +0530, Sumit Garg wrote:
> > On Wed, 3 Jun 2020 at 13:55, Petr Mladek <pmladek@xxxxxxxx> wrote:
> > >
> > > On Wed 2020-06-03 12:52:15, Sumit Garg wrote:
> > > > In kgdb context, calling console handlers aren't safe due to locks used
> > > > in those handlers which could in turn lead to a deadlock. Although, using
> > > > oops_in_progress increases the chance to bypass locks in most console
> > > > handlers but it might not be sufficient enough in case a console uses
> > > > more locks (VT/TTY is good example).
> > > >
> > > > Currently when a driver provides both polling I/O and a console then kdb
> > > > will output using the console. We can increase robustness by using the
> > > > currently active polling I/O driver (which should be lockless) instead
> > > > of the corresponding console. For several common cases (e.g. an
> > > > embedded system with a single serial port that is used both for console
> > > > output and debugger I/O) this will result in no console handler being
> > > > used.
> > > >
> > > > In order to achieve this we need to reverse the order of preference to
> > > > use dbg_io_ops (uses polling I/O mode) over console APIs. So we just
> > > > store "struct console" that represents debugger I/O in dbg_io_ops and
> > > > while emitting kdb messages, skip console that matches dbg_io_ops
> > > > console in order to avoid duplicate messages. After this change,
> > > > "is_console" param becomes redundant and hence removed.
> > > >
> > > > diff --git a/drivers/tty/serial/kgdboc.c b/drivers/tty/serial/kgdboc.c
> > > > index 4139698..6e182aa 100644
> > > > --- a/drivers/tty/serial/kgdboc.c
> > > > +++ b/drivers/tty/serial/kgdboc.c
> > > > @@ -558,6 +557,7 @@ static int __init kgdboc_earlycon_init(char *opt)
> > > >       }
> > > >
> > > >       earlycon = con;
> > > > +     kgdboc_earlycon_io_ops.cons = con;
> > > >       pr_info("Going to register kgdb with earlycon '%s'\n", con->name);
> > > >       if (kgdb_register_io_module(&kgdboc_earlycon_io_ops) != 0) {
> > > >               earlycon = NULL;
> > >
> > > Should we clear kgdboc_earlycon_io_ops.cons here when
> > > kgdb_register_io_module() failed?
> > >
> >
> > AFAIK, kgdboc_earlycon_io_ops won't be used at any later stage in case
> > registration fails. So IMO, it would be a redundant assignment unless
> > I missed something.
>
> Or, putting it another way, earlycon is a redundant (albeit better
> maintained) copy of kgdboc_earlycon_io_ops.cons. So I think the best
> thing to do is entirely replace earlycon with
> kgdboc_earlycon_io_ops.cons and then properly set it to NULL!
>

Sounds reasonable, will replace earlycon instead.

-Sumit

>
> Daniel.