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

[Sumit Garg]

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.

-Sumit

> > diff --git a/include/linux/kgdb.h b/include/linux/kgdb.h
> > index c62d764..529116b 100644
> > --- a/include/linux/kgdb.h
> > +++ b/include/linux/kgdb.h
> > @@ -276,8 +276,7 @@ struct kgdb_arch {
> >   * the I/O driver.
> >   * @post_exception: Pointer to a function that will do any cleanup work
> >   * for the I/O driver.
> > - * @is_console: 1 if the end device is a console 0 if the I/O device is
> > - * not a console
> > + * @cons: valid if the I/O device is a console; else NULL.
> >   */
> >  struct kgdb_io {
> >       const char              *name;
> > @@ -288,7 +287,7 @@ struct kgdb_io {
> >       void                    (*deinit) (void);
> >       void                    (*pre_exception) (void);
> >       void                    (*post_exception) (void);
> > -     int                     is_console;
> > +     struct console          *cons;
>
> Nit: I would call it "con". The trailing 's' makes me feel that that the
>      variable points to an array or list of consoles.
>
> >  };
> >
>
> Otherwise, it looks pretty straightforward.
>
> Best Regards,
> Petr