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    SubjectRe: [PATCH v5 4/4] kdb: Switch to use safer dbg_io_ops over console APIs
    On Wed, 3 Jun 2020 at 17:12, Daniel Thompson <daniel.thompson@linaro.org> 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@suse.com> 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.

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