linux-kernel - Re: [PATCH v4 3/4] kdb: Make kdb_printf() console handling more robust

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Date:   Tue, 2 Jun 2020 14:32:27 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     Sumit Garg <sumit.garg@...aro.org>
Cc:     Daniel Thompson <daniel.thompson@...aro.org>,
        kgdb-bugreport@...ts.sourceforge.net,
        Jason Wessel <jason.wessel@...driver.com>,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 3/4] kdb: Make kdb_printf() console handling more robust

Hi,

On Fri, May 29, 2020 at 4:27 AM Sumit Garg <sumit.garg@...aro.org> wrote:
>
> While rounding up CPUs via NMIs, its possible that a rounded up CPU
> maybe holding a console port lock leading to kgdb master CPU stuck in
> a deadlock during invocation of console write operations. A similar
> deadlock could also be possible while using synchronous breakpoints.
>
> So in order to avoid such a deadlock, set oops_in_progress to encourage
> the console drivers to disregard their internal spin locks: in the
> current calling context the risk of deadlock is a bigger problem than
> risks due to re-entering the console driver. We operate directly on
> oops_in_progress rather than using bust_spinlocks() because the calls
> bust_spinlocks() makes on exit are not appropriate for this calling
> context.
>
> Suggested-by: Petr Mladek <pmladek@...e.com>
> Signed-off-by: Sumit Garg <sumit.garg@...aro.org>
> ---
>  kernel/debug/kdb/kdb_io.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
>
> diff --git a/kernel/debug/kdb/kdb_io.c b/kernel/debug/kdb/kdb_io.c
> index fad38eb..9e5a40d 100644
> --- a/kernel/debug/kdb/kdb_io.c
> +++ b/kernel/debug/kdb/kdb_io.c
> @@ -566,7 +566,18 @@ static void kdb_msg_write(char *msg, int msg_len)
>         for_each_console(c) {
>                 if (!(c->flags & CON_ENABLED))
>                         continue;
> +               /*
> +                * Set oops_in_progress to encourage the console drivers to
> +                * disregard their internal spin locks: in the current calling
> +                * context the risk of deadlock is a bigger problem than risks
> +                * due to re-entering the console driver. We operate directly on
> +                * oops_in_progress rather than using bust_spinlocks() because
> +                * the calls bust_spinlocks() makes on exit are not appropriate
> +                * for this calling context.
> +                */
> +               ++oops_in_progress;
>                 c->write(c, msg, msg_len);
> +               --oops_in_progress;

This seems sane to me, especially when combined with your next patch
that tries really hard not to run this flow.  ;-)

Reviewed-by: Douglas Anderson <dianders@...omium.org>