linux-kernel - Re: [PATCH v3 3/4] kdb: Make kdb_printf robust to run in NMI context

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Date:   Wed, 27 May 2020 15:26:17 +0100
From:   Daniel Thompson <daniel.thompson@...aro.org>
To:     Sumit Garg <sumit.garg@...aro.org>
Cc:     kgdb-bugreport@...ts.sourceforge.net, jason.wessel@...driver.com,
        dianders@...omium.org, pmladek@...e.com,
        sergey.senozhatsky@...il.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 3/4] kdb: Make kdb_printf robust to run in NMI context

On Wed, May 27, 2020 at 11:55:58AM +0530, Sumit Garg wrote:
> While rounding up CPUs via NMIs, its possible that a rounded up CPU

This problem does not just impact NMI roundup (breakpoints, including
implicit breakpoint-on-oops can have the same effect).


> maybe holding a console port lock leading to kgdb master CPU stuck in
> a deadlock during invocation of console write operations. So in order
> to avoid such a deadlock, enable oops_in_progress prior to invocation
> of console handlers.
> 
> Suggested-by: Petr Mladek <pmladek@...e.com>
> Signed-off-by: Sumit Garg <sumit.garg@...aro.org>
> ---
>  kernel/debug/kdb/kdb_io.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/kernel/debug/kdb/kdb_io.c b/kernel/debug/kdb/kdb_io.c
> index 349dfcc..f848482 100644
> --- a/kernel/debug/kdb/kdb_io.c
> +++ b/kernel/debug/kdb/kdb_io.c
> @@ -566,7 +566,17 @@ static void kdb_msg_write(char *msg, int msg_len)
>  	for_each_console(c) {
>  		if (!(c->flags & CON_ENABLED))
>  			continue;
> +		/*
> +		 * While rounding up CPUs via NMIs, its possible that

Ditto.

> +		 * 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. So in order
> +		 * to avoid such a deadlock, enable oops_in_progress
> +		 * prior to invocation of console handlers.

Actually looking at this comment as a whole I think it spends to many
words on what and not enough on why (e.g. what the tradeoffs are and
why we are not using bust_spinlocks() which would be a more obvious
approach).

  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.


Daniel.


> +		 */
> +		++oops_in_progress;
>  		c->write(c, msg, msg_len);
> +		--oops_in_progress;
>  		touch_nmi_watchdog();
>  	}
>  }
> -- 
> 2.7.4
>