linux-kernel - Re: [PATCH V6] kdb: Fix the deadlock issue in KDB debugging.

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Date: Tue, 26 Mar 2024 09:22:57 +0100
From: Greg KH <gregkh@...uxfoundation.org>
To: liu.yec@....com
Cc: daniel.thompson@...aro.org, jirislaby@...nel.org, dianders@...omium.org,
	jason.wessel@...driver.com, kgdb-bugreport@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org, linux-serial@...r.kernel.org
Subject: Re: [PATCH V6] kdb: Fix the deadlock issue in KDB debugging.

On Tue, Mar 26, 2024 at 03:40:14PM +0800, liu.yec@....com wrote:
> Note that we still need to  a workqueue since we cannot resync
> the keyboard state from the hardirq context provided by irq_work.

I think you are missing a word between "to" and "a", right?

> That must be done from task context for the calls into the input
> subystem. Hence we must defer the work twice. First to safely
> switch from the debug trap (NMI-like context) to hardirq and
> then, secondly, to get from hardirq to the system workqueue.
> 
> Signed-off-by: LiuYe <liu.yeC@....com>
> Co-authored-by: Daniel Thompson <daniel.thompson@...aro.org>
> Signed-off-by: Daniel Thompson <daniel.thompson@...aro.org>
> 
> ---
> V5 -> V6: Replace with a more professional and accurate answer.
> V4 -> V5: Answer why schedule another work in the irq_work and not do the job directly.
> V3 -> V4: Add changelogs
> V2 -> V3: Add description information
> V1 -> V2: using irq_work to solve this properly.
> ---
> ---
>  drivers/tty/serial/kgdboc.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/tty/serial/kgdboc.c b/drivers/tty/serial/kgdboc.c
> index 7ce7bb164..161b25ecc 100644
> --- a/drivers/tty/serial/kgdboc.c
> +++ b/drivers/tty/serial/kgdboc.c
> @@ -22,6 +22,7 @@
>  #include <linux/module.h>
>  #include <linux/platform_device.h>
>  #include <linux/serial_core.h>
> +#include <linux/irq_work.h>
>  
>  #define MAX_CONFIG_LEN		40
>  
> @@ -99,10 +100,17 @@ static void kgdboc_restore_input_helper(struct work_struct *dummy)
>  
>  static DECLARE_WORK(kgdboc_restore_input_work, kgdboc_restore_input_helper);
>  
> +static void kgdboc_queue_restore_input_helper(struct irq_work *unused)
> +{
> +	schedule_work(&kgdboc_restore_input_work);

As this is a "two stage deferment" or something like that, it should be
documented in the code exactly why this is needed and what is happening,
otherwise it looks very odd.

thanks,

greg k-h