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Date:   Fri, 22 May 2020 11:38:23 +0100
From:   Daniel Thompson <daniel.thompson@linaro.org>
To:     Sumit Garg <sumit.garg@linaro.org>
Cc:     kgdb-bugreport@lists.sourceforge.net, jason.wessel@windriver.com,
        dianders@chromium.org, pmladek@suse.com,
        sergey.senozhatsky@gmail.com, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] kdb: Make kdb_printf robust to run in NMI context
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On Fri, May 22, 2020 at 03:32:26PM +0530, Sumit Garg 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. So in order
> to avoid such a deadlock, invoke bust_spinlocks() prior to invocation
> of console handlers.
> 
> Also, add a check for console port to be enabled prior to invocation of
> corresponding handler.
> 
> Suggested-by: Petr Mladek <pmladek@suse.com>
> Suggested-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
> Signed-off-by: Sumit Garg <sumit.garg@linaro.org>
> ---
>  kernel/debug/kdb/kdb_io.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/kernel/debug/kdb/kdb_io.c b/kernel/debug/kdb/kdb_io.c
> index 924bc92..e32ece6 100644
> --- a/kernel/debug/kdb/kdb_io.c
> +++ b/kernel/debug/kdb/kdb_io.c
> @@ -699,7 +699,11 @@ int vkdb_printf(enum kdb_msgsrc src, const char *fmt, va_list ap)
>  			}
>  		}
>  		for_each_console(c) {
> +			if (!(c->flags & CON_ENABLED))
> +				continue;
> +			bust_spinlocks(1);
>  			c->write(c, cp, retlen - (cp - kdb_buffer));
> +			bust_spinlocks(0);

I think it might actually be better to directly manipulate oops_in_progress
here.

bust_spinlocks(0) will call unblank_screen() and console_unblank() and
I don't think calling these is worth the risk (and whilst kicking klogd
is safe I think its also not required).


Daniel.


>  			touch_nmi_watchdog();
>  		}
>  	}
> @@ -761,7 +765,11 @@ int vkdb_printf(enum kdb_msgsrc src, const char *fmt, va_list ap)
>  			}
>  		}
>  		for_each_console(c) {
> +			if (!(c->flags & CON_ENABLED))
> +				continue;
> +			bust_spinlocks(1);
>  			c->write(c, moreprompt, strlen(moreprompt));
> +			bust_spinlocks(0);
>  			touch_nmi_watchdog();
>  		}
>  
> -- 
> 2.7.4
>