linux-kernel - Re: [PATCH v2 1/5] printk: honor the max_reason field in kmsg

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Date:   Thu, 14 May 2020 10:49:55 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Pavel Tatashin <pasha.tatashin@...een.com>
Cc:     jmorris@...ei.org, sashal@...nel.org, linux-kernel@...r.kernel.org,
        sergey.senozhatsky@...il.com, rostedt@...dmis.org,
        keescook@...omium.org, anton@...msg.org, ccross@...roid.com,
        tony.luck@...el.com, robh+dt@...nel.org, devicetree@...r.kernel.org
Subject: Re: [PATCH v2 1/5] printk: honor the max_reason field in kmsg_dumper

On Tue 2020-05-05 11:45:06, Pavel Tatashin wrote:
> kmsg_dump() allows to dump kmesg buffer for various system events: oops,
> panic, reboot, etc. It provides an interface to register a callback call
> for clients, and in that callback interface there is a field "max_reason"
> which gets ignored unless always_kmsg_dump is passed as kernel parameter.
> 
> Allow clients to decide max_reason, and keep the current behavior when
> max_reason is not set.
> 
> Signed-off-by: Pavel Tatashin <pasha.tatashin@...een.com>
> ---
>  include/linux/kmsg_dump.h |  1 +
>  kernel/printk/printk.c    | 15 +++++++++++----
>  2 files changed, 12 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/kmsg_dump.h b/include/linux/kmsg_dump.h
> index 2e7a1e032c71..cfc042066be7 100644
> --- a/include/linux/kmsg_dump.h
> +++ b/include/linux/kmsg_dump.h
> @@ -28,6 +28,7 @@ enum kmsg_dump_reason {
>  	KMSG_DUMP_RESTART,
>  	KMSG_DUMP_HALT,
>  	KMSG_DUMP_POWEROFF,
> +	KMSG_DUMP_MAX
>  };
>  
>  /**
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index 9a9b6156270b..1aab69a8a2bf 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -3157,12 +3157,19 @@ void kmsg_dump(enum kmsg_dump_reason reason)
>  	struct kmsg_dumper *dumper;
>  	unsigned long flags;
>  
> -	if ((reason > KMSG_DUMP_OOPS) && !always_kmsg_dump)
> -		return;
> -
>  	rcu_read_lock();
>  	list_for_each_entry_rcu(dumper, &dump_list, list) {
> -		if (dumper->max_reason && reason > dumper->max_reason)
> +		enum kmsg_dump_reason cur_reason = dumper->max_reason;

If this code is still in the next version, please, rename this variable
to max_reason or so.

"cur" is ambiguous. It might be current dumper or current message
which confused me later in the code ;-)

Best Regards,
Petr

> +
> +		/*
> +		 * If client has not provided a specific max_reason, default
> +		 * to KMSG_DUMP_OOPS, unless always_kmsg_dump was set.
> +		 */
> +		if (cur_reason == KMSG_DUMP_UNDEF) {
> +			cur_reason = always_kmsg_dump ? KMSG_DUMP_MAX :
> +							KMSG_DUMP_OOPS;
> +		}
> +		if (reason > cur_reason)
>  			continue;
>  
>  		/* initialize iterator with data about the stored records */
> -- 
> 2.25.1