linux-kernel - Re: [PATCH v6 1/2] printk: Add function to replay kernel log on consoles

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Message-ID: <Zk4MtXxbzGrQhSFA@pathway.suse.cz>
Date: Wed, 22 May 2024 17:18:13 +0200
From: Petr Mladek <pmladek@...e.com>
To: Sreenath Vijayan <sreenath.vijayan@...y.com>
Cc: john.ogness@...utronix.de, corbet@....net, gregkh@...uxfoundation.org,
	jirislaby@...nel.org, rdunlap@...radead.org, rostedt@...dmis.org,
	senozhatsky@...omium.org, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-serial@...r.kernel.org,
	taichi.shimoyashiki@...y.com, daniel.palmer@...y.com,
	anandakumar.balasubramaniam@...y.com
Subject: Re: [PATCH v6 1/2] printk: Add function to replay kernel log on
 consoles


On Wed 2024-03-13 15:50:52, Sreenath Vijayan wrote:
> Add a generic function console_replay_all() for replaying
> the kernel log on consoles, in any context. It would allow
> viewing the logs on an unresponsive terminal via sysrq.
> 
> Reuse the existing code from console_flush_on_panic() for
> resetting the sequence numbers, by introducing a new helper
> function __console_rewind_all(). It is safe to be called
> under console_lock().
> 
> Try to acquire lock on the console subsystem without waiting.
> If successful, reset the sequence number to oldest available
> record on all consoles and call console_unlock() which will
> automatically flush the messages to the consoles.
> 
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -4259,6 +4271,23 @@ void kmsg_dump_rewind(struct kmsg_dump_iter *iter)
>  }
>  EXPORT_SYMBOL_GPL(kmsg_dump_rewind);
>  
> +/**
> + * console_replay_all - replay kernel log on consoles

It should rather be called "console_try_replay_all()" to make it clear
that it is just the best effort.

> + *
> + * Try to obtain lock on console subsystem and replay all
> + * available records in printk buffer on the consoles.
> + * Does nothing if lock is not obtained.
> + *
> + * Context: Any context.

This should be:

 * Context: Any, except for NMI

Basically only lockless code is safe in NMI which is not the case here.

> + */
> +void console_replay_all(void)
> +{
> +	if (console_trylock()) {
> +		__console_rewind_all();
> +		/* Consoles are flushed as part of console_unlock(). */
> +		console_unlock();
> +	}
> +}
>  #endif

Otherwise, it looks good. With the two changes:

Reviewed-by: Petr Mladek <pmladek@...e.com>

I am sorry for the late review. I have been snowed under tasks.
Also I had healthy problems.

I have seen a mail that Greg has queued the patch in tty-next.
I am not sure if it still can be fixed. It will be perfectly fine
to change this by a followup patch.

Best Regards,
Petr