linux-kernel - Re: [PATCH printk 18/18] printk: Handle dropped message smarter

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Message-ID: <YzEoYPSC5Qf2aL92@google.com>
Date:   Mon, 26 Sep 2022 13:19:44 +0900
From:   Sergey Senozhatsky <senozhatsky@...omium.org>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH printk 18/18] printk: Handle dropped message smarter

On (22/09/24 02:10), John Ogness wrote:
> +/**
> + * cons_print_dropped - Print 'dropped' message if required
> + * @desc:	Pointer to the output descriptor
> + *
> + * Prints the 'dropped' message info the output buffer if @desc->dropped is
> + * not 0 and the regular format is requested. Extended format does not
> + * need this message because it prints the sequence numbers.
> + *
> + * In regular format the extended message buffer is not in use.
> + * So print into it at the beginning and move the resulting string
> + * just in front of the regular text buffer so that the message can
> + * be printed in one go.
> + *
> + * In case of a message this returns with @desc->outbuf and @desc->len
> + * updated. If no message is required then @desc is not modified.
> + */
> +static void cons_print_dropped(struct cons_outbuf_desc *desc)
> +{
> +	struct cons_text_buf *txtbuf = desc->txtbuf;
> +	size_t len;
> +
> +	if (!desc->dropped || desc->extmsg)
> +		return;
> +
> +	if (WARN_ON_ONCE(desc->outbuf != txtbuf->text))
> +		return;
> +
> +	/* Print it into ext_text which is unused */
> +	len = snprintf(txtbuf->ext_text, DROPPED_TEXT_MAX,
> +		       "** %lu printk messages dropped **\n", desc->dropped);
> +	desc->dropped = 0;
> +
> +	/* Copy it just below text so it goes out with one write */
> +	memcpy(txtbuf->text - len, txtbuf->ext_text, len);
> +
> +	/* Update the descriptor */
> +	desc->len += len;
> +	desc->outbuf -= len;

Oh, hmm. This does not look to me as a simplification. Quite
the opposite, moving cons_text_buf::text pointer to point to
cons_text_buf::text - strlen("... dropped messages...") looks
somewhat fragile.

Is printing 'dropped' and outbuf messages in one go such an
important feature?