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Date:   Thu, 13 Aug 2020 02:30:02 +0206
From:   John Ogness <john.ogness@...utronix.de>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        kexec@...ts.infradead.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: POC: Alternative solution: Re: [PATCH 0/4] printk: reimplement LOG_CONT handling

On 2020-08-12, Petr Mladek <pmladek@...e.com> wrote:
> So, I have one crazy idea to add one more state bit so that we
> could have:
>
>   + committed: set when the data are written into the data ring.
>   + final: set when the data block could not longer get reopened
>   + reuse: set when the desctiptor/data block could get reused
>
> "final" bit will define when the descriptor could not longer
> get reopened (cleared committed bit) and the data block could
> not get extended.

I had not thought of extending data blocks. That is clever!

I implemented this solution for myself and am currently running more
tests. Some things that I changed from your suggestion:

1. I created a separate prb_reserve_cont() function. The reason for this
is because the caller needs to understand what is happening. The caller
is getting an existing record with existing data and must append new
data. The @text_len field of the info reports how long the existing data
is. So the LOG_CONT handling code in printk.c looks something like this:

        if (lflags & LOG_CONT) {
                struct prb_reserved_entry e;
                struct printk_record r;

                prb_rec_init_wr(&r, text_len, 0);

                if (prb_reserve_cont(&e, prb, &r, caller_id)) {
                        memcpy(&r.text_buf[r.info->text_len], text, text_len);
                        r.info->text_len += text_len;

                        if (lflags & LOG_NEWLINE)
                                r.info->flags |= LOG_NEWLINE;

                        if (r.info->flags & LOG_NEWLINE)
                                prb_commit_finalize(&e);
                        else
                                prb_commit(&e);

                        return text_len;
                }
        }

This seemed simpler than trying to extend prb_reserve() to secretly
support LOG_CONT records.

2. I haven't yet figured out how to preserve calling context when a
newline appears. For example:

pr_info("text");
pr_cont(" 1");
pr_cont(" 2\n");
pr_cont("3");
pr_cont(" 4\n");

For "3" the calling context (info, timestamp) is lost because with "2"
the record is finalized. Perhaps the above is invalid usage of LOG_CONT?

3. There are some memory barriers introduced, but it looks like it
shouldn't add too much complexity.

I will continue to refine my working version and post a patch so that we
have something to work with. This looks to be the most promising way
forward. Thanks.

John Ogness

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