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From:   John Ogness <john.ogness@linutronix.de>
To:     Petr Mladek <pmladek@suse.com>
Cc:     Sergey Senozhatsky <senozhatsky@chromium.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH printk v2 3/4] printk: Skip unfinalized records in panic
In-Reply-To: <ZS_5Xd7zPWvSHuqq@alley>
References: <20231013204340.1112036-1-john.ogness@linutronix.de>
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Date:   Wed, 18 Oct 2023 17:56:52 +0206
Message-ID: <87zg0g53qb.fsf@jogness.linutronix.de>
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On 2023-10-18, Petr Mladek <pmladek@suse.com> wrote:
> So it is the _last_ finalized id from the timing POV. If there are
> more CPUs storing and finalizing the messages in parallel then
> it might change forth and back. There might be earlier non-finalized
> records and newer finalized ones.
>
> It means that prb_next_seq() really is the best effort and
> the description is not valid:

Well, the description was valid until prb_next_seq() was optimized and
converted to best-effort with:

commit f244b4dc53e5 ("printk: ringbuffer: Improve prb_next_seq() performance")

> It would be great to document these subtle details especially when
> we are going to depend on them.

Going through the various call sites of prb_next_seq(), I would argue
that the above optimization introduced some bugs. I will investigate if
prb_next_seq() can be fixed to match its description because the current
users already depend on that.

WRT to this series, I have put together an alternative implementation
that does not use prb_next_seq().

John