linux-kernel - Re: [PATCH v4 0/4] printk: replace ringbuffer

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Date:   Thu, 09 Jul 2020 09:09:31 +0206
From:   John Ogness <john.ogness@...utronix.de>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrea Parri <parri.andrea@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Paul McKenney <paulmck@...nel.org>, kexec@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 0/4] printk: replace ringbuffer

On 2020-07-08, Petr Mladek <pmladek@...e.com> wrote:
> OK, I think that we are ready to try this in linux-next.
> I am going to push it there via printk/linux.git.
>
> [...]
> 
> Of course, there are still many potential problems. The following comes
> to my mind:
>
> [...]
>
>    + Debugging tools accessing the buffer directly would need to
>      understand the new structure. Fortunately John provided
>      patches for the most prominent ones.

The next series in the printk-rework (move LOG_CONT handling from
writers to readers) makes some further changes that, while not
incompatible, could affect the output of existing tools. It may be a
good idea to let the new ringbuffer sit in linux-next until the next
series has been discussed/reviewed/merged. After the next series,
everything will be in place (with regard to userspace tools) to finish
the rework.

As reminder, here are all the steps planned for the full rework:

1. replace ringbuffer
2. implement NMI-safe LOG_CONT (i.e. move handling to readers)
3. remove logbuf_lock
4. remove safe buffers
5. implement per-console printing kthreads
6. implement emergency mode and write_atomic() support
7. implement write_atomic() for 8250 UART

Some of the steps may be combined into a single series if the changes
are not too dramatic.

John Ogness