Re: [PATCH v3 2/2] Add XFS messages to printk index

[Dave Chinner <david@fromorbit.com>]

On Wed, Mar 30, 2022 at 12:47:39PM -0400, Steven Rostedt wrote:
> On Wed, 30 Mar 2022 12:52:58 +0100
> Chris Down <chris@chrisdown.name> wrote:
> 
> > The policy, as with all debugfs APIs by default, is that it's completely 
> > unstable and there are no API stability guarantees whatsoever. That's why 
> > there's no extensive documentation for users: because this is a feature for 
> > kernel developers.
> > 
> > 0: https://lwn.net/Articles/309298/
> 
> That article you reference states the opposite of what you said. And I got
> burnt by it before. Because Linus stated, if it is available for users, it
> is an ABI.
> 
> From the article above:
> 
> "Linus put it this way:
> 
>    The fact that something is documented (whether correctly or not) has
>    absolutely _zero_ impact on anything at all. What makes something an ABI is
>    that it's useful and available. The only way something isn't an ABI is by
>    _explicitly_ making sure that it's not available even by mistake in a
>    stable form for binary use. Example: kernel internal data structures and
>    function calls. We make sure that you simply _cannot_ make a binary that
>    works across kernel versions. That is the only way for an ABI to not form."
> 
> IOW, files in debugfs are available for users, and if something is written
> that depends on it and it is useful, it becomes ABI.

Yup, that's exactly what happened with powertop and the tracepoints
it used and why I pointed to it as is the canonical example of
information exposed from within debugfs unintentionally becoming
stable KABI....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com