linux-kernel - Re: [PATCH v5 4/4] printk: use the lockless ringbuffer

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Date:   Mon, 20 Jul 2020 11:40:52 +0200
From:   Marco Elver <elver@...gle.com>
To:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:     John Ogness <john.ogness@...utronix.de>,
        Petr Mladek <pmladek@...e.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...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,
        LKML <linux-kernel@...r.kernel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Andy Shevchenko <andy.shevchenko@...il.com>
Subject: Re: [PATCH v5 4/4] printk: use the lockless ringbuffer

On Mon, 20 Jul 2020 at 10:41, Sergey Senozhatsky
<sergey.senozhatsky@...il.com> wrote:
>
> On (20/07/20 08:43), Marco Elver wrote:
> > On Sun, Jul 19, 2020 at 12:43PM +0900, Sergey Senozhatsky wrote:
> >
> > As I said, a number of debugging tools use them to format reports to be
> > more readable (visually separate title and report body, and separate
> > parts of the report). Also, such reports are often parsed by CI systems,
> > and by changing the reports, these CI systems may break. But those are
> > just the usecases I'm acutely aware of
>
> Can you give example of such CI systems? // that's a real question //

None of ours should break; I agree the CI system is brittle if it
relies on newlines. Parsed and displayed reports are changing, however
-- what irks me is now all the reports sent to the LKML look ugly.

Some random KASAN reports (just compare formatting):
next (ugly): https://lore.kernel.org/lkml/000000000000c87b7305aadb6dba@google.com/
mainline (normal):
https://lore.kernel.org/lkml/000000000000f4ef6a05aa92ec6c@google.com/

The same problem exists with lockdep reports, KCSAN reports, ... If
newline-printks to insert blank lines are now banned, what are we to
do? Send dozens of patches to switch everyone to printk(" \n")? Or
some better suggestion? I cannot yet see how that is an improvement.
(And if the behaviour is not reverted, please document the new
behaviour.)

That also doesn't yet address the ~400 other newline-printk users, and
somebody needs to do the due diligence to understand if it's just a
flush, or an intentional blank line.

Thanks,
-- Marco