linux-kernel - Re: [PATCH 0/4] printk: reimplement LOG

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Date:   Sat, 18 Jul 2020 16:48:55 +0206
From:   John Ogness <john.ogness@...utronix.de>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Petr Mladek <pmladek@...e.com>,
        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: [PATCH 0/4] printk: reimplement LOG_CONT handling

On 2020-07-17, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> Make sure you test the case of "fast concurrent readers". The last
> time we did things like this, it was a disaster, because a concurrent
> reader would see and return the _incomplete_ line, and the next entry
> was still being generated on another CPU.
>
> The reader would then decide to return that incomplete line, because
> it had something.
>
> And while in theory this could then be handled properly in user space,
> in practice it wasn't. So you'd see a lot of logging tools that would
> then report all those continuations as separate log events.
>
> Which is the whole point of LOG_CONT - for that *not* to happen.

I expect this is handled correctly since the reader is not given any
parts until a full line is ready, but I will put more focus on testing
this to make sure. Thanks for the regression and testing tips.

> So this is just a heads-up that I will not pull something that breaks
> LOG_CONT because it thinks "user space can handle it". No. User space
> does not handle it, and we need to handle it for the user.

Understood. Petr and Sergey are also strict about this. We are making a
serious effort to avoid breaking things for userspace.

John Ogness