linux-kernel - Re: [RFC PATCH v1 00/25] printk: new implementation

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Date:   Fri, 8 Mar 2019 11:00:16 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Daniel Wang <wonderfly@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Alan Cox <gnomes@...rguk.ukuu.org.uk>,
        Jiri Slaby <jslaby@...e.com>,
        Peter Feiner <pfeiner@...gle.com>,
        linux-serial@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [RFC PATCH v1 00/25] printk: new implementation

On Thu 2019-03-07 10:53:48, John Ogness wrote:
> On 2019-03-04, Sergey Senozhatsky <sergey.senozhatsky.work@...il.com> wrote:
> > If there are setups which can be fully !atomic (in terms of console
> > output) then we, essentially, have a fully preemptible kthread printk
> > implementation.
> 
> Correct. I've mentioned in another response[0] some ideas about what
> could be done to aid this.
> 
> I understand that fully preemptible kthread printing is unacceptable for
> you. Since all current console drivers are already irq safe, I'm
> wondering if using irq_work to handle the emergency printing for console
> drivers without write_atomic() would help. (If the printk caller is in a
> context that write() supports, then write() could be called directly.)
> This would also demand that the irq-safe requirements for write() are
> not relaxed. The printk-kthread might still be faster than irq_work, but
> it might increase reliability if an irq_work is triggered as an extra
> precaution.

It is getting more and more complicated. The messages would be pushed
directly, from irq, and kthread. It would depend how the code would
look like but I am not much optimistic.

Note that you could not pass any data to the irq_work handler.
It would need to iterate over the logbuffer and take care of
all non-handled emergency messages.

Anyway, we could solve this later. We need to keep the current
console_unlock() handling as a fallback until enough consoles
support the direct mode anyway.

Best Regards,
Petr