linux-kernel - Re: [RFC][PATCHv2 2/4] printk: move printk

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Message-ID: <20181016121752.GA2537@hirez.programming.kicks-ass.net>
Date:   Tue, 16 Oct 2018 14:17:52 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        linux-kernel@...r.kernel.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][PATCHv2 2/4] printk: move printk_safe macros to printk
 header

On Tue, Oct 16, 2018 at 01:40:06PM +0200, Petr Mladek wrote:
> On Tue 2018-10-16 09:27:19, Peter Zijlstra wrote:

> > Instead of this tinkering around the edges, why don't you make the main
> > logbuf a lockless ringbuffer and then delegate the actual printing of
> > that buffer to a kthread, except for earlycon, which can do synchronous
> > output.
> 
> In fact, there is no problem with printk log buffer. This patchset
> tries to avoid deadlock caused by console-specific locks
> used by console->write() methods.
> 
> By other words, we neither need printk_safe or lockless log buffer
> to fix this prolem. Instead, we need either printk_deferred context
> or lockless consoles.

If you have a lockless buffer and a trailing printk thread, that's
deferred.

And earlycon _should_ be lockless (yes, I know, some suck)

But if you do this deferred nonsense that's currently in printk, then
you risk never seeing the message -- the same problem I have with the
whole printk_safe thing too.

Printing _after_ the fact is horrible for robustness.