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

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Date:   Wed, 17 Oct 2018 09:57:28 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     linux-kernel@...r.kernel.org, Petr Mladek <pmladek@...e.com>,
        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 Wed, Oct 17, 2018 at 01:32:51PM +0900, Sergey Senozhatsky wrote:
> This probably will be a bit more hairy. logbuf is written to by many
> sources and is read from by many sides, including user-space [both read()
> and write()]. So we will need more flags/magic around memcpy(). A simple,
> "grab the logbuf entry, set the proper offset to point to the next available
> logbuf record and then do memcpy()" won't suffice. We need a flag for
> "memcpy() complete, we can read this entry". Otherwise:

Sure, but lockless buffers mostly have reserve and commit stages anyway.
Exactly to avoid that problem.

> All right. OK. So we are on the same page here:

> - Have more opinions on this. People please speak out.
> - Have clear "let's do it" from Cc-ed people.
> 
> 
> If we are really doing this, then let's split it and have
> incremental changes. Namely, what I suggest is:

I'd start by replacing logbuf with the lockless buffer and ripping out
the current nmi/safe/etc.. bollocks.

There is absolutely no point what so ever in doing anything until that
is sorted.

After that, move the console output to a kthread, but keep earlycon (if
set) synchronous.

Also avoid the whole panic/warn special case when earlycon is set, no
point in doing dodgy crap when you know you have a good option already.