Re: [RFC][PATCHv2 2/4] printk: move printk_safe macros to printk header

From: Peter Zijlstra
Date: Wed Oct 17 2018 - 03:57:47 EST

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.