Re: [PATCH RESEND 1/3] printk: convert byte-buffer to variable-length record buffer

[Kay Sievers <kay@vrfy.org>]

On Fri, 2012-05-11 at 02:38 +0200, Kay Sievers wrote:
> On Thu, May 10, 2012 at 10:52 PM, Linus Torvalds <torvalds@linux-foundation.org> wrote:
> 
> > The change to verify that 'current' matches the previous printout is
> > so far the *only* sane semantic change I've seen in this thread.
> > Everything else has been pure garbage.
> 
> Ok, let's continue with sane things then. :)
> 
> Here is a stupid kernel module which creates 3 racy printk() loops, 2 of
> them do continuation, one does an atomic print. The continuation one
> prints one char per printk.
>   http://people.freedesktop.org/~kay/printk-race/printk-race.c
> 
> The lines all try to look like:
>   (AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA)
>   (CCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCC)
>   (XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX)
> 
> Every loop has it's own uppercase character. The line enclosed in '()'.
> 'A' is the atomic one, the cont ones are 'C' and 'X'.

Having only one continuation user racing against an atomic printk user
looks like this:

Current kernel:
[   54.062900] (AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA)
[   54.068200] (AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA)
[   54.071698] (CC(AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA)
[   54.074103] C(AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA)
[   54.076052] C(AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA)
[   54.078116] C(AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA)
[   54.080096] C(AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA)
[   54.082060] CC(AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA)
[   54.086121] C(AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA)
  http://people.freedesktop.org/~kay/printk-race/printk-out-before-2.txt

Record buffer, with cont buffer owner:
[   33.431129] (AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA)
[   33.435112] (AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA)
[   33.439131] (AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA)
[   33.441820] (CCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCC)
[   33.445292] (AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA)
[   33.450131] (AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA)
[   33.453127] (AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA)
[   33.456084] (AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA)
[   33.459143] (AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA)
[   33.462161] (AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA)
[   33.465165] (AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA)
  http://people.freedesktop.org/~kay/printk-race/printk-out-after-2.txt

Kay