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

[Kay Sievers <kay@vrfy.org>]

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'.

Our current kernel look like:
[   99.056806] XC(AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA)
[   99.059772] XC(AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA)
[   99.062168] XCCX(AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA)
[   99.066479] CXXC(AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA)
[   99.070063] XC(AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA)
[   99.073081] XCCXCXCXCXCXCXCXCXCXCXCXCXCXCXCCXCXCXCXCXCXCXCXCXCXCXCXCXCXCXCXCXCXCXCXC)
[   99.148325] (CCXCXCX)
[   99.155468] (XXCXCXCXCCXCXCXCXCXCXCXCXCXCXCXCXCXCXCXCXCXCCXCXCXCXCXCXCXCXCXCXCXCXCXCXCXCXCXCXCXCXCXCXCXCXCXXCXCXCXCXCXCXCXCXC)
[   99.277357] (CCXCXCXCX)
  http://people.freedesktop.org/~kay/printk-race/printk-out-before.txt


The kernel with the record buffer, and the 'current' who owns the cont buffer
looks like:
[   78.080866] (AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA)
[   78.082790] (CC
[   78.084152] (XX
[   78.084497] (AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA)
[   78.085494] C
[   78.086125] X
[   78.087188] C
[   78.088669] X
[   78.089803] C
[   78.091510] X
[   78.091924] (AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA)
[   78.094112] CC
[   78.096171] X
  http://people.freedesktop.org/~kay/printk-race/printk-out-after.txt

Thanks,
Kay