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

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Message-ID: <1336696707.947.56.camel@mop>
Date:	Fri, 11 May 2012 02:38:27 +0200
From:	Kay Sievers <kay@...y.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Joe Perches <joe@...ches.com>, Ted Ts'o <tytso@....edu>,
	Ingo Molnar <mingo@...nel.org>,
	Jonathan Corbet <corbet@....net>,
	Sasha Levin <levinsasha928@...il.com>,
	Greg Kroah-Hartmann <greg@...ah.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH RESEND 1/3] printk: convert byte-buffer to
 variable-length record buffer

On Thu, May 10, 2012 at 10:52 PM, Linus Torvalds <torvalds@...ux-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

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