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

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Date:	Thu, 29 Nov 2012 15:08:39 +0100
From:	Kay Sievers <kay@...y.org>
To:	mtk.manpages@...il.com
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Greg Kroah-Hartmann <greg@...ah.com>,
	Ingo Molnar <mingo@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RESEND 1/3] printk: convert byte-buffer to variable-length
 record buffer

On Thu, Nov 29, 2012 at 2:37 PM, Michael Kerrisk (man-pages)
<mtk.manpages@...il.com> wrote:
> On Thu, Nov 29, 2012 at 2:28 PM, Kay Sievers <kay@...y.org> wrote:
>> On Thu, Nov 29, 2012 at 2:18 PM, Michael Kerrisk (man-pages)
>> <mtk.manpages@...il.com> wrote:
>>> On Wed, Nov 28, 2012 at 6:51 PM, Kay Sievers <kay@...y.org> wrote:
>>
>>>> Before:
>>>>   syslog(SYSLOG_ACTION_SIZE_UNREAD, 0, 0) = 286965
>>>>   syslog(SYSLOG_ACTION_READ_CLEAR, "<12>"..., 1000000) = 24000
>>>>   syslog(SYSLOG_ACTION_SIZE_UNREAD, 0, 0) = 286965
>>>>
>>>> After:
>>>>   syslog(SYSLOG_ACTION_SIZE_UNREAD, 0, 0) = 90402
>>>>   syslog(SYSLOG_ACTION_READ_CLEAR, "<5>"..., 1000000) = 90402
>>>>   syslog(SYSLOG_ACTION_SIZE_UNREAD, 0, 0) = 0
>>
>>> I'm going to call my report yesterday bogus. Somewhere along the way,
>>> I got confused while testing something, and my statement about 2.6.31
>>> behavior is wrong: the 2.6.31 and 3.5 behaviors are the same. As such,
>>> your patch is unneeded. Sorry for wasting your time.
>>
>> I think you have been right with your report. The above pasted
>> before/after from the patch commit text is actually a result of real
>> testing with current git. And your initial description sounds right,
>> and the patch seems to produce the expected results here. I just
>> confused the numbers in your report and wrongly parsed 2.6 > 3.6.
>>
>> Hmm, at least do far we did not blame anybody else than ourselves as
>> confused. One of us at least is right, and it looks you have been, and
>> I also think the patch is at least intended to be right. :)
>
> Okay -- I'm pretty sure I am right about being wrong ;-).
>
> Could you do some comparative testing please between 3.5 and pre-3.5.
> I have a little test program below. When I rechecked 2.6.31 and 3.5
> using this program I found the behavior was the same, which is why I
> conclude my report is wrong. (And also, your proposed patch in
> response to my bogus report produces different behavior from 2.6.31).

Oh, seems you are right.

The old kernel does not return 0, while it probably should. The
current kernel seems to do the same thing.

But the behaviour with the patch stills seems like the better and the
obvious and expected behaviour to me. :)

Thanks,
Kay

######### old fedora kernel (pre-new-logging) ###############
# uname -a
Linux nja 3.4.4-3.fc17.x86_64

# echo 1234567890 > /dev/kmsg

# ./syslog-test 9
SYSLOG_ACTION_SIZE_UNREAD
Number of unread bytes: 95299

# ./syslog-test 4
SYSLOG_ACTION_READ_CLEAR
Return value: 29
<4>[   54.933282] 1234567890
Ring buffer cleared

# ./syslog-test 9
SYSLOG_ACTION_SIZE_UNREAD
Number of unread bytes: 95299

######### current fedora kernel (new logging) ############

# uname -a
Linux nja 3.6.6-3.fc18.x86_64

# echo 1234567890 > /dev/kmsg

# ./syslog-test 9
SYSLOG_ACTION_SIZE_UNREAD
Number of unread bytes: 286822

# ./syslog-test 4
SYSLOG_ACTION_READ_CLEAR
Return value: 31
<12>[259301.067699] 1234567890
Ring buffer cleared

# ./syslog-test 9
SYSLOG_ACTION_SIZE_UNREAD
Number of unread bytes: 286822

######### git kernel with the above patch applied #############

# uname -a
Linux nja 3.7.0-rc7+

# echo 1234567890 > /dev/kmsg

# ./syslog-test 9
SYSLOG_ACTION_SIZE_UNREAD
Number of unread bytes: 30

# ./syslog-test 4
SYSLOG_ACTION_READ_CLEAR
Return value: 30
<12>[   69.591745] 1234567890
Ring buffer cleared

# ./syslog-test 9
SYSLOG_ACTION_SIZE_UNREAD
Number of unread bytes: 0
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