linux-kernel - Re: Regression in v4.19.106 breaking waking up of readers of /proc/kmsg and /dev/kmsg

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Date:   Thu, 27 Feb 2020 14:08:34 +0000
From:   Lech Perczak <l.perczak@...lintechnologies.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Theodore Ts'o <tytso@....edu>, Arnd Bergmann <arnd@...db.de>,
        Krzysztof Drobiński 
        <k.drobinski@...lintechnologies.com>,
        Pawel Lenkow <p.lenkow@...lintechnologies.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Subject: Re: Regression in v4.19.106 breaking waking up of readers of
 /proc/kmsg and /dev/kmsg

W dniu 27.02.2020 o 13:39, Lech Perczak pisze:
> W dniu 27.02.2020 o 13:36, Greg Kroah-Hartman pisze:
>> On Thu, Feb 27, 2020 at 11:09:49AM +0000, Lech Perczak wrote:
>>> Hello,
>>>
>>> After upgrading kernel on our boards from v4.19.105 to v4.19.106 we found out that syslog fails to read the messages after ones read initially after opening /proc/kmsg just after booting.
>>> I also found out, that output of 'dmesg --follow' also doesn't react on new printks appearing for whatever reason - to read new messages, reopening /proc/kmsg or /dev/kmsg was needed.
>>> I bisected this down to commit 15341b1dd409749fa5625e4b632013b6ba81609b ("char/random: silence a lockdep splat with printk()"), and reverting it on top of v4.19.106 restored correct behaviour.
>> That is really really odd.
> Very odd it is indeed.
>>> My test scenario for bisecting was:
>>> 1. run 'dmesg --follow' as root
>>> 2. run 'echo t > /proc/sysrq-trigger'
>>> 3. If trace appears in dmesg output -> good, otherwise, bad. If trace doesn't appear in output of 'dmesg --follow', re-running it will show the trace.
>>>
>>> I ran my tests on Debian 10.3 with configuration based directly on one from 4.19.0-8-amd64 (4.19.98-1) in Qemu.
>>> I could reproduce the same issue on several boards with x86 and ARMv7 CPUs alike, with 100% reproducibility.
>>>
>>> I haven't yet digged into why exactly this commit breaks notifications for readers of /proc/kmsg and /dev/kmsg, but as reverting it fixed the issue, I'm pretty sure this is the one. It is possible that the same happened in 5.4 line, bu I hadn't had a chance to test this as well yet.
>> I can revert this, but it feels like there is something else going wrong
>> here.  Can you try the 5.4 tree to see if that too has your same
>> problem?
> Yes, I'll check it in a short while.
I checked v5.4.22 just now and didn't observe the issue. Maybe this commit wasn't destined for 4.19.y due to intricacies of locking inside printk() :-)

From my side, there is no need to rush with the revert, as I can do this locally and drop the revert with next rebase-to-upstream, which we do very often.
OTOH, the issue is likely to affect a lot of users, especially ones using distros tracking this branch like Debian 10 mentioned earlier,
once they pick it up the change, as the kernel log content recorded by syslog will be affected, and 'dmesg --follow' behaviour will be quite surprising.
>> thanks,
>>
>> greg k-h
Also CC'ed Sergey, maybe he will have a clue about this.

-- 
Pozdrawiam/With kind regards,
Lech Perczak

Sr. Software Engineer
Camlin Technologies Poland Limited Sp. z o.o.