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 12:39:34 +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>
Subject: Re: Regression in v4.19.106 breaking waking up of readers of
 /proc/kmsg and /dev/kmsg

W dniu 27.02.2020 o 13:36, 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.
>
> thanks,
>
> greg k-h

-- 
Pozdrawiam/With kind regards,
Lech Perczak

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