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Date:   Fri, 28 Feb 2020 11:04:16 +0100
From:   Petr Mladek <pmladek@suse.com>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Cc:     Lech Perczak <l.perczak@camlintechnologies.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Theodore Ts'o <tytso@mit.edu>, Arnd Bergmann <arnd@arndb.de>,
        Krzysztof =?utf-8?Q?Drobi=C5=84ski?= 
        <k.drobinski@camlintechnologies.com>,
        Pawel Lenkow <p.lenkow@camlintechnologies.com>,
        John Ogness <john.ogness@linutronix.de>,
        Tejun Heo <tj@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>
Subject: Re: Regression in v4.19.106 breaking waking up of readers of
 /proc/kmsg and /dev/kmsg
Message-ID: <20200228100416.6bwathdtopwat5wy@pathway.suse.cz>
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On Fri 2020-02-28 12:13:06, Sergey Senozhatsky wrote:
> Cc-ing Petr, Steven, John
> 
> https://lore.kernel.org/lkml/e9358218-98c9-2866-8f40-5955d093dc1b@camlintechnologies.com
> 
> On (20/02/27 14:08), Lech Perczak wrote:
> > 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 have reproduced the problem with a kernel based on v4.19.106
and I see the following in the log:

[    0.028250] clocksource: refined-jiffies: mask: 0xffffffff max_cycles: 0xffffffff, max_idle_ns: 7645519600211568 ns
[    0.028263] random: get_random_bytes called from start_kernel+0x9e/0x4f6 with crng_init=0
[    0.028268] setup_percpu: NR_CPUS:8192 nr_cpumask_bits:4 nr_cpu_ids:4 nr_node_ids:1
[    0.028407] percpu: Embedded 44 pages/cpu s142216 r8192 d29816 u524288
[    0.028411] pcpu-alloc: s142216 r8192 d29816 u524288 alloc=1*2097152
[    0.028412] pcpu-alloc: [0] 0 1 2 3 

Note that percpu stuff is initialized after printk_deferred(). And the
deferred console is scheduled by:

void defer_console_output(void)
{
	preempt_disable();
	__this_cpu_or(printk_pending, PRINTK_PENDING_OUTPUT);
	irq_work_queue(this_cpu_ptr(&wake_up_klogd_work));
	preempt_enable();
}

I am afraid that the patch creates some mess via the non-initialized
per-cpu variable.

I see that x86 has some support for EARLY_PER_CPU stuff but it seems
to be arch-specific.

I do not see a reliable way to detect when per-cpu variables are
initialized. Adding Tejun and PeterZ into CC if they have any
idea.

I suggest to revert the patch until we have some easy and safe solution.

Best Regards,
Petr