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Date:   Wed, 6 Feb 2019 09:22:14 +0800
From:   tomli@tomli.me
To:     Aaro Koskinen <aaro.koskinen@iki.fi>,
        "Paul E. McKenney" <paulmck@linux.ibm.com>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
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        linux-kernel@vger.kernel.org, mingo@redhat.com, rjw@rjwysocki.net,
        viresh.kumar@linaro.org, linux-mips@vger.kernel.org
Subject: Re: [REGRESSION 4.20-rc1] 45975c7d21a1 ("rcu: Define RCU-sched API
 in terms of RCU for Tree RCU PREEMPT builds")
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> OK, thanks. This looks slightly different from the Loongson problem:
> 
> - In Loongson, we don't get stuck in RCU, but in
>   cpufreq_dbs_governor_stop -> irq_work_sync(). 
> 
> - I run non-preemptible kernel, and my system still gets stuck.
> 
> What is common is that it's UP with i8259 PIC.
> 
> A.

Now it's an interesting case. Because on my machine, the problem I
encountered seems to be the identical one of the original thread,
disabling preempting can effectively solve the lockup. Also, my
issue is not only occuring on 4.20-rc1, but also on earlier kernels,
with a lower probability.

But on your machine, you have another non-identical, but closely-
related issue. It seems the timing-dependent lockup of i8259 PIC can
be triggered in different ways.

The conclusion is clear though, there's a real lockup condition in
i8259 PIC driver, and it's causing real issues.  Aaro, have you tried
submitting your i8259 patch to the mainline? Despite your concerns
about its underlying cause, I think a fix should be submitted. If there
are no objections from the maintainers, I suggest submitting it to the
mainline upstream, and send it to linux-stable, requesting it to be
applied on 3.16, 4.4, 4.9, 4.14, 4.19, 4.20 stable branches. If you
are busy, I can help submitting.

Tom Li