linux-kernel - Re: [BUG RT] dump-capture kernel not executed for panic in interrupt context

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Message-ID: <c6b095af-fc92-420f-303f-d2efd9f28873@jv-coder.de>
Date:   Fri, 21 Aug 2020 12:25:33 +0200
From:   Joerg Vehlow <lkml@...coder.de>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Huang Ying <ying.huang@...el.com>
Cc:     linux-kernel@...r.kernel.org,
        Joerg Vehlow <joerg.vehlow@...-tech.de>
Subject: Re: [BUG RT] dump-capture kernel not executed for panic in interrupt
 context

Hi Andrew and Others (please read at least the part with @RT developers),

> Yup, mutex_trylock() from interrupt is improper.  Well dang, that's a
> bit silly.  Presumably the 2006 spin_lock_mutex() wasn't taken with
> irqs-off.
>
> Ho hum, did you look at switching the kexec code back to the xchg
> approach?
>
I looked into reverting to the xchg approach, but that seems to be
not a good solution anymore, because the mutex is used in many places,
a lot with waiting locks and I guess that would require spinning now,
if we do this with bare xchg.

Instead I thought about using a spinlock, because they are supposed
to be used in interrupt context as well, if I understand the documentation
correctly ([1]).
@RT developers
Unfortunately the rt patches seem to interpret it a bit different and
spin_trylock uses __rt_mutex_trylock again, with the same consequences as
with the current code.

I tried raw_spinlocks, but it looks like they result in a deadlock at
least in the rt kernel. Thiy may be because of memory allocations in the
critical sections, that are not allowed if I understand it correctly.

I have no clue how to fix it at this point.

Jörg

[1] https://kernel.readthedocs.io/en/sphinx-samples/kernel-locking.html