linux-kernel - Re: [PATCH v2] panic, kexec: Make __crash

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Message-ID: <YrQzB7sPD8BNxSFq@linutronix.de>
Date:   Thu, 23 Jun 2022 11:31:51 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Valentin Schneider <vschneid@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kexec@...ts.infradead.org,
        linux-rt-users@...r.kernel.org,
        Eric Biederman <ebiederm@...ssion.com>,
        Arnd Bergmann <arnd@...db.de>, Petr Mladek <pmladek@...e.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Juri Lelli <jlelli@...hat.com>,
        "Luis Claudio R. Goncalves" <lgoncalv@...hat.com>
Subject: Re: [PATCH v2] panic, kexec: Make __crash_kexec() NMI safe

On 2022-06-20 12:15:20 [+0100], Valentin Schneider wrote:
> Attempting to get a crash dump out of a debug PREEMPT_RT kernel via an NMI
> panic() doesn't work. The cause of that lies in the PREEMPT_RT definition
> of mutex_trylock():
> 
…

> Fixes: 6ce47fd961fa ("rtmutex: Warn if trylock is called from hard/softirq context")
> Signed-off-by: Valentin Schneider <vschneid@...hat.com>

Reviewed-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>

> ---
> v1 -> v2
> ++++++++
> 
> o Changed from Peterson-like synchronization to simpler atomic_cmpxchg
>   (Petr)
> o Slightly reworded changelog
> o Added Fixes: tag. Technically should be up to since kexec can happen
>   in an NMI, but that isn't such a clear target

RT-wise it would be needed for each release.
There is also a mutex_unlock() in case an image is missing. This can go
via the scheduler if there is a waiter which does not look good with the
NMI in the picture.

Sebastian