[PATCH v2] panic, kexec: Make __crash_kexec() NMI safe

[Sebastian Andrzej Siewior]

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 at redhat.com>

Reviewed-by: Sebastian Andrzej Siewior <bigeasy at linutronix.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