[V5 PATCH 2/4] panic/x86: Allow cpus to save registers even if they are looping in NMI context

[河合英宏 / KAWAI，HIDEHIRO]

> @@ -357,7 +358,15 @@ static void default_do_nmi(struct pt_regs *regs)
>  	}
> 
>  	/* Non-CPU-specific NMI: NMI sources can be processed on any CPU */
> -	raw_spin_lock(&nmi_reason_lock);
> +
> +	/*
> +	 * Another CPU may be processing panic routines with holding
> +	 * nmi_reason_lock.  Check IPI issuance from the panicking CPU
> +	 * and call the callback directly.
> +	 */
> +	while (!raw_spin_trylock(&nmi_reason_lock))
> +		poll_crash_ipi_and_callback(regs);
> +
>  	reason = x86_platform.get_nmi_reason();

I noticed this logic is unneeded until applying PATCH 4/4.
Currently, unknown NMI can be broadcast to all CPUs, but in that case,
panic()/nmi_panic() are called after releasing nmi_reason_lock.
So CPUs can't loop infinitely here.

PATCH 4/4 allows us to broadcast external NMIs to all CPUs, and it
causes infinite loop in raw_spin_lock(&nmi_reason_lock).  So the above
changes are needed.

I'll move these chagnes to a later patch in the next version.

Thanks,

--
Hidehiro Kawai
Hitachi, Ltd. Research & Development Group