[V3 PATCH 3/4] kexec: Fix race between panic() and crash_kexec() called directly

[Peter Zijlstra]

On Thu, Aug 06, 2015 at 02:45:43PM +0900, Hidehiro Kawai wrote:
>  void crash_kexec(struct pt_regs *regs)
>  {
> +	int old_cpu, this_cpu;
> +
> +	/*
> +	 * `old_cpu == -1' means we are the first comer and crash_kexec()
> +	 * was called without entering panic().
> +	 * `old_cpu == this_cpu' means crash_kexec() was called from panic().
> +	 */
> +	this_cpu = raw_smp_processor_id();
> +	old_cpu = atomic_cmpxchg(&panic_cpu, -1, this_cpu);
> +	if (old_cpu != -1 && old_cpu != this_cpu)
> +		return;

This allows recursive calling of crash_kexec(), the Changelog did not
mention that. Is this really required?

> +
>  	/* Take the kexec_mutex here to prevent sys_kexec_load
>  	 * running on one cpu from replacing the crash kernel
>  	 * we are using after a panic on a different cpu.