linux-kernel - Re: [PATCH 1/3] nmi: create generic NMI backtrace implementation

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Date:	Thu, 16 Jul 2015 10:11:24 +0100
From:	Daniel Thompson <daniel.thompson@...aro.org>
To:	Russell King <rmk+kernel@....linux.org.uk>,
	linux-arm-kernel@...ts.infradead.org, x86@...nel.org
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] nmi: create generic NMI backtrace implementation

On 15/07/15 21:39, Russell King wrote:
> +void nmi_trigger_all_cpu_backtrace(bool include_self,
> +				   void (*raise)(cpumask_t *mask))
> +{
> +	struct nmi_seq_buf *s;
> +	int i, cpu, this_cpu = get_cpu();
> +
> +	if (test_and_set_bit(0, &backtrace_flag)) {
> +		/*
> +		 * If there is already a trigger_all_cpu_backtrace() in progress
> +		 * (backtrace_flag == 1), don't output double cpu dump infos.
> +		 */
> +		put_cpu();
> +		return;
> +	}
> +
> +	cpumask_copy(to_cpumask(backtrace_mask), cpu_online_mask);
> +	if (!include_self)
> +		cpumask_clear_cpu(this_cpu, to_cpumask(backtrace_mask));
> +
> +	cpumask_copy(&printtrace_mask, to_cpumask(backtrace_mask));
> +
> +	/*
> +	 * Set up per_cpu seq_buf buffers that the NMIs running on the other
> +	 * CPUs will write to.
> +	 */
> +	for_each_cpu(cpu, to_cpumask(backtrace_mask)) {
> +		s = &per_cpu(nmi_print_seq, cpu);
> +		seq_buf_init(&s->seq, s->buffer, NMI_BUF_SIZE);
> +	}
> +
> +	if (!cpumask_empty(to_cpumask(backtrace_mask))) {
> +		pr_info("Sending NMI to %s CPUs:\n",
> +			(include_self ? "all" : "other"));
> +		raise(to_cpumask(backtrace_mask));

On ARM, this code could be running with IRQs locked and with raise() 
implemented using IRQs. In such as case the IPI will not be raised until 
the function exists (and perhaps never). Thanks to the timeout we will 
exit but we end up needlessly failing to print a backtrace for the 
calling CPU.

The solution I used for this was to special case the current CPU and 
call nmi_cpu_backtrace() directly. Originally I made this logic arm only 
but I can't really see any reason for this to be arch specific so the 
logic to do that should probably be included here.

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