Date: Thu, 16 Jul 2015 10:37:44 +0100
From: Russell King - ARM Linux <linux@arm.linux.org.uk>
To: Daniel Thompson <daniel.thompson@linaro.org>
Cc: linux-arm-kernel@lists.infradead.org, x86@kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/3] nmi: create generic NMI backtrace implementation
Message-ID: <20150716093744.GI7557@n2100.arm.linux.org.uk>
References: <20150715203911.GF7557@n2100.arm.linux.org.uk>
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 <55A7753C.9020708@linaro.org>
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On Thu, Jul 16, 2015 at 10:11:24AM +0100, Daniel Thompson wrote:
> 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.

That can be implemented in the arch raise() method if needed - most
architectures shouldn't need it as if they are properly raising a NMI
which is, by definition, deliverable with normal IRQs disabled.

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