v4.19.13 snapshot.
diff --git a/lib/nmi_backtrace.c b/lib/nmi_backtrace.c
new file mode 100644
index 0000000..15ca78e
--- /dev/null
+++ b/lib/nmi_backtrace.c
@@ -0,0 +1,110 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * NMI backtrace support
+ *
+ * Gratuitously copied from arch/x86/kernel/apic/hw_nmi.c by Russell King,
+ * with the following header:
+ *
+ * HW NMI watchdog support
+ *
+ * started by Don Zickus, Copyright (C) 2010 Red Hat, Inc.
+ *
+ * Arch specific calls to support NMI watchdog
+ *
+ * Bits copied from original nmi.c file
+ */
+#include <linux/cpumask.h>
+#include <linux/delay.h>
+#include <linux/kprobes.h>
+#include <linux/nmi.h>
+#include <linux/cpu.h>
+#include <linux/sched/debug.h>
+
+#ifdef arch_trigger_cpumask_backtrace
+/* For reliability, we're prepared to waste bits here. */
+static DECLARE_BITMAP(backtrace_mask, NR_CPUS) __read_mostly;
+
+/* "in progress" flag of arch_trigger_cpumask_backtrace */
+static unsigned long backtrace_flag;
+
+/*
+ * When raise() is called it will be passed a pointer to the
+ * backtrace_mask. Architectures that call nmi_cpu_backtrace()
+ * directly from their raise() functions may rely on the mask
+ * they are passed being updated as a side effect of this call.
+ */
+void nmi_trigger_cpumask_backtrace(const cpumask_t *mask,
+ bool exclude_self,
+ void (*raise)(cpumask_t *mask))
+{
+ int i, 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), mask);
+ if (exclude_self)
+ cpumask_clear_cpu(this_cpu, to_cpumask(backtrace_mask));
+
+ /*
+ * Don't try to send an NMI to this cpu; it may work on some
+ * architectures, but on others it may not, and we'll get
+ * information at least as useful just by doing a dump_stack() here.
+ * Note that nmi_cpu_backtrace(NULL) will clear the cpu bit.
+ */
+ if (cpumask_test_cpu(this_cpu, to_cpumask(backtrace_mask)))
+ nmi_cpu_backtrace(NULL);
+
+ if (!cpumask_empty(to_cpumask(backtrace_mask))) {
+ pr_info("Sending NMI from CPU %d to CPUs %*pbl:\n",
+ this_cpu, nr_cpumask_bits, to_cpumask(backtrace_mask));
+ raise(to_cpumask(backtrace_mask));
+ }
+
+ /* Wait for up to 10 seconds for all CPUs to do the backtrace */
+ for (i = 0; i < 10 * 1000; i++) {
+ if (cpumask_empty(to_cpumask(backtrace_mask)))
+ break;
+ mdelay(1);
+ touch_softlockup_watchdog();
+ }
+
+ /*
+ * Force flush any remote buffers that might be stuck in IRQ context
+ * and therefore could not run their irq_work.
+ */
+ printk_safe_flush();
+
+ clear_bit_unlock(0, &backtrace_flag);
+ put_cpu();
+}
+
+bool nmi_cpu_backtrace(struct pt_regs *regs)
+{
+ int cpu = smp_processor_id();
+
+ if (cpumask_test_cpu(cpu, to_cpumask(backtrace_mask))) {
+ if (regs && cpu_in_idle(instruction_pointer(regs))) {
+ pr_warn("NMI backtrace for cpu %d skipped: idling at %pS\n",
+ cpu, (void *)instruction_pointer(regs));
+ } else {
+ pr_warn("NMI backtrace for cpu %d\n", cpu);
+ if (regs)
+ show_regs(regs);
+ else
+ dump_stack();
+ }
+ cpumask_clear_cpu(cpu, to_cpumask(backtrace_mask));
+ return true;
+ }
+
+ return false;
+}
+NOKPROBE_SYMBOL(nmi_cpu_backtrace);
+#endif