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+Using RCU to Protect Dynamic NMI Handlers
+
+
+Although RCU is usually used to protect read-mostly data structures,
+it is possible to use RCU to provide dynamic non-maskable interrupt
+handlers, as well as dynamic irq handlers. This document describes
+how to do this, drawing loosely from Zwane Mwaikambo's NMI-timer
+work in "arch/i386/oprofile/nmi_timer_int.c" and in
+"arch/i386/kernel/traps.c".
+
+The relevant pieces of code are listed below, each followed by a
+brief explanation.
+
+ static int dummy_nmi_callback(struct pt_regs *regs, int cpu)
+ {
+ return 0;
+ }
+
+The dummy_nmi_callback() function is a "dummy" NMI handler that does
+nothing, but returns zero, thus saying that it did nothing, allowing
+the NMI handler to take the default machine-specific action.
+
+ static nmi_callback_t nmi_callback = dummy_nmi_callback;
+
+This nmi_callback variable is a global function pointer to the current
+NMI handler.
+
+ fastcall void do_nmi(struct pt_regs * regs, long error_code)
+ {
+ int cpu;
+
+ nmi_enter();
+
+ cpu = smp_processor_id();
+ ++nmi_count(cpu);
+
+ if (!rcu_dereference(nmi_callback)(regs, cpu))
+ default_do_nmi(regs);
+
+ nmi_exit();
+ }
+
+The do_nmi() function processes each NMI. It first disables preemption
+in the same way that a hardware irq would, then increments the per-CPU
+count of NMIs. It then invokes the NMI handler stored in the nmi_callback
+function pointer. If this handler returns zero, do_nmi() invokes the
+default_do_nmi() function to handle a machine-specific NMI. Finally,
+preemption is restored.
+
+Strictly speaking, rcu_dereference() is not needed, since this code runs
+only on i386, which does not need rcu_dereference() anyway. However,
+it is a good documentation aid, particularly for anyone attempting to
+do something similar on Alpha.
+
+Quick Quiz: Why might the rcu_dereference() be necessary on Alpha,
+ given that the code referenced by the pointer is read-only?
+
+
+Back to the discussion of NMI and RCU...
+
+ void set_nmi_callback(nmi_callback_t callback)
+ {
+ rcu_assign_pointer(nmi_callback, callback);
+ }
+
+The set_nmi_callback() function registers an NMI handler. Note that any
+data that is to be used by the callback must be initialized up -before-
+the call to set_nmi_callback(). On architectures that do not order
+writes, the rcu_assign_pointer() ensures that the NMI handler sees the
+initialized values.
+
+ void unset_nmi_callback(void)
+ {
+ rcu_assign_pointer(nmi_callback, dummy_nmi_callback);
+ }
+
+This function unregisters an NMI handler, restoring the original
+dummy_nmi_handler(). However, there may well be an NMI handler
+currently executing on some other CPU. We therefore cannot free
+up any data structures used by the old NMI handler until execution
+of it completes on all other CPUs.
+
+One way to accomplish this is via synchronize_sched(), perhaps as
+follows:
+
+ unset_nmi_callback();
+ synchronize_sched();
+ kfree(my_nmi_data);
+
+This works because synchronize_sched() blocks until all CPUs complete
+any preemption-disabled segments of code that they were executing.
+Since NMI handlers disable preemption, synchronize_sched() is guaranteed
+not to return until all ongoing NMI handlers exit. It is therefore safe
+to free up the handler's data as soon as synchronize_sched() returns.
+
+
+Answer to Quick Quiz
+
+ Why might the rcu_dereference() be necessary on Alpha, given
+ that the code referenced by the pointer is read-only?
+
+ Answer: The caller to set_nmi_callback() might well have
+ initialized some data that is to be used by the
+ new NMI handler. In this case, the rcu_dereference()
+ would be needed, because otherwise a CPU that received
+ an NMI just after the new handler was set might see
+ the pointer to the new NMI handler, but the old
+ pre-initialized version of the handler's data.
+
+ More important, the rcu_dereference() makes it clear
+ to someone reading the code that the pointer is being
+ protected by RCU.
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