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/x86/oprofile/nmi_timer_int.c" and in
"arch/x86/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.

	void do_nmi(struct pt_regs * regs, long error_code)
	{
		int cpu;

		nmi_enter();

		cpu = smp_processor_id();
		++nmi_count(cpu);

		if (!rcu_dereference_sched(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.

In theory, rcu_dereference_sched() is not needed, since this code runs
only on i386, which in theory does not need rcu_dereference_sched()
anyway.  However, in practice it is a good documentation aid, particularly
for anyone attempting to do something similar on Alpha or on systems
with aggressive optimizing compilers.

Quick Quiz:  Why might the rcu_dereference_sched() 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_rcu(), perhaps as
follows:

	unset_nmi_callback();
	synchronize_rcu();
	kfree(my_nmi_data);

This works because (as of v4.20) synchronize_rcu() blocks until all
CPUs complete any preemption-disabled segments of code that they were
executing.
Since NMI handlers disable preemption, synchronize_rcu() 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_rcu() returns.

Important note: for this to work, the architecture in question must
invoke nmi_enter() and nmi_exit() on NMI entry and exit, respectively.


Answer to Quick Quiz

	Why might the rcu_dereference_sched() 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_sched() 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.

		This same sad story can happen on other CPUs when using
		a compiler with aggressive pointer-value speculation
		optimizations.

		More important, the rcu_dereference_sched() makes it
		clear to someone reading the code that the pointer is
		being protected by RCU-sched.