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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.