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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/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_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.
-
-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.