1Using RCU to Protect Dynamic NMI Handlers
2
3
4Although RCU is usually used to protect read-mostly data structures,
5it is possible to use RCU to provide dynamic non-maskable interrupt
6handlers, as well as dynamic irq handlers.  This document describes
7how to do this, drawing loosely from Zwane Mwaikambo's NMI-timer
8work in "arch/x86/oprofile/nmi_timer_int.c" and in
9"arch/x86/kernel/traps.c".
10
11The relevant pieces of code are listed below, each followed by a
12brief explanation.
13
14	static int dummy_nmi_callback(struct pt_regs *regs, int cpu)
15	{
16		return 0;
17	}
18
19The dummy_nmi_callback() function is a "dummy" NMI handler that does
20nothing, but returns zero, thus saying that it did nothing, allowing
21the NMI handler to take the default machine-specific action.
22
23	static nmi_callback_t nmi_callback = dummy_nmi_callback;
24
25This nmi_callback variable is a global function pointer to the current
26NMI handler.
27
28	void do_nmi(struct pt_regs * regs, long error_code)
29	{
30		int cpu;
31
32		nmi_enter();
33
34		cpu = smp_processor_id();
35		++nmi_count(cpu);
36
37		if (!rcu_dereference_sched(nmi_callback)(regs, cpu))
38			default_do_nmi(regs);
39
40		nmi_exit();
41	}
42
43The do_nmi() function processes each NMI.  It first disables preemption
44in the same way that a hardware irq would, then increments the per-CPU
45count of NMIs.  It then invokes the NMI handler stored in the nmi_callback
46function pointer.  If this handler returns zero, do_nmi() invokes the
47default_do_nmi() function to handle a machine-specific NMI.  Finally,
48preemption is restored.
49
50In theory, rcu_dereference_sched() is not needed, since this code runs
51only on i386, which in theory does not need rcu_dereference_sched()
52anyway.  However, in practice it is a good documentation aid, particularly
53for anyone attempting to do something similar on Alpha or on systems
54with aggressive optimizing compilers.
55
56Quick Quiz:  Why might the rcu_dereference_sched() be necessary on Alpha,
57	     given that the code referenced by the pointer is read-only?
58
59
60Back to the discussion of NMI and RCU...
61
62	void set_nmi_callback(nmi_callback_t callback)
63	{
64		rcu_assign_pointer(nmi_callback, callback);
65	}
66
67The set_nmi_callback() function registers an NMI handler.  Note that any
68data that is to be used by the callback must be initialized up -before-
69the call to set_nmi_callback().  On architectures that do not order
70writes, the rcu_assign_pointer() ensures that the NMI handler sees the
71initialized values.
72
73	void unset_nmi_callback(void)
74	{
75		rcu_assign_pointer(nmi_callback, dummy_nmi_callback);
76	}
77
78This function unregisters an NMI handler, restoring the original
79dummy_nmi_handler().  However, there may well be an NMI handler
80currently executing on some other CPU.  We therefore cannot free
81up any data structures used by the old NMI handler until execution
82of it completes on all other CPUs.
83
84One way to accomplish this is via synchronize_rcu(), perhaps as
85follows:
86
87	unset_nmi_callback();
88	synchronize_rcu();
89	kfree(my_nmi_data);
90
91This works because (as of v4.20) synchronize_rcu() blocks until all
92CPUs complete any preemption-disabled segments of code that they were
93executing.
94Since NMI handlers disable preemption, synchronize_rcu() is guaranteed
95not to return until all ongoing NMI handlers exit.  It is therefore safe
96to free up the handler's data as soon as synchronize_rcu() returns.
97
98Important note: for this to work, the architecture in question must
99invoke nmi_enter() and nmi_exit() on NMI entry and exit, respectively.
100
101
102Answer to Quick Quiz
103
104	Why might the rcu_dereference_sched() be necessary on Alpha, given
105	that the code referenced by the pointer is read-only?
106
107	Answer: The caller to set_nmi_callback() might well have
108		initialized some data that is to be used by the new NMI
109		handler.  In this case, the rcu_dereference_sched() would
110		be needed, because otherwise a CPU that received an NMI
111		just after the new handler was set might see the pointer
112		to the new NMI handler, but the old pre-initialized
113		version of the handler's data.
114
115		This same sad story can happen on other CPUs when using
116		a compiler with aggressive pointer-value speculation
117		optimizations.
118
119		More important, the rcu_dereference_sched() makes it
120		clear to someone reading the code that the pointer is
121		being protected by RCU-sched.
122