Based on kernel version 2.6.25. Page generated on 2008-04-18 21:22 EST.
1 Using RCU to Protect Dynamic NMI Handlers 2 3 4 Although RCU is usually used to protect read-mostly data structures, 5 it is possible to use RCU to provide dynamic non-maskable interrupt 6 handlers, as well as dynamic irq handlers. This document describes 7 how to do this, drawing loosely from Zwane Mwaikambo's NMI-timer 8 work in "arch/i386/oprofile/nmi_timer_int.c" and in 9 "arch/i386/kernel/traps.c". 10 11 The relevant pieces of code are listed below, each followed by a 12 brief explanation. 13 14 static int dummy_nmi_callback(struct pt_regs *regs, int cpu) 15 { 16 return 0; 17 } 18 19 The dummy_nmi_callback() function is a "dummy" NMI handler that does 20 nothing, but returns zero, thus saying that it did nothing, allowing 21 the NMI handler to take the default machine-specific action. 22 23 static nmi_callback_t nmi_callback = dummy_nmi_callback; 24 25 This nmi_callback variable is a global function pointer to the current 26 NMI 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(nmi_callback)(regs, cpu)) 38 default_do_nmi(regs); 39 40 nmi_exit(); 41 } 42 43 The do_nmi() function processes each NMI. It first disables preemption 44 in the same way that a hardware irq would, then increments the per-CPU 45 count of NMIs. It then invokes the NMI handler stored in the nmi_callback 46 function pointer. If this handler returns zero, do_nmi() invokes the 47 default_do_nmi() function to handle a machine-specific NMI. Finally, 48 preemption is restored. 49 50 Strictly speaking, rcu_dereference() is not needed, since this code runs 51 only on i386, which does not need rcu_dereference() anyway. However, 52 it is a good documentation aid, particularly for anyone attempting to 53 do something similar on Alpha. 54 55 Quick Quiz: Why might the rcu_dereference() be necessary on Alpha, 56 given that the code referenced by the pointer is read-only? 57 58 59 Back to the discussion of NMI and RCU... 60 61 void set_nmi_callback(nmi_callback_t callback) 62 { 63 rcu_assign_pointer(nmi_callback, callback); 64 } 65 66 The set_nmi_callback() function registers an NMI handler. Note that any 67 data that is to be used by the callback must be initialized up -before- 68 the call to set_nmi_callback(). On architectures that do not order 69 writes, the rcu_assign_pointer() ensures that the NMI handler sees the 70 initialized values. 71 72 void unset_nmi_callback(void) 73 { 74 rcu_assign_pointer(nmi_callback, dummy_nmi_callback); 75 } 76 77 This function unregisters an NMI handler, restoring the original 78 dummy_nmi_handler(). However, there may well be an NMI handler 79 currently executing on some other CPU. We therefore cannot free 80 up any data structures used by the old NMI handler until execution 81 of it completes on all other CPUs. 82 83 One way to accomplish this is via synchronize_sched(), perhaps as 84 follows: 85 86 unset_nmi_callback(); 87 synchronize_sched(); 88 kfree(my_nmi_data); 89 90 This works because synchronize_sched() blocks until all CPUs complete 91 any preemption-disabled segments of code that they were executing. 92 Since NMI handlers disable preemption, synchronize_sched() is guaranteed 93 not to return until all ongoing NMI handlers exit. It is therefore safe 94 to free up the handler's data as soon as synchronize_sched() returns. 95 96 97 Answer to Quick Quiz 98 99 Why might the rcu_dereference() be necessary on Alpha, given 100 that the code referenced by the pointer is read-only? 101 102 Answer: The caller to set_nmi_callback() might well have 103 initialized some data that is to be used by the 104 new NMI handler. In this case, the rcu_dereference() 105 would be needed, because otherwise a CPU that received 106 an NMI just after the new handler was set might see 107 the pointer to the new NMI handler, but the old 108 pre-initialized version of the handler's data. 109 110 More important, the rcu_dereference() makes it clear 111 to someone reading the code that the pointer is being 112 protected by RCU.