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/i386/oprofile/nmi_timer_int.c" and in 9"arch/i386/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(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 50Strictly speaking, rcu_dereference() is not needed, since this code runs 51only on i386, which does not need rcu_dereference() anyway. However, 52it is a good documentation aid, particularly for anyone attempting to 53do something similar on Alpha. 54 55Quick Quiz: Why might the rcu_dereference() be necessary on Alpha, 56 given that the code referenced by the pointer is read-only? 57 58 59Back 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 66The set_nmi_callback() function registers an NMI handler. Note that any 67data that is to be used by the callback must be initialized up -before- 68the call to set_nmi_callback(). On architectures that do not order 69writes, the rcu_assign_pointer() ensures that the NMI handler sees the 70initialized values. 71 72 void unset_nmi_callback(void) 73 { 74 rcu_assign_pointer(nmi_callback, dummy_nmi_callback); 75 } 76 77This function unregisters an NMI handler, restoring the original 78dummy_nmi_handler(). However, there may well be an NMI handler 79currently executing on some other CPU. We therefore cannot free 80up any data structures used by the old NMI handler until execution 81of it completes on all other CPUs. 82 83One way to accomplish this is via synchronize_sched(), perhaps as 84follows: 85 86 unset_nmi_callback(); 87 synchronize_sched(); 88 kfree(my_nmi_data); 89 90This works because synchronize_sched() blocks until all CPUs complete 91any preemption-disabled segments of code that they were executing. 92Since NMI handlers disable preemption, synchronize_sched() is guaranteed 93not to return until all ongoing NMI handlers exit. It is therefore safe 94to free up the handler's data as soon as synchronize_sched() returns. 95 96Important note: for this to work, the architecture in question must 97invoke irq_enter() and irq_exit() on NMI entry and exit, respectively. 98 99 100Answer to Quick Quiz 101 102 Why might the rcu_dereference() be necessary on Alpha, given 103 that the code referenced by the pointer is read-only? 104 105 Answer: The caller to set_nmi_callback() might well have 106 initialized some data that is to be used by the 107 new NMI handler. In this case, the rcu_dereference() 108 would be needed, because otherwise a CPU that received 109 an NMI just after the new handler was set might see 110 the pointer to the new NMI handler, but the old 111 pre-initialized version of the handler's data. 112 113 More important, the rcu_dereference() makes it clear 114 to someone reading the code that the pointer is being 115 protected by RCU. 116