Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 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/x86/oprofile/nmi_timer_int.c" and in 9 "arch/x86/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_sched(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 In theory, rcu_dereference_sched() is not needed, since this code runs 51 only on i386, which in theory does not need rcu_dereference_sched() 52 anyway. However, in practice it is a good documentation aid, particularly 53 for anyone attempting to do something similar on Alpha or on systems 54 with aggressive optimizing compilers. 55 56 Quick 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 60 Back 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 67 The set_nmi_callback() function registers an NMI handler. Note that any 68 data that is to be used by the callback must be initialized up -before- 69 the call to set_nmi_callback(). On architectures that do not order 70 writes, the rcu_assign_pointer() ensures that the NMI handler sees the 71 initialized values. 72 73 void unset_nmi_callback(void) 74 { 75 rcu_assign_pointer(nmi_callback, dummy_nmi_callback); 76 } 77 78 This function unregisters an NMI handler, restoring the original 79 dummy_nmi_handler(). However, there may well be an NMI handler 80 currently executing on some other CPU. We therefore cannot free 81 up any data structures used by the old NMI handler until execution 82 of it completes on all other CPUs. 83 84 One way to accomplish this is via synchronize_sched(), perhaps as 85 follows: 86 87 unset_nmi_callback(); 88 synchronize_sched(); 89 kfree(my_nmi_data); 90 91 This works because synchronize_sched() blocks until all CPUs complete 92 any preemption-disabled segments of code that they were executing. 93 Since NMI handlers disable preemption, synchronize_sched() is guaranteed 94 not to return until all ongoing NMI handlers exit. It is therefore safe 95 to free up the handler's data as soon as synchronize_sched() returns. 96 97 Important note: for this to work, the architecture in question must 98 invoke nmi_enter() and nmi_exit() on NMI entry and exit, respectively. 99 100 101 Answer to Quick Quiz 102 103 Why might the rcu_dereference_sched() be necessary on Alpha, given 104 that the code referenced by the pointer is read-only? 105 106 Answer: The caller to set_nmi_callback() might well have 107 initialized some data that is to be used by the new NMI 108 handler. In this case, the rcu_dereference_sched() would 109 be needed, because otherwise a CPU that received an NMI 110 just after the new handler was set might see the pointer 111 to the new NMI handler, but the old pre-initialized 112 version of the handler's data. 113 114 This same sad story can happen on other CPUs when using 115 a compiler with aggressive pointer-value speculation 116 optimizations. 117 118 More important, the rcu_dereference_sched() makes it 119 clear to someone reading the code that the pointer is 120 being protected by RCU-sched.