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Diffstat (limited to 'Documentation/RCU/NMI-RCU.txt')
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diff --git a/Documentation/RCU/NMI-RCU.txt b/Documentation/RCU/NMI-RCU.txt deleted file mode 100644 index 881353fd5bff..000000000000 --- a/Documentation/RCU/NMI-RCU.txt +++ /dev/null @@ -1,121 +0,0 @@ -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_rcu(), perhaps as -follows: - - unset_nmi_callback(); - synchronize_rcu(); - kfree(my_nmi_data); - -This works because (as of v4.20) synchronize_rcu() blocks until all -CPUs complete any preemption-disabled segments of code that they were -executing. -Since NMI handlers disable preemption, synchronize_rcu() 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_rcu() 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. |