Linux RT(2)－硬实时Linux(RT-Preempt Patch)的中断线程化
特别声明：本系列文章LiAnLab.org著作权所有，转载请注明出处。by @宋宝华Barry
底半部：线程化IRQ
线程化中断的支持在2009年已经进入Linux官方内核，详见Thomas Gleixner的patch:
http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=3aa551c9b4c40018f0e261a178e3d25478dc04a9
该patch提供一个能力，驱动可以通过
int request_threaded_irq(unsigned int irq, irq_handler_t handler,
irq_handler_t thread_fn, unsigned long irqflags,
const char *devname, void *dev_id)
申请一个线程化的IRQ，kernel会为中断的底版本创建一个名字为irq/%d-%s的线程，%d对应着中断号。其中顶半部（硬中断）handler在做完必要的处理工作之后，会返回IRQ_WAKE_THREAD，之后kernel会唤醒irq/%d-%s线程，而该kernel线程会调用thread_fn函数，因此，该线程成为底半部。在后续维护的过程中，笔者曾参与进一步完善该功能的讨论，后续patch包括nested、oneshot等的支持，详见patch：
http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=399b5da29b9f851eb7b96e2882097127f003e87c
http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=70aedd24d20e75198f5a0b11750faabbb56924e2
http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=b25c340c195447afb1860da580fe2a85a6b652c5
该机制目前在kernel中使用已经十分广泛，可以认为是继softirq（含tasklet）和workqueue之后的又一大中断底半部方式。
顶半部：强制线程化
在使能Linux RT-Preempt后，默认情况下会强制透过request_irq()申请的IRQ的顶半部函数在线程中执行，我们都知道request_irq的原型为：
static inline int __must_check
request_irq(unsigned int irq, irq_handler_t handler, unsigned long flags,
const char *name, void *dev)
{
return request_threaded_irq(irq, handler, NULL, flags, name, dev);
}
这意味着通过request_irq()申请的IRQ，在没有Rt-Preepmt的情况下，kernel并不会为其创建irq线程，因为它在最终调用request_threaded_irq()的时候传递的thread_fn为NULL。
如果使能了RT-Preempt Patch的情况下，其中的genirq-force-threading.patch会强制ARM使用threaded irq：
Index: linux-stable/arch/arm/Kconfig
===================================================================
--- linux-stable.orig/arch/arm/Kconfig
+++ linux-stable/arch/arm/Kconfig
@@ -40,6 +40,7 @@ config ARM
select GENERIC_IRQ_SHOW
select ARCH_WANT_IPC_PARSE_VERSION
select HARDIRQS_SW_RESEND
+ select IRQ_FORCED_THREADING
select CPU_PM if (SUSPEND || CPU_IDLE)
select GENERIC_PCI_IOMAP
select HAVE_BPF_JIT
在RT-Preempt Patch中，会针对使能了IRQ_FORCED_THREADING的情况，对这一原先没有线程化IRQ的case进行强制线程化，代码见__setup_irq()：
887 static int
888 __setup_irq(unsigned int irq, struct irq_desc *desc, struct irqaction *new)
889 {
890 ...
903
904 /*
905 * Check whether the interrupt nests into another interrupt
906 * thread.
907 */
908 nested = irq_settings_is_nested_thread(desc);
909 if (nested) {
910 ...
920 } else {
921 if (irq_settings_can_thread(desc))
922 irq_setup_forced_threading(new);
923 }
925 /*
926 * Create a handler thread when a thread function is supplied
927 * and the interrupt does not nest into another interrupt
928 * thread.
929 */
930 if (new->thread_fn && !nested) {
931 struct task_struct *t;
932
933 t = kthread_create(irq_thread, new, "irq/%d-%s", irq,
934 new->name);
935 ...
939 /*
940 * We keep the reference to the task struct even if
941 * the thread dies to avoid that the interrupt code
942 * references an already freed task_struct.
943 */
944 get_task_struct(t);
945 new->thread = t;
946
