text: /*
   * Copyright (C) 2010 Red Hat, Inc., Peter Zijlstra <pzijlstr@redhat.com>
   *
   * Provides a framework for enqueueing and running callbacks from hardirq
   * context. The enqueueing is NMI-safe.
   */
  
  #include <linux/bug.h>
  #include <linux/kernel.h>
  #include <linux/export.h>
  #include <linux/irq_work.h>
  #include <linux/percpu.h>
  #include <linux/hardirq.h>
  #include <linux/irqflags.h>
  #include <linux/sched.h>
  #include <linux/tick.h>
  #include <linux/cpu.h>
  #include <linux/notifier.h>
  #include <asm/processor.h>
  
  
  static DEFINE_PER_CPU(struct llist_head, irq_work_list);
  static DEFINE_PER_CPU(int, irq_work_raised);
  
  /*
   * Claim the entry so that no one else will poke at it.
   */
  static bool irq_work_claim(struct irq_work *work)
  {
  	unsigned long flags, oflags, nflags;
  
  	/*
  	 * Start with our best wish as a premise but only trust any
  	 * flag value after cmpxchg() result.
  	 */
  	flags = work->flags & ~IRQ_WORK_PENDING;
  	for (;;) {
  		nflags = flags | IRQ_WORK_FLAGS;
  		oflags = cmpxchg(&work->flags, flags, nflags);
  		if (oflags == flags)
  			break;
  		if (oflags & IRQ_WORK_PENDING)
  			return false;
  		flags = oflags;
  		cpu_relax();
  	}
  
  	return true;
  }
  
  void __weak arch_irq_work_raise(void)
  {
  	/*
  	 * Lame architectures will get the timer tick callback
  	 */
  }
  
  /*
   * Enqueue the irq_work @entry unless it's already pending
   * somewhere.
   *
   * Can be re-enqueued while the callback is still in progress.
   */
  bool irq_work_queue(struct irq_work *work)
  {
  	/* Only queue if not already pending */
  	if (!irq_work_claim(work))
  		return false;
  
  	/* Queue the entry and raise the IPI if needed. */
  	preempt_disable();
  
  	llist_add(&work->llnode, &__get_cpu_var(irq_work_list));
  
  	/*
  	 * If the work is not "lazy" or the tick is stopped, raise the irq
  	 * work interrupt (if supported by the arch), otherwise, just wait
  	 * for the next tick.
  	 */
  	if (!(work->flags & IRQ_WORK_LAZY) || tick_nohz_tick_stopped()) {
  		if (!this_cpu_cmpxchg(irq_work_raised, 0, 1))
  			arch_irq_work_raise();
  	}
  
  	preempt_enable();
  
  	return true;
  }
  EXPORT_SYMBOL_GPL(irq_work_queue);
  
  bool irq_work_needs_cpu(void)
  {
  	struct llist_head *this_list;
  
  	this_list = &__get_cpu_var(irq_work_list);
  	if (llist_empty(this_list))
  		return false;
  
  	/* All work should have been flushed before going offline */
  	WARN_ON_ONCE(cpu_is_offline(smp_processor_id()));
  
  	return true;
  }
  
  static void __irq_work_run(void)
  {
  	unsigned long flags;
  	struct irq_work *work;
  	struct llist_head *this_list;
  	struct llist_node *llnode;
  
  
  	/*
  	 * Reset the "raised" state right before we check the list because
  	 * an NMI may enqueue after we find the list empty from the runner.
  	 */
  	__this_cpu_write(irq_work_raised, 0);
  	barrier();
  
  	this_list = &__get_cpu_var(irq_work_list);
  	if (llist_empty(this_list))
  		return;
  
  	BUG_ON(!irqs_disabled());
  
  	llnode = llist_del_all(this_list);
  	while (llnode != NULL) {
  		work = llist_entry(llnode, struct irq_work, llnode);
  
  		llnode = llist_next(llnode);
  
  		/*
  		 * Clear the PENDING bit, after this point the @work
  		 * can be re-used.
  		 * Make it immediately visible so that other CPUs trying
  		 * to claim that work don't rely on us to handle their data
  		 * while we are in the middle of the func.
  		 */
  		flags = work->flags & ~IRQ_WORK_PENDING;
  		xchg(&work->flags, flags);
  
  		work->func(work);
  		/*
  		 * Clear the BUSY bit and return to the free state if
  		 * no-one else claimed it meanwhile.
  		 */
  		(void)cmpxchg(&work->flags, flags, flags & ~IRQ_WORK_BUSY);
  	}
  }
  
  /*
   * Run the irq_work entries on this cpu. Requires to be ran from hardirq
   * context with local IRQs disabled.
   */
  void irq_work_run(void)
  {
  	BUG_ON(!in_irq());
  	__irq_work_run();
  }
  EXPORT_SYMBOL_GPL(irq_work_run);
  
  /*
   * Synchronize against the irq_work @entry, ensures the entry is not
   * currently in use.
   */
  void irq_work_sync(struct irq_work *work)
  {
  	WARN_ON_ONCE(irqs_disabled());
  
  	while (work->flags & IRQ_WORK_BUSY)
  		cpu_relax();
  }
  EXPORT_SYMBOL_GPL(irq_work_sync);
  
  #ifdef CONFIG_HOTPLUG_CPU
  static int irq_work_cpu_notify(struct notifier_block *self,
  			       unsigned long action, void *hcpu)
  {
  	long cpu = (long)hcpu;
  
  	switch (action) {
  	case CPU_DYING:
  		/* Called from stop_machine */
  		if (WARN_ON_ONCE(cpu != smp_processor_id()))
  			break;
  		__irq_work_run();
  		break;
  	default:
  		break;
  	}
  	return NOTIFY_OK;
  }
  
  static struct notifier_block cpu_notify;
  
  static __init int irq_work_init_cpu_notifier(void)
  {
  	cpu_notify.notifier_call = irq_work_cpu_notify;
  	cpu_notify.priority = 0;
  	register_cpu_notifier(&cpu_notify);
  	return 0;
  }
  device_initcall(irq_work_init_cpu_notifier);
  
  #endif /* CONFIG_HOTPLUG_CPU */
  