/*
   * 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/kernel.h>
  #include <linux/module.h>
  #include <linux/irq_work.h>
  #include <linux/hardirq.h>
  
  /*
   * An entry can be in one of four states:
   *
   * free	     NULL, 0 -> {claimed}       : free to be used
   * claimed   NULL, 3 -> {pending}       : claimed to be enqueued
   * pending   next, 3 -> {busy}          : queued, pending callback
   * busy      NULL, 2 -> {free, claimed} : callback in progress, can be claimed
   *
   * We use the lower two bits of the next pointer to keep PENDING and BUSY
   * flags.
   */
  
  #define IRQ_WORK_PENDING	1UL
  #define IRQ_WORK_BUSY		2UL
  #define IRQ_WORK_FLAGS		3UL
  
  static inline bool irq_work_is_set(struct irq_work *entry, int flags)
  {
  	return (unsigned long)entry->next & flags;
  }
  
  static inline struct irq_work *irq_work_next(struct irq_work *entry)
  {
  	unsigned long next = (unsigned long)entry->next;
  	next &= ~IRQ_WORK_FLAGS;
  	return (struct irq_work *)next;
  }
  
  static inline struct irq_work *next_flags(struct irq_work *entry, int flags)
  {
  	unsigned long next = (unsigned long)entry;
  	next |= flags;
  	return (struct irq_work *)next;
  }
  
  static DEFINE_PER_CPU(struct irq_work *, irq_work_list);
  
  /*
   * Claim the entry so that no one else will poke at it.
   */
  static bool irq_work_claim(struct irq_work *entry)
  {
  	struct irq_work *next, *nflags;
  
  	do {
  		next = entry->next;
  		if ((unsigned long)next & IRQ_WORK_PENDING)
  			return false;
  		nflags = next_flags(next, IRQ_WORK_FLAGS);
  	} while (cmpxchg(&entry->next, next, nflags) != next);
  
  	return true;
  }
  
  
  void __weak arch_irq_work_raise(void)
  {
  	/*
  	 * Lame architectures will get the timer tick callback
  	 */
  }
  
  /*
   * Queue the entry and raise the IPI if needed.
   */
  static void __irq_work_queue(struct irq_work *entry)
  {

  	struct irq_work *next;

  	preempt_disable();

  
  	do {

  		next = __this_cpu_read(irq_work_list);

  		/* Can assign non-atomic because we keep the flags set. */
  		entry->next = next_flags(next, IRQ_WORK_FLAGS);

  	} while (this_cpu_cmpxchg(irq_work_list, next, entry) != next);

  
  	/* The list was empty, raise self-interrupt to start processing. */
  	if (!irq_work_next(entry))
  		arch_irq_work_raise();

  	preempt_enable();

  }
  
  /*
   * Enqueue the irq_work @entry, returns true on success, failure when the
   * @entry was already enqueued by someone else.
   *
   * Can be re-enqueued while the callback is still in progress.
   */
  bool irq_work_queue(struct irq_work *entry)
  {
  	if (!irq_work_claim(entry)) {
  		/*
  		 * Already enqueued, can't do!
  		 */
  		return false;
  	}
  
  	__irq_work_queue(entry);
  	return true;
  }
  EXPORT_SYMBOL_GPL(irq_work_queue);
  
  /*
   * Run the irq_work entries on this cpu. Requires to be ran from hardirq
   * context with local IRQs disabled.
   */
  void irq_work_run(void)
  {

  	struct irq_work *list;

  	if (this_cpu_read(irq_work_list) == NULL)

  		return;
  
  	BUG_ON(!in_irq());
  	BUG_ON(!irqs_disabled());

  	list = this_cpu_xchg(irq_work_list, NULL);

  	while (list != NULL) {
  		struct irq_work *entry = list;
  
  		list = irq_work_next(list);
  
  		/*
  		 * Clear the PENDING bit, after this point the @entry
  		 * can be re-used.
  		 */
  		entry->next = next_flags(NULL, IRQ_WORK_BUSY);
  		entry->func(entry);
  		/*
  		 * Clear the BUSY bit and return to the free state if
  		 * no-one else claimed it meanwhile.
  		 */

  		(void)cmpxchg(&entry->next,
  			      next_flags(NULL, IRQ_WORK_BUSY),
  			      NULL);

  	}
  }
  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 *entry)
  {
  	WARN_ON_ONCE(irqs_disabled());
  
  	while (irq_work_is_set(entry, IRQ_WORK_BUSY))
  		cpu_relax();
  }
  EXPORT_SYMBOL_GPL(irq_work_sync);