/*
   * 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/export.h>

  #include <linux/irq_work.h>

  #include <linux/percpu.h>

  #include <linux/hardirq.h>

  #include <asm/processor.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

   */
  
  #define IRQ_WORK_PENDING	1UL
  #define IRQ_WORK_BUSY		2UL
  #define IRQ_WORK_FLAGS		3UL

  static DEFINE_PER_CPU(struct llist_head, irq_work_list);

  
  /*
   * Claim the entry so that no one else will poke at it.
   */

  static bool irq_work_claim(struct irq_work *work)

  {

  	unsigned long flags, nflags;

  	for (;;) {
  		flags = work->flags;
  		if (flags & IRQ_WORK_PENDING)

  			return false;

  		nflags = flags | IRQ_WORK_FLAGS;
  		if (cmpxchg(&work->flags, flags, nflags) == flags)
  			break;
  		cpu_relax();
  	}

  
  	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 *work)

  {

  	bool empty;

  	preempt_disable();

  	empty = llist_add(&work->llnode, &__get_cpu_var(irq_work_list));

  	/* The list was empty, raise self-interrupt to start processing. */

  	if (empty)

  		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 *work)

  {

  	if (!irq_work_claim(work)) {

  		/*
  		 * Already enqueued, can't do!
  		 */
  		return false;
  	}

  	__irq_work_queue(work);

  	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 *work;
  	struct llist_head *this_list;
  	struct llist_node *llnode;

  	this_list = &__get_cpu_var(irq_work_list);
  	if (llist_empty(this_list))

  		return;
  
  	BUG_ON(!in_irq());
  	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.
  		 */

  		work->flags = IRQ_WORK_BUSY;
  		work->func(work);

  		/*
  		 * Clear the BUSY bit and return to the free state if
  		 * no-one else claimed it meanwhile.
  		 */

  		(void)cmpxchg(&work->flags, IRQ_WORK_BUSY, 0);

  	}
  }
  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);