#ifndef __LINUX_SEQLOCK_H
  #define __LINUX_SEQLOCK_H
  /*
   * Reader/writer consistent mechanism without starving writers. This type of
   * lock for data where the reader wants a consistent set of information
   * and is willing to retry if the information changes. There are two types
   * of readers:
   * 1. Sequence readers which never block a writer but they may have to retry
   *    if a writer is in progress by detecting change in sequence number.
   *    Writers do not wait for a sequence reader.
   * 2. Locking readers which will wait if a writer or another locking reader
   *    is in progress. A locking reader in progress will also block a writer
   *    from going forward. Unlike the regular rwlock, the read lock here is
   *    exclusive so that only one locking reader can get it.
   *
   * This is not as cache friendly as brlock. Also, this may not work well
   * for data that contains pointers, because any writer could
   * invalidate a pointer that a reader was following.
   *
   * Expected non-blocking reader usage:
   * 	do {
   *	    seq = read_seqbegin(&foo);
   * 	...
   *      } while (read_seqretry(&foo, seq));
   *
   *
   * On non-SMP the spin locks disappear but the writer still needs
   * to increment the sequence variables because an interrupt routine could
   * change the state of the data.
   *
   * Based on x86_64 vsyscall gettimeofday 
   * by Keith Owens and Andrea Arcangeli
   */
  
  #include <linux/spinlock.h>
  #include <linux/preempt.h>
  #include <linux/lockdep.h>
  #include <asm/processor.h>
  
  /*
   * Version using sequence counter only.
   * This can be used when code has its own mutex protecting the
   * updating starting before the write_seqcountbeqin() and ending
   * after the write_seqcount_end().
   */
  typedef struct seqcount {
  	unsigned sequence;
  #ifdef CONFIG_DEBUG_LOCK_ALLOC
  	struct lockdep_map dep_map;
  #endif
  } seqcount_t;
  
  static inline void __seqcount_init(seqcount_t *s, const char *name,
  					  struct lock_class_key *key)
  {
  	/*
  	 * Make sure we are not reinitializing a held lock:
  	 */
  	lockdep_init_map(&s->dep_map, name, key, 0);
  	s->sequence = 0;
  }
  
  #ifdef CONFIG_DEBUG_LOCK_ALLOC
  # define SEQCOUNT_DEP_MAP_INIT(lockname) \
  		.dep_map = { .name = #lockname } \
  
  # define seqcount_init(s)				\
  	do {						\
  		static struct lock_class_key __key;	\
  		__seqcount_init((s), #s, &__key);	\
  	} while (0)
  
  static inline void seqcount_lockdep_reader_access(const seqcount_t *s)
  {
  	seqcount_t *l = (seqcount_t *)s;
  	unsigned long flags;
  
  	local_irq_save(flags);
  	seqcount_acquire_read(&l->dep_map, 0, 0, _RET_IP_);
  	seqcount_release(&l->dep_map, 1, _RET_IP_);
  	local_irq_restore(flags);
  }
  
  #else
  # define SEQCOUNT_DEP_MAP_INIT(lockname)
  # define seqcount_init(s) __seqcount_init(s, NULL, NULL)
  # define seqcount_lockdep_reader_access(x)
  #endif
  
  #define SEQCNT_ZERO(lockname) { .sequence = 0, SEQCOUNT_DEP_MAP_INIT(lockname)}
  
  
  /**
   * __read_seqcount_begin - begin a seq-read critical section (without barrier)
   * @s: pointer to seqcount_t
   * Returns: count to be passed to read_seqcount_retry
   *
   * __read_seqcount_begin is like read_seqcount_begin, but has no smp_rmb()
   * barrier. Callers should ensure that smp_rmb() or equivalent ordering is
   * provided before actually loading any of the variables that are to be
   * protected in this critical section.
   *
   * Use carefully, only in critical code, and comment how the barrier is
   * provided.
   */
  static inline unsigned __read_seqcount_begin(const seqcount_t *s)
  {
  	unsigned ret;
  
  repeat:
  	ret = ACCESS_ONCE(s->sequence);
  	if (unlikely(ret & 1)) {
  		cpu_relax();
  		goto repeat;
  	}
  	return ret;
  }
  
  /**
   * raw_read_seqcount_begin - start seq-read critical section w/o lockdep
   * @s: pointer to seqcount_t
   * Returns: count to be passed to read_seqcount_retry
   *
   * raw_read_seqcount_begin opens a read critical section of the given
   * seqcount, but without any lockdep checking. Validity of the critical
   * section is tested by checking read_seqcount_retry function.
   */
  static inline unsigned raw_read_seqcount_begin(const seqcount_t *s)
  {
  	unsigned ret = __read_seqcount_begin(s);
  	smp_rmb();
  	return ret;
  }
  
  /**
   * read_seqcount_begin - begin a seq-read critical section
   * @s: pointer to seqcount_t
   * Returns: count to be passed to read_seqcount_retry
   *
   * read_seqcount_begin opens a read critical section of the given seqcount.
   * Validity of the critical section is tested by checking read_seqcount_retry
   * function.
   */
  static inline unsigned read_seqcount_begin(const seqcount_t *s)
  {
  	seqcount_lockdep_reader_access(s);
  	return raw_read_seqcount_begin(s);
  }
  
  /**
   * raw_seqcount_begin - begin a seq-read critical section
   * @s: pointer to seqcount_t
   * Returns: count to be passed to read_seqcount_retry
   *
   * raw_seqcount_begin opens a read critical section of the given seqcount.
   * Validity of the critical section is tested by checking read_seqcount_retry
   * function.
   *
   * Unlike read_seqcount_begin(), this function will not wait for the count
   * to stabilize. If a writer is active when we begin, we will fail the
   * read_seqcount_retry() instead of stabilizing at the beginning of the
   * critical section.
   */
  static inline unsigned raw_seqcount_begin(const seqcount_t *s)
  {
  	unsigned ret = ACCESS_ONCE(s->sequence);
  	smp_rmb();
  	return ret & ~1;
  }
  
  /**
   * __read_seqcount_retry - end a seq-read critical section (without barrier)
   * @s: pointer to seqcount_t
   * @start: count, from read_seqcount_begin
   * Returns: 1 if retry is required, else 0
   *
   * __read_seqcount_retry is like read_seqcount_retry, but has no smp_rmb()
   * barrier. Callers should ensure that smp_rmb() or equivalent ordering is
   * provided before actually loading any of the variables that are to be
   * protected in this critical section.
   *
   * Use carefully, only in critical code, and comment how the barrier is
   * provided.
   */
  static inline int __read_seqcount_retry(const seqcount_t *s, unsigned start)
  {
  	return unlikely(s->sequence != start);
  }
  
  /**
   * read_seqcount_retry - end a seq-read critical section
   * @s: pointer to seqcount_t
   * @start: count, from read_seqcount_begin
   * Returns: 1 if retry is required, else 0
   *
   * read_seqcount_retry closes a read critical section of the given seqcount.
   * If the critical section was invalid, it must be ignored (and typically
   * retried).
   */
  static inline int read_seqcount_retry(const seqcount_t *s, unsigned start)
  {
  	smp_rmb();
  	return __read_seqcount_retry(s, start);
  }
  
  
  
  static inline void raw_write_seqcount_begin(seqcount_t *s)
  {
  	s->sequence++;
  	smp_wmb();
  }
  
  static inline void raw_write_seqcount_end(seqcount_t *s)
  {
  	smp_wmb();
  	s->sequence++;
  }
  
  /*
   * Sequence counter only version assumes that callers are using their
   * own mutexing.
   */
  static inline void write_seqcount_begin_nested(seqcount_t *s, int subclass)
  {
  	raw_write_seqcount_begin(s);
  	seqcount_acquire(&s->dep_map, subclass, 0, _RET_IP_);
  }
  
  static inline void write_seqcount_begin(seqcount_t *s)
  {
  	write_seqcount_begin_nested(s, 0);
  }
  
  static inline void write_seqcount_end(seqcount_t *s)
  {
  	seqcount_release(&s->dep_map, 1, _RET_IP_);
  	raw_write_seqcount_end(s);
  }
  
  /**
   * write_seqcount_barrier - invalidate in-progress read-side seq operations
   * @s: pointer to seqcount_t
   *
   * After write_seqcount_barrier, no read-side seq operations will complete
   * successfully and see data older than this.
   */
  static inline void write_seqcount_barrier(seqcount_t *s)
  {
  	smp_wmb();
  	s->sequence+=2;
  }
  
  typedef struct {
  	struct seqcount seqcount;
  	spinlock_t lock;
  } seqlock_t;
  
  /*
   * These macros triggered gcc-3.x compile-time problems.  We think these are
   * OK now.  Be cautious.
   */
  #define __SEQLOCK_UNLOCKED(lockname)			\
  	{						\
  		.seqcount = SEQCNT_ZERO(lockname),	\
  		.lock =	__SPIN_LOCK_UNLOCKED(lockname)	\
  	}
  
  #define seqlock_init(x)					\
  	do {						\
  		seqcount_init(&(x)->seqcount);		\
  		spin_lock_init(&(x)->lock);		\
  	} while (0)
  
  #define DEFINE_SEQLOCK(x) \
  		seqlock_t x = __SEQLOCK_UNLOCKED(x)
  
  /*
   * Read side functions for starting and finalizing a read side section.
   */
  static inline unsigned read_seqbegin(const seqlock_t *sl)
  {
  	return read_seqcount_begin(&sl->seqcount);
  }
  
  static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start)
  {
  	return read_seqcount_retry(&sl->seqcount, start);
  }
  
  /*
   * Lock out other writers and update the count.
   * Acts like a normal spin_lock/unlock.
   * Don't need preempt_disable() because that is in the spin_lock already.
   */
  static inline void write_seqlock(seqlock_t *sl)
  {
  	spin_lock(&sl->lock);
  	write_seqcount_begin(&sl->seqcount);
  }
  
  static inline void write_sequnlock(seqlock_t *sl)
  {
  	write_seqcount_end(&sl->seqcount);
  	spin_unlock(&sl->lock);
  }
  
  static inline void write_seqlock_bh(seqlock_t *sl)
  {
  	spin_lock_bh(&sl->lock);
  	write_seqcount_begin(&sl->seqcount);
  }
  
  static inline void write_sequnlock_bh(seqlock_t *sl)
  {
  	write_seqcount_end(&sl->seqcount);
  	spin_unlock_bh(&sl->lock);
  }
  
  static inline void write_seqlock_irq(seqlock_t *sl)
  {
  	spin_lock_irq(&sl->lock);
  	write_seqcount_begin(&sl->seqcount);
  }
  
  static inline void write_sequnlock_irq(seqlock_t *sl)
  {
  	write_seqcount_end(&sl->seqcount);
  	spin_unlock_irq(&sl->lock);
  }
  
  static inline unsigned long __write_seqlock_irqsave(seqlock_t *sl)
  {
  	unsigned long flags;
  
  	spin_lock_irqsave(&sl->lock, flags);
  	write_seqcount_begin(&sl->seqcount);
  	return flags;
  }
  
  #define write_seqlock_irqsave(lock, flags)				\
  	do { flags = __write_seqlock_irqsave(lock); } while (0)
  
  static inline void
  write_sequnlock_irqrestore(seqlock_t *sl, unsigned long flags)
  {
  	write_seqcount_end(&sl->seqcount);
  	spin_unlock_irqrestore(&sl->lock, flags);
  }
  
  /*
   * A locking reader exclusively locks out other writers and locking readers,
   * but doesn't update the sequence number. Acts like a normal spin_lock/unlock.
   * Don't need preempt_disable() because that is in the spin_lock already.
   */
  static inline void read_seqlock_excl(seqlock_t *sl)
  {
  	spin_lock(&sl->lock);
  }
  
  static inline void read_sequnlock_excl(seqlock_t *sl)
  {
  	spin_unlock(&sl->lock);
  }
  
  /**
   * read_seqbegin_or_lock - begin a sequence number check or locking block
   * @lock: sequence lock
   * @seq : sequence number to be checked
   *
   * First try it once optimistically without taking the lock. If that fails,
   * take the lock. The sequence number is also used as a marker for deciding
   * whether to be a reader (even) or writer (odd).
   * N.B. seq must be initialized to an even number to begin with.
   */
  static inline void read_seqbegin_or_lock(seqlock_t *lock, int *seq)
  {
  	if (!(*seq & 1))	/* Even */
  		*seq = read_seqbegin(lock);
  	else			/* Odd */
  		read_seqlock_excl(lock);
  }
  
  static inline int need_seqretry(seqlock_t *lock, int seq)
  {
  	return !(seq & 1) && read_seqretry(lock, seq);
  }
  
  static inline void done_seqretry(seqlock_t *lock, int seq)
  {
  	if (seq & 1)
  		read_sequnlock_excl(lock);
  }
  
  static inline void read_seqlock_excl_bh(seqlock_t *sl)
  {
  	spin_lock_bh(&sl->lock);
  }
  
  static inline void read_sequnlock_excl_bh(seqlock_t *sl)
  {
  	spin_unlock_bh(&sl->lock);
  }
  
  static inline void read_seqlock_excl_irq(seqlock_t *sl)
  {
  	spin_lock_irq(&sl->lock);
  }
  
  static inline void read_sequnlock_excl_irq(seqlock_t *sl)
  {
  	spin_unlock_irq(&sl->lock);
  }
  
  static inline unsigned long __read_seqlock_excl_irqsave(seqlock_t *sl)
  {
  	unsigned long flags;
  
  	spin_lock_irqsave(&sl->lock, flags);
  	return flags;
  }
  
  #define read_seqlock_excl_irqsave(lock, flags)				\
  	do { flags = __read_seqlock_excl_irqsave(lock); } while (0)
  
  static inline void
  read_sequnlock_excl_irqrestore(seqlock_t *sl, unsigned long flags)
  {
  	spin_unlock_irqrestore(&sl->lock, flags);
  }
  
  #endif /* __LINUX_SEQLOCK_H */