include/linux/seqlock.h - kernel/lockdown - Git at Google

 #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 = READ_ONCE(s->sequence);
 	if (unlikely(ret & 1)) {
 		cpu_relax();
 		goto repeat;
 	}
 	return ret;
 }

 /**
  * raw_read_seqcount - Read the raw seqcount
  * @s: pointer to seqcount_t
  * Returns: count to be passed to read_seqcount_retry
  *
  * raw_read_seqcount opens a read critical section of the given
  * seqcount without any lockdep checking and without checking or
  * masking the LSB. Calling code is responsible for handling that.
  */
 static inline unsigned raw_read_seqcount(const seqcount_t *s)
 {
 	unsigned ret = READ_ONCE(s->sequence);
 	smp_rmb();
 	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 = READ_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++;
 }

 /*
  * raw_write_seqcount_latch - redirect readers to even/odd copy
  * @s: pointer to seqcount_t
  */
 static inline void raw_write_seqcount_latch(seqcount_t *s)
 {
        smp_wmb();      /* prior stores before incrementing "sequence" */
        s->sequence++;
        smp_wmb();      /* increment "sequence" before following stores */
 }

 /*
  * 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_invalidate - invalidate in-progress read-side seq operations
  * @s: pointer to seqcount_t
  *
  * After write_seqcount_invalidate, no read-side seq operations will complete
  * successfully and see data older than this.
  */
 static inline void write_seqcount_invalidate(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);
 }

 static inline unsigned long
 read_seqbegin_or_lock_irqsave(seqlock_t *lock, int *seq)
 {
 	unsigned long flags = 0;

 	if (!(*seq & 1))	/* Even */
 		*seq = read_seqbegin(lock);
 	else			/* Odd */
 		read_seqlock_excl_irqsave(lock, flags);

 	return flags;
 }

 static inline void
 done_seqretry_irqrestore(seqlock_t *lock, int seq, unsigned long flags)
 {
 	if (seq & 1)
 		read_sequnlock_excl_irqrestore(lock, flags);
 }
 #endif /* __LINUX_SEQLOCK_H */
	#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 = READ_ONCE(s->sequence);
	if (unlikely(ret & 1)) {
	cpu_relax();
	goto repeat;
	}
	return ret;
	}

	/**
	* raw_read_seqcount - Read the raw seqcount
	* @s: pointer to seqcount_t
	* Returns: count to be passed to read_seqcount_retry
	*
	* raw_read_seqcount opens a read critical section of the given
	* seqcount without any lockdep checking and without checking or
	* masking the LSB. Calling code is responsible for handling that.
	*/
	static inline unsigned raw_read_seqcount(const seqcount_t *s)
	{
	unsigned ret = READ_ONCE(s->sequence);
	smp_rmb();
	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 = READ_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++;
	}

	/*
	* raw_write_seqcount_latch - redirect readers to even/odd copy
	* @s: pointer to seqcount_t
	*/
	static inline void raw_write_seqcount_latch(seqcount_t *s)
	{
	smp_wmb(); /* prior stores before incrementing "sequence" */
	s->sequence++;
	smp_wmb(); /* increment "sequence" before following stores */
	}

	/*
	* 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_invalidate - invalidate in-progress read-side seq operations
	* @s: pointer to seqcount_t
	*
	* After write_seqcount_invalidate, no read-side seq operations will complete
	* successfully and see data older than this.
	*/
	static inline void write_seqcount_invalidate(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);
	}

	static inline unsigned long
	read_seqbegin_or_lock_irqsave(seqlock_t lock, int seq)
	{
	unsigned long flags = 0;

	if (!(seq & 1)) / Even */
	*seq = read_seqbegin(lock);
	else /* Odd */
	read_seqlock_excl_irqsave(lock, flags);

	return flags;
	}

	static inline void
	done_seqretry_irqrestore(seqlock_t *lock, int seq, unsigned long flags)
	{
	if (seq & 1)
	read_sequnlock_excl_irqrestore(lock, flags);
	}
	#endif /* __LINUX_SEQLOCK_H */