自旋锁

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#Linux #驱动 #锁

除了信号量的另一个使用的比较多的互斥方法是自旋锁。自旋锁只有两种情况：锁住和解锁。与信号量未获得信号量时进入休眠状态不同，自旋锁会不断的去检测是否能锁上。

因为自旋锁不会进入休眠状态，所以自旋锁的临界区代码不能进入休眠状态或放弃处理器（被高优先级的线程抢占或中断），因为自旋锁的临界区代码进入休眠状态，可能其他试图获得自旋锁的线程在一直检测自旋锁，而不会放弃处理器，所以自旋锁的临界区必需是原子操作。

拥有自旋锁的代码不会被别的线程抢占。自旋锁的执行时间必须很短。

使用锁（信号量或自旋锁）时必须注意不要引起死锁，引起死锁的几种情况：

1）获得锁的代码又调用了需要获取同一个锁的函数

2）两个线程都需要获得锁a和b，但是获得锁的顺序相反

1）定义和初始化

spinlock_t mylock= SPIN_LOCK_UNLOCKED;

或

spin_lock_init(lock);

2)锁住自旋锁

spin_lock(lock);

spin_lock_irq(lock)

spin_lock_bh(lock)

read_lock_irqsave(lock, flags)

3)解锁自旋锁

spin_unlock(lock);

spin_unlock_irq(lock)

spin_unlock_bh(lock)

read_unlock_irqrestore(lock, flags)

spin_trylock(lock)

spin_trylock_bh(lock)

读取者/写入者自旋锁

读取者/写入者与自旋锁的关系与读取者/写入者信号量和信号量的关系一样，使用方法也一样。

1）定义和初始化

rwlock_t my_rwlock = RW_LOCK_UNLOCKED;

或

rwlock_init(lock)

2)读锁

read_lock(lock)

read_lock_irqsave(lock)

read_lock_irq(lock)

read_lock_bh(lock)

read_unlock(lock)

read_unlock_irqrestore(lock)

read_unlock_irq(lock)

read_unlock_bh(lock)

3）写锁

write_lock(lock)

write_lock_irqsave(lock)

write_lock_irq(lock)

write_lock_bh(lock)

write_unlock(lock)

write_unlock_irqrestore(lock)

write_unlock_irq(lock)

write_unlock_bh(lock)

<linux/spinlock.h>头文件

#ifndef __LINUX_SPINLOCK_H
#define __LINUX_SPINLOCK_H

/*
 * include/linux/spinlock.h - generic spinlock/rwlock declarations
 *
 * here's the role of the various spinlock/rwlock related include files:
 *
 * on SMP builds:
 *
 *  asm/spinlock_types.h: contains the raw_spinlock_t/raw_rwlock_t and the
 *                        initializers
 *
 *  linux/spinlock_types.h:
 *                        defines the generic type and initializers
 *
 *  asm/spinlock.h:       contains the __raw_spin_*()/etc. lowlevel
 *                        implementations, mostly inline assembly code
 *
 *   (also included on UP-debug builds:)
 *
 *  linux/spinlock_api_smp.h:
 *                        contains the prototypes for the _spin_*() APIs.
 *
 *  linux/spinlock.h:     builds the final spin_*() APIs.
 *
 * on UP builds:
 *
 *  linux/spinlock_type_up.h:
 *                        contains the generic, simplified UP spinlock type.
 *                        (which is an empty structure on non-debug builds)
 *
 *  linux/spinlock_types.h:
 *                        defines the generic type and initializers
 *
 *  linux/spinlock_up.h:
 *                        contains the __raw_spin_*()/etc. version of UP
 *                        builds. (which are NOPs on non-debug, non-preempt
 *                        builds)
 *
 *   (included on UP-non-debug builds:)
 *
 *  linux/spinlock_api_up.h:
 *                        builds the _spin_*() APIs.
 *
 *  linux/spinlock.h:     builds the final spin_*() APIs.
 */

#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 

#include 

/*
 * Must define these before including other files, inline functions need them
 */
#define LOCK_SECTION_NAME ".text.lock."KBUILD_BASENAME

#define LOCK_SECTION_START(extra)               \
        ".subsection 1\n\t"                     \
        extra                                   \
        ".ifndef " LOCK_SECTION_NAME "\n\t"     \
        LOCK_SECTION_NAME ":\n\t"               \
        ".endif\n"

#define LOCK_SECTION_END                        \
        ".previous\n\t"

#define __lockfunc __attribute__((section(".spinlock.text")))

/*
 * Pull the raw_spinlock_t and raw_rwlock_t definitions:
 */
#include 

extern int __lockfunc generic__raw_read_trylock(raw_rwlock_t *lock);

/*
 * Pull the __raw*() functions/declarations (UP-nondebug doesnt need them):
 */
#ifdef CONFIG_SMP
# include 
#else
# include 
#endif

#ifdef CONFIG_DEBUG_SPINLOCK
  extern void __spin_lock_init(spinlock_t *lock, const char *name,
    		       struct lock_class_key *key);
# define spin_lock_init(lock)					\
do {								\
	static struct lock_class_key __key;			\
								\
	__spin_lock_init((lock), #lock, &__key);		\
} while (0)

#else
# define spin_lock_init(lock)					\
	do { *(lock) = SPIN_LOCK_UNLOCKED; } while (0)
#endif

#ifdef CONFIG_DEBUG_SPINLOCK
  extern void __rwlock_init(rwlock_t *lock, const char *name,
			    struct lock_class_key *key);
# define rwlock_init(lock)					\
do {								\
	static struct lock_class_key __key;			\
								\
	__rwlock_init((lock), #lock, &__key);			\
} while (0)
#else
# define rwlock_init(lock)					\
	do { *(lock) = RW_LOCK_UNLOCKED; } while (0)
#endif

#define spin_is_locked(lock)	__raw_spin_is_locked(&(lock)->raw_lock)

#ifdef CONFIG_GENERIC_LOCKBREAK
#define spin_is_contended(lock) ((lock)->break_lock)
#else

#ifdef __raw_spin_is_contended
#define spin_is_contended(lock)	__raw_spin_is_contended(&(lock)->raw_lock)
#else
#define spin_is_contended(lock)	(((void)(lock), 0))
#endif /*__raw_spin_is_contended*/
#endif

/* The lock does not imply full memory barrier. */
#ifndef ARCH_HAS_SMP_MB_AFTER_LOCK
static inline void smp_mb__after_lock(void) { smp_mb(); }
#endif

/**
 * spin_unlock_wait - wait until the spinlock gets unlocked
 * @lock: the spinlock in question.
 */
#define spin_unlock_wait(lock)	__raw_spin_unlock_wait(&(lock)->raw_lock)

#ifdef CONFIG_DEBUG_SPINLOCK
 extern void _raw_spin_lock(spinlock_t *lock);
#define _raw_spin_lock_flags(lock, flags) _raw_spin_lock(lock)
 extern int _raw_spin_trylock(spinlock_t *lock);
 extern void _raw_spin_unlock(spinlock_t *lock);
 extern void _raw_read_lock(rwlock_t *lock);
#define _raw_read_lock_flags(lock, flags) _raw_read_lock(lock)
 extern int _raw_read_trylock(rwlock_t *lock);
 extern void _raw_read_unlock(rwlock_t *lock);
 extern void _raw_write_lock(rwlock_t *lock);
#define _raw_write_lock_flags(lock, flags) _raw_write_lock(lock)
 extern int _raw_write_trylock(rwlock_t *lock);
 extern void _raw_write_unlock(rwlock_t *lock);
#else
# define _raw_spin_lock(lock)		__raw_spin_lock(&(lock)->raw_lock)
# define _raw_spin_lock_flags(lock, flags) \
		__raw_spin_lock_flags(&(lock)->raw_lock, *(flags))
# define _raw_spin_trylock(lock)	__raw_spin_trylock(&(lock)->raw_lock)
# define _raw_spin_unlock(lock)		__raw_spin_unlock(&(lock)->raw_lock)
# define _raw_read_lock(rwlock)		__raw_read_lock(&(rwlock)->raw_lock)
# define _raw_read_lock_flags(lock, flags) \
		__raw_read_lock_flags(&(lock)->raw_lock, *(flags))
# define _raw_read_trylock(rwlock)	__raw_read_trylock(&(rwlock)->raw_lock)
# define _raw_read_unlock(rwlock)	__raw_read_unlock(&(rwlock)->raw_lock)
# define _raw_write_lock(rwlock)	__raw_write_lock(&(rwlock)->raw_lock)
# define _raw_write_lock_flags(lock, flags) \
		__raw_write_lock_flags(&(lock)->raw_lock, *(flags))
# define _raw_write_trylock(rwlock)	__raw_write_trylock(&(rwlock)->raw_lock)
# define _raw_write_unlock(rwlock)	__raw_write_unlock(&(rwlock)->raw_lock)
#endif

#define read_can_lock(rwlock)		__raw_read_can_lock(&(rwlock)->raw_lock)
#define write_can_lock(rwlock)		__raw_write_can_lock(&(rwlock)->raw_lock)

/*
 * Define the various spin_lock and rw_lock methods.  Note we define these
 * regardless of whether CONFIG_SMP or CONFIG_PREEMPT are set. The various
 * methods are defined as nops in the case they are not required.
 */
#define spin_trylock(lock)		__cond_lock(lock, _spin_trylock(lock))
#define read_trylock(lock)		__cond_lock(lock, _read_trylock(lock))
#define write_trylock(lock)		__cond_lock(lock, _write_trylock(lock))

#define spin_lock(lock)			_spin_lock(lock)

#ifdef CONFIG_DEBUG_LOCK_ALLOC
# define spin_lock_nested(lock, subclass) _spin_lock_nested(lock, subclass)
# define spin_lock_nest_lock(lock, nest_lock)				\
	 do {								\
		 typecheck(struct lockdep_map *, &(nest_lock)->dep_map);\
		 _spin_lock_nest_lock(lock, &(nest_lock)->dep_map);	\
	 } while (0)
#else
# define spin_lock_nested(lock, subclass) _spin_lock(lock)
# define spin_lock_nest_lock(lock, nest_lock) _spin_lock(lock)
#endif

#define write_lock(lock)		_write_lock(lock)
#define read_lock(lock)			_read_lock(lock)

#if defined(CONFIG_SMP) || defined(CONFIG_DEBUG_SPINLOCK)

#define spin_lock_irqsave(lock, flags)			\
	do {						\
		typecheck(unsigned long, flags);	\
		flags = _spin_lock_irqsave(lock);	\
	} while (0)
#define read_lock_irqsave(lock, flags)			\
	do {						\
		typecheck(unsigned long, flags);	\
		flags = _read_lock_irqsave(lock);	\
	} while (0)
#define write_lock_irqsave(lock, flags)			\
	do {						\
		typecheck(unsigned long, flags);	\
		flags = _write_lock_irqsave(lock);	\
	} while (0)

#ifdef CONFIG_DEBUG_LOCK_ALLOC
#define spin_lock_irqsave_nested(lock, flags, subclass)			\
	do {								\
		typecheck(unsigned long, flags);			\
		flags = _spin_lock_irqsave_nested(lock, subclass);	\
	} while (0)
#else
#define spin_lock_irqsave_nested(lock, flags, subclass)			\
	do {								\
		typecheck(unsigned long, flags);			\
		flags = _spin_lock_irqsave(lock);			\
	} while (0)
#endif

#else

#define spin_lock_irqsave(lock, flags)			\
	do {						\
		typecheck(unsigned long, flags);	\
		_spin_lock_irqsave(lock, flags);	\
	} while (0)
#define read_lock_irqsave(lock, flags)			\
	do {						\
		typecheck(unsigned long, flags);	\
		_read_lock_irqsave(lock, flags);	\
	} while (0)
#define write_lock_irqsave(lock, flags)			\
	do {						\
		typecheck(unsigned long, flags);	\
		_write_lock_irqsave(lock, flags);	\
	} while (0)
#define spin_lock_irqsave_nested(lock, flags, subclass)	\
	spin_lock_irqsave(lock, flags)

#endif

#define spin_lock_irq(lock)		_spin_lock_irq(lock)
#define spin_lock_bh(lock)		_spin_lock_bh(lock)
#define read_lock_irq(lock)		_read_lock_irq(lock)
#define read_lock_bh(lock)		_read_lock_bh(lock)
#define write_lock_irq(lock)		_write_lock_irq(lock)
#define write_lock_bh(lock)		_write_lock_bh(lock)
#define spin_unlock(lock)		_spin_unlock(lock)
#define read_unlock(lock)		_read_unlock(lock)
#define write_unlock(lock)		_write_unlock(lock)
#define spin_unlock_irq(lock)		_spin_unlock_irq(lock)
#define read_unlock_irq(lock)		_read_unlock_irq(lock)
#define write_unlock_irq(lock)		_write_unlock_irq(lock)

#define spin_unlock_irqrestore(lock, flags)		\
	do {						\
		typecheck(unsigned long, flags);	\
		_spin_unlock_irqrestore(lock, flags);	\
	} while (0)
#define spin_unlock_bh(lock)		_spin_unlock_bh(lock)

#define read_unlock_irqrestore(lock, flags)		\
	do {						\
		typecheck(unsigned long, flags);	\
		_read_unlock_irqrestore(lock, flags);	\
	} while (0)
#define read_unlock_bh(lock)		_read_unlock_bh(lock)

#define write_unlock_irqrestore(lock, flags)		\
	do {						\
		typecheck(unsigned long, flags);	\
		_write_unlock_irqrestore(lock, flags);	\
	} while (0)
#define write_unlock_bh(lock)		_write_unlock_bh(lock)

#define spin_trylock_bh(lock)	__cond_lock(lock, _spin_trylock_bh(lock))

#define spin_trylock_irq(lock) \
({ \
	local_irq_disable(); \
	spin_trylock(lock) ? \
	1 : ({ local_irq_enable(); 0;  }); \
})

#define spin_trylock_irqsave(lock, flags) \
({ \
	local_irq_save(flags); \
	spin_trylock(lock) ? \
	1 : ({ local_irq_restore(flags); 0; }); \
})

#define write_trylock_irqsave(lock, flags) \
({ \
	local_irq_save(flags); \
	write_trylock(lock) ? \
	1 : ({ local_irq_restore(flags); 0; }); \
})

/*
 * Pull the atomic_t declaration:
 * (asm-mips/atomic.h needs above definitions)
 */
#include 
/**
 * atomic_dec_and_lock - lock on reaching reference count zero
 * @atomic: the atomic counter
 * @lock: the spinlock in question
 *
 * Decrements @atomic by 1.  If the result is 0, returns true and locks
 * @lock.  Returns false for all other cases.
 */
extern int _atomic_dec_and_lock(atomic_t *atomic, spinlock_t *lock);
#define atomic_dec_and_lock(atomic, lock) \
		__cond_lock(lock, _atomic_dec_and_lock(atomic, lock))

/**
 * spin_can_lock - would spin_trylock() succeed?
 * @lock: the spinlock in question.
 */
#define spin_can_lock(lock)	(!spin_is_locked(lock))

/*
 * Pull the _spin_*()/_read_*()/_write_*() functions/declarations:
 */
#if defined(CONFIG_SMP) || defined(CONFIG_DEBUG_SPINLOCK)
# include 
#else
# include 
#endif

#endif /* __LINUX_SPINLOCK_H */

<spinlock_types.h>头文件

#ifndef __LINUX_SPINLOCK_TYPES_H
#define __LINUX_SPINLOCK_TYPES_H

/*
 * include/linux/spinlock_types.h - generic spinlock type definitions
 *                                  and initializers
 *
 * portions Copyright 2005, Red Hat, Inc., Ingo Molnar
 * Released under the General Public License (GPL).
 */

#if defined(CONFIG_SMP)
# include 
#else
# include 
#endif

#include 

typedef struct {
    raw_spinlock_t raw_lock;
#ifdef CONFIG_GENERIC_LOCKBREAK
	unsigned int break_lock;
#endif
#ifdef CONFIG_DEBUG_SPINLOCK
	unsigned int magic, owner_cpu;
	void *owner;
#endif
#ifdef CONFIG_DEBUG_LOCK_ALLOC
	struct lockdep_map dep_map;
#endif
} spinlock_t;

#define SPINLOCK_MAGIC		0xdead4ead

typedef struct {
	raw_rwlock_t raw_lock;
#ifdef CONFIG_GENERIC_LOCKBREAK
	unsigned int break_lock;
#endif
#ifdef CONFIG_DEBUG_SPINLOCK
	unsigned int magic, owner_cpu;
	void *owner;
#endif
#ifdef CONFIG_DEBUG_LOCK_ALLOC
	struct lockdep_map dep_map;
#endif
} rwlock_t;

#define RWLOCK_MAGIC		0xdeaf1eed

#define SPINLOCK_OWNER_INIT	((void *)-1L)

#ifdef CONFIG_DEBUG_LOCK_ALLOC
# define SPIN_DEP_MAP_INIT(lockname)	.dep_map = { .name = #lockname }
#else
# define SPIN_DEP_MAP_INIT(lockname)
#endif

#ifdef CONFIG_DEBUG_LOCK_ALLOC
# define RW_DEP_MAP_INIT(lockname)	.dep_map = { .name = #lockname }
#else
# define RW_DEP_MAP_INIT(lockname)
#endif

#ifdef CONFIG_DEBUG_SPINLOCK
# define __SPIN_LOCK_UNLOCKED(lockname)					\
	(spinlock_t)	{	.raw_lock = __RAW_SPIN_LOCK_UNLOCKED,	\
				.magic = SPINLOCK_MAGIC,		\
				.owner = SPINLOCK_OWNER_INIT,		\
				.owner_cpu = -1,			\
				SPIN_DEP_MAP_INIT(lockname) }
#define __RW_LOCK_UNLOCKED(lockname)					\
	(rwlock_t)	{	.raw_lock = __RAW_RW_LOCK_UNLOCKED,	\
				.magic = RWLOCK_MAGIC,			\
				.owner = SPINLOCK_OWNER_INIT,		\
				.owner_cpu = -1,			\
				RW_DEP_MAP_INIT(lockname) }
#else
# define __SPIN_LOCK_UNLOCKED(lockname) \
	(spinlock_t)	{	.raw_lock = __RAW_SPIN_LOCK_UNLOCKED,	\
				SPIN_DEP_MAP_INIT(lockname) }
#define __RW_LOCK_UNLOCKED(lockname) \
	(rwlock_t)	{	.raw_lock = __RAW_RW_LOCK_UNLOCKED,	\
				RW_DEP_MAP_INIT(lockname) }
#endif

/*
 * SPIN_LOCK_UNLOCKED and RW_LOCK_UNLOCKED defeat lockdep state tracking and
 * are hence deprecated.
 * Please use DEFINE_SPINLOCK()/DEFINE_RWLOCK() or
 * __SPIN_LOCK_UNLOCKED()/__RW_LOCK_UNLOCKED() as appropriate.
 */
#define SPIN_LOCK_UNLOCKED	__SPIN_LOCK_UNLOCKED(old_style_spin_init)
#define RW_LOCK_UNLOCKED	__RW_LOCK_UNLOCKED(old_style_rw_init)

#define DEFINE_SPINLOCK(x)	spinlock_t x = __SPIN_LOCK_UNLOCKED(x)
#define DEFINE_RWLOCK(x)	rwlock_t x = __RW_LOCK_UNLOCKED(x)

#endif /* __LINUX_SPINLOCK_TYPES_H */

seqlock

seqlock适用于读很多，写很少的场合。但保护的对象不能有指针变量。

1)定义和初始化

seqlock_t my_seqlock = SEQLOCK_UNLOCKED;

或

seqlock_init(x)

2）读

do {

seq = read_seqbegin(&my_seqlock);

/*处理任务*/

} while read_seqretry(&my_seqlock, seq);

read_seqbegin(lock)

read_seqbegin_irqsave(lock, flags)

read_seqretry_irqrestore(lock, iv, flags)

3)写

void write_seqlock(seqlock_t *sl)

void write_sequnlock(seqlock_t *sl)

int write_tryseqlock(seqlock_t *sl)

#define write_seqlock_irqsave(lock, flags)               \
   do { local_irq_save(flags); write_seqlock(lock); } while (0)
#define write_seqlock_irq(lock)                       \
   do { local_irq_disable();   write_seqlock(lock); } while (0)
#define write_seqlock_bh(lock)                       \
        do { local_bh_disable();    write_seqlock(lock); } while (0)

#define write_sequnlock_irqrestore(lock, flags)               \
   do { write_sequnlock(lock); local_irq_restore(flags); } while(0)
#define write_sequnlock_irq(lock)                   \
   do { write_sequnlock(lock); local_irq_enable(); } while(0)
#define write_sequnlock_bh(lock)                   \
   do { write_sequnlock(lock); local_bh_enable(); } while(0)

#define read_seqbegin_irqsave(lock, flags)               \
   ({ local_irq_save(flags);   read_seqbegin(lock); })

#define read_seqretry_irqrestore(lock, iv, flags)           \

<linux/seqlock.h>头文件内容

#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.  Readers never
 * block but they may have to retry if a writer is in
 * progress. Writers do not wait for readers. 
 *
 * This is not as cache friendly as brlock. Also, this will not work
 * for data that contains pointers, because any writer could
 * invalidate a pointer that a reader was following.
 *
 * Expected 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 
#include 

typedef struct {
	unsigned sequence;
	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) \
		 { 0, __SPIN_LOCK_UNLOCKED(lockname) }

#define SEQLOCK_UNLOCKED \
		 __SEQLOCK_UNLOCKED(old_style_seqlock_init)

#define seqlock_init(x)					\
	do {						\
		(x)->sequence = 0;			\
		spin_lock_init(&(x)->lock);		\
	} while (0)

#define DEFINE_SEQLOCK(x) \
		seqlock_t x = __SEQLOCK_UNLOCKED(x)

/* 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);
	++sl->sequence;
	smp_wmb();
}

static inline void write_sequnlock(seqlock_t *sl)
{
	smp_wmb();
	sl->sequence++;
	spin_unlock(&sl->lock);
}

static inline int write_tryseqlock(seqlock_t *sl)
{
	int ret = spin_trylock(&sl->lock);

	if (ret) {
		++sl->sequence;
		smp_wmb();
	}
	return ret;
}

/* Start of read calculation -- fetch last complete writer token */
static __always_inline unsigned read_seqbegin(const seqlock_t *sl)
{
	unsigned ret;

repeat:
	ret = ACCESS_ONCE(sl->sequence);
	if (unlikely(ret & 1)) {
		cpu_relax();
		goto repeat;
	}
	smp_rmb();

	return ret;
}

/*
 * Test if reader processed invalid data.
 *
 * If sequence value changed then writer changed data while in section.
 */
static __always_inline int read_seqretry(const seqlock_t *sl, unsigned start)
{
	smp_rmb();

	return unlikely(sl->sequence != start);
}


/*
 * 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;
} seqcount_t;

#define SEQCNT_ZERO { 0 }
#define seqcount_init(x)	do { *(x) = (seqcount_t) SEQCNT_ZERO; } while (0)

/**
 * __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 = s->sequence;
	if (unlikely(ret & 1)) {
		cpu_relax();
		goto repeat;
	}
	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)
{
	unsigned ret = __read_seqcount_begin(s);
	smp_rmb();
	return ret;
}

/**
 * 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);
}


/*
 * Sequence counter only version assumes that callers are using their
 * own mutexing.
 */
static inline void write_seqcount_begin(seqcount_t *s)
{
	s->sequence++;
	smp_wmb();
}

static inline void write_seqcount_end(seqcount_t *s)
{
	smp_wmb();
	s->sequence++;
}

/**
 * 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;
}

/*
 * Possible sw/hw IRQ protected versions of the interfaces.
 */
#define write_seqlock_irqsave(lock, flags)				\
	do { local_irq_save(flags); write_seqlock(lock); } while (0)
#define write_seqlock_irq(lock)						\
	do { local_irq_disable();   write_seqlock(lock); } while (0)
#define write_seqlock_bh(lock)						\
        do { local_bh_disable();    write_seqlock(lock); } while (0)

#define write_sequnlock_irqrestore(lock, flags)				\
	do { write_sequnlock(lock); local_irq_restore(flags); } while(0)
#define write_sequnlock_irq(lock)					\
	do { write_sequnlock(lock); local_irq_enable(); } while(0)
#define write_sequnlock_bh(lock)					\
	do { write_sequnlock(lock); local_bh_enable(); } while(0)

#define read_seqbegin_irqsave(lock, flags)				\
	({ local_irq_save(flags);   read_seqbegin(lock); })

#define read_seqretry_irqrestore(lock, iv, flags)			\
	({								\
		int ret = read_seqretry(lock, iv);			\
		local_irq_restore(flags);				\
		ret;							\
	})

#endif /* __LINUX_SEQLOCK_H */

避免使用锁的方法：

1）循环缓冲区：邮箱机制/命名管道/circular buffer

2）atomic_t数据类型以及相关操作函数<asm/atomic.h>