一个很容混淆的地方是，误以为用户程序锁API的实现和接口，最终也会调用内核相关的锁操作提供的API。

应用程序锁API接口

主要的API有：
pthread_mutex_lock;

相关说明如下：

NAME
pthread_mutex_lock -- lock a mutex
SYNOPSIS
#include <pthread.h>
int
pthread_mutex_lock(pthread_mutex_t *mutex);
DESCRIPTION
The pthread_mutex_lock() function locks mutex.
If the mutex is already locked, the calling thread will block until the mutex becomes
available.
RETURN VALUES
If successful, pthread_mutex_lock() will return zero, otherwise an error number will be returned to indicate the error.
ERRORS
The pthread_mutex_lock() function will fail if:
[EINVAL]
The value specified by mutex is invalid.
[EDEADLK]
A deadlock would occur if the thread blocked waiting for mutex.

pthread_mutex_unlock;

相关说明如下：

PTHREAD_MUTEX_UNLOCK(3)
BSD Library Functions Manual
PTHREAD_MUTEX_UNLOCK(3)
NAME
pthread_mutex_unlock -- unlock a mutex
SYNOPSIS
#include <pthread.h>
int
pthread_mutex_unlock(pthread_mutex_t *mutex);
DESCRIPTION
If the current thread holds the lock on mutex, then the pthread_mutex_unlock() function unlocks mutex.
Calling pthread_mutex_unlock() with a mutex that the calling thread does not hold will result in undefined behavior.
RETURN VALUES
If successful, pthread_mutex_unlock() will return zero, otherwise an error number will be returned to indicate the error.
ERRORS
The pthread_mutex_unlock() function will fail if:
[EINVAL]
The value specified by mutex is invalid.
[EPERM]
The current thread does not hold a lock on mutex.
SEE ALSO
pthread_mutex_destroy(3), pthread_mutex_init(3), pthread_mutex_lock(3), pthread_mutex_trylock(3)
STANDARDS
The pthread_mutex_unlock() function conforms to ISO/IEC 9945-1:1996 (``POSIX.1'').

用户态锁的实现

相关数据结构

用户锁相关的操作都涉及到同一个数据结构mutex，mutex 数据结构在中定义，主要的成员如下：

typedef union
{
struct __pthread_mutex_s
{
int __lock;
unsigned int __count;
int __owner;
#ifdef __x86_64__
unsigned int __nusers;
#endif
/* KIND must stay at this position in the structure to maintain
binary compatibility with static initializers.
*/
int __kind;
#ifdef __x86_64__
short __spins;
short __elision;
__pthread_list_t __list;
# define __PTHREAD_MUTEX_HAVE_PREV
1
/* Mutex __spins initializer used by PTHREAD_MUTEX_INITIALIZER.
*/
# define __PTHREAD_SPINS
0, 0
#else
#endif
} __data;
char __size[__SIZEOF_PTHREAD_MUTEX_T];
long int __align;
} pthread_mutex_t
————————————————

主要的流程

参考链接

介绍用户态中pthead_mutex_lock() 的具体实现
https://blog.csdn.net/hzhzh007/article/details/6535437 ： 果如所料，最底层也是基于用户态的lock cmxhg 指令来实现的；
https://blog.csdn.net/luoyuyou/article/details/73498640