一個很容混淆的地方是,誤以爲用戶程序鎖API的實現和接口,最終也會調用內核相關的鎖操作提供的API。
應用程序鎖API接口
主要的API有:
pthread_mutex_lock;
相關說明如下:
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;
相關說明如下:
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'').
用戶態鎖的實現
相關數據結構
用戶鎖相關的操作都涉及到同一個數據結構mutex,mutex 數據結構在中定義,主要的成員如下:
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
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主要的流程
參考鏈接
介紹用戶態中pthead_mutex_lock() 的具體實現
https://blog.csdn.net/hzhzh007/article/details/6535437 : 果如所料,最底層也是基於用戶態的lock cmxhg 指令來實現的;
https://blog.csdn.net/luoyuyou/article/details/73498640