Linux内核同步，进程，线程同步各种方法
包括我自己在内，很多人对内核，进程，线程同步都不是很清楚，下面稍微总结一下：
内核同步：
主要是防止多核处理器同时访问修改某段代码，或者在对设备驱动程序进行临界区保护。主要有一下几种方式：
1. Mutex（互斥）
头文件：
#include <linux/mutex.h>
初始化方法：
DEFINE_MUTEX(name);或者
void mutex_init(struct mutex *lock);
使用方法：
void mutex_lock (struct mutex *lock);
Tries to lock the mutex, sleeps otherwise.
Caution: can't be interrupted, resulting in processes you cannot kill!
int mutex_lock_interruptible (struct mutex *lock);
Same, but can be interrupted. If interrupted, returns a non zero value and doesn't hold the lock. Test the return value!!!
int mutex_trylock (struct mutex *lock);
Never waits. Returns a non zero value if the mutex is not available.int mutex_is_locked(struct mutex *lock);Just tells whether the mutex is locked or not.
void mutex_unlock (struct mutex *lock);
Releases the lock. Make sure you do it as quickly as possible!
2. Reader/writer semphopres 读写信号量
Allow shared access by unlimited readers, or by only 1 writer. Writers get priority.
允许有限数量的读访问，但是只能有一个写访问。
void init_rwsem (struct rw_semaphore *sem);
void down_read (struct rw_semaphore *sem);
int down_read_trylock (struct rw_semaphore *sem);
int up_read (struct rw_semaphore *sem);
void down_write (struct rw_semaphore *sem);
int down_write_trylock (struct rw_semaphore *sem);
int up_write (struct rw_semaphore *sem);
Well suited for rare writes, holding the semaphore briefly. Otherwise, readers get starved, waiting too long for the semaphore to be released.
3. Spinlocks 自旋锁
初始化：
Static
spinlock_t my_lock = SPIN_LOCK_UNLOCKED;
Dynamic
void spin_lock_init (spinlock_t *lock);
使用：
void spin_[un]lock (spinlock_t *lock);
Doesn't disable interrupts. Used for locking in process context (critical sections in which you do not want to sleep).
void spin_lock_irqsave / spin_unlock_irqrestore (spinlock_t *lock, unsigned long flags);
Disables / restores IRQs on the local CPU.
Typically used when the lock can be accessed in both process and interrupt context, to prevent preemption by interrupts
进程同步/通信
1. Semaphore 信号量
简单过程：
semaphore sv = 1;
loop forever {
P(sv);
critical code section;
V(sv);
noncritical code section;
}
头文件以及函数：
#include <sys/sem.h>
int semctl(int sem_id, int sem_num, int command, ...);
int semget(key_t key, int num_sems, int sem_flags);
int semop(int sem_id, struct sembuf *sem_ops, size_t num_sem_ops);
2.Share Memory 共享内存
头文件以及函数
#include <sys/shm.h>
void *shmat(int shm_id, const void *shm_addr, int shmflg);
int shmctl(int shm_id, int cmd, struct shmid_ds *buf);
int shmdt(const void *shm_addr);
int shmget(key_t key, size_t size, int shmflg);
3.Message Queues 消息队列
头文件以及函数
#include <sys/msg.h>
int msgctl(int msqid, int cmd, struct msqid_ds *buf);
int msgget(key_t key, int msgflg);
int msgrcv(int msqid, void *msg_ptr, size_t msg_sz, long int msgtype, int msgflg);
int msgsnd(int msqid, const void *msg_ptr, size_t msg_sz, int msgflg);
线程同步
1. semophore 信号量
简单用法：
#include <semaphore.h>
sem_t bin_sem;
res = sem_init(&bin_sem, 0, 0);
sem_wait(&bin_sem);
sem_post(&bin_sem);
sem_destroy(&bin_sem);
2. Mutex 互斥
头文件以及函数
#include <pthread.h>
int pthread_mutex_init(pthread_mutex_t *mutex, const pthread_mutexattr_t *mutexattr);
int pthread_mutex_lock(pthread_mutex_t *mutex));
int pthread_mutex_unlock(pthread_mutex_t *mutex);
int pthread_mutex_destroy(pthread_mutex_t *mutex);
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