互斥鎖是Linux下多線程資源保護的常用手段，但是在時序複雜的情況下，很容易會出現死鎖的情況。
可以通過設置鎖的屬性，避免同一條線程重複上鎖導致死鎖的問題。
通過int pthread_mutexattr_settype(pthread_mutexattr_t *attr, int type)接口設置
一般是以下四種屬性：
PTHREAD_MUTEX_NORMAL
This type of mutex does not detect deadlock. A thread attempting to relock this mutex without first unlocking it will deadlock. Attempting to unlock a mutex locked by a different thread results in undefined behaviour. Attempting to unlock an unlocked mutex results in undefined behaviour.
PTHREAD_MUTEX_ERRORCHECK
This type of mutex provides error checking. A thread attempting to relock this mutex without first unlocking it will return with an error. A thread attempting to unlock a mutex which another thread has locked will return with an error. A thread attempting to unlock an unlocked mutex will return with an error.
PTHREAD_MUTEX_RECURSIVE
A thread attempting to relock this mutex without first unlocking it will succeed in locking the mutex. The relocking deadlock which can occur with mutexes of type PTHREAD_MUTEX_NORMAL cannot occur with this type of mutex. Multiple locks of this mutex require the same number of unlocks to release the mutex before another thread can acquire the mutex. A thread attempting to unlock a mutex which another thread has locked will return with an error. A thread attempting to unlock an unlocked mutex will return with an error.
PTHREAD_MUTEX_DEFAULT
Attempting to recursively lock a mutex of this type results in undefined behaviour. Attempting to unlock a mutex of this type which was not locked by the calling thread results in undefined behaviour. Attempting to unlock a mutex of this type which is not locked results in undefined behaviour. An implementation is allowed to map this mutex to one of the other mutex types.
這裏主要指同一條線程重複上鎖，不同線程上鎖，無論設置什麼屬性，當鎖已經被鎖定後都會互斥阻塞。
使用PTHREAD_MUTEX_RECURSIVE屬性，當同一條線程在沒有解鎖的情況下嘗試再次鎖定會返回成功。
