guard

Posted 2020-12-16 janly

#include "1.h"

#include "pthread.h"


static pthread_mutex_t __guard_mutex;
static pthread_once_t __once_control = PTHREAD_ONCE_INIT;

static void makeRecusiveMutex() // 将 __guard_mutex 初始化为递归锁
{
    pthread_mutexattr_t recursiveMutexAttr;
    pthread_mutexattr_init(&recursiveMutexAttr);
    pthread_mutexattr_settype(&recursiveMutexAttr, PTHREAD_MUTEX_RECURSIVE);
    pthread_mutex_init(&__guard_mutex, &recursiveMutexAttr);
}

//__attribute__((noinline))
static pthread_mutex_t* guard_mutex()
{
    pthread_once(&__once_control, &makeRecusiveMutex); // 一次性初始化 __guard_mutex
    return &__guard_mutex;
}

// helper functions for getting/setting flags in guard_object
static bool initializerHasRun(uint64_t* guard_object)
{
    // 取最低字节作为是否已初始化的标志
    return ( *((uint8_t*)guard_object) != 0 );
}

static void setInitializerHasRun(uint64_t* guard_object)
{
    *((uint8_t*)guard_object)  = 1;
}

static bool inUse(uint64_t* guard_object)
{
    // 取次低字节作为 guard_object 是否正在被当前线程使用的标志
    return ( ((uint8_t*)guard_object)[1] != 0 );
}

static void setInUse(uint64_t* guard_object)
{
    ((uint8_t*)guard_object)[1] = 1;
}

static void setNotInUse(uint64_t* guard_object)
{
    ((uint8_t*)guard_object)[1] = 0;
}


//
// Returns 1 if the caller needs to run the initializer and then either
// call __cxa_guard_release() or __cxa_guard_abort().  If zero is returned,
// then the initializer has already been run.  This function blocks
// if another thread is currently running the initializer.  This function
// aborts if called again on the same guard object without an intervening
// call to __cxa_guard_release() or __cxa_guard_abort().
//
int __cxa_guard_acquire(uint64_t* guard_object)
{
    // Double check that the initializer has not already been run
    if ( initializerHasRun(guard_object) ) // 如果对象已被初始化
        return 0;

    // We now need to acquire a lock that allows only one thread
    // to run the initializer.  If a different thread calls
    // __cxa_guard_acquire() with the same guard object, we want
    // that thread to block until this thread is done running the
    // initializer and calls __cxa_guard_release().  But if the same
    // thread calls __cxa_guard_acquire() with the same guard object,
    // we want to abort.
    // To implement this we have one global pthread recursive mutex
    // shared by all guard objects, but only one at a time.

    int result = ::pthread_mutex_lock(guard_mutex());
    if ( result != 0 ) {
//        abort_message("__cxa_guard_acquire(): pthread_mutex_lock "
//                      "failed with %d ", result);
    }
    // At this point all other threads will block in __cxa_guard_acquire()

    // Check if another thread has completed initializer run
    if ( initializerHasRun(guard_object) ) { // 再次判断， 对象是否已被其他线程初始化
        int result = ::pthread_mutex_unlock(guard_mutex());
        if ( result != 0 ) {
//            abort_message("__cxa_guard_acquire(): pthread_mutex_unlock "
//                          "failed with %d ", result);
        }
        return 0;
    }

    // The pthread mutex is recursive to allow other lazy initialized
    // function locals to be evaluated during evaluation of this one.
    // But if the same thread can call __cxa_guard_acquire() on the
    // *same* guard object again, we call abort();
    if ( inUse(guard_object) ) { // 防止同一线程对对象多次初始化
//        abort_message("__cxa_guard_acquire(): initializer for function "
//                      "local static variable called enclosing function ");
    }

    // mark this guard object as being in use
    setInUse(guard_object);

    // return non-zero to tell caller to run initializer
    return 1;
}



//
// Sets the first byte of the guard_object to a non-zero value.
// Releases any locks acquired by __cxa_guard_acquire().
//
void __cxa_guard_release(uint64_t* guard_object)
{
    // first mark initalizer as having been run, so
    // other threads won‘t try to re-run it.
    setInitializerHasRun(guard_object);

    // release global mutex
    int result = ::pthread_mutex_unlock(guard_mutex());
    if ( result != 0 ) {
//        abort_message("__cxa_guard_acquire(): pthread_mutex_unlock "
//                      "failed with %d ", result);
    }
}



//
// Releases any locks acquired by __cxa_guard_acquire().
//
//void __cxxabiv1::__cxa_guard_abort(uint64_t* guard_object) // 初始化异常时被调用
//{
//    int result = ::pthread_mutex_unlock(guard_mutex());
//    if ( result != 0 ) {
//        abort_message("__cxa_guard_abort(): pthread_mutex_unlock "
//                      "failed with %d ", result);
//    }
//    // now reset state, so possible to try to initialize again
//    setNotInUse(guard_object);
//}