v81.01 鸿蒙内核源码分析(读写锁) | 内核如何实现多读单写 原创 精华

typedef struct OsRwlock {//读写锁
    INT32 magic:24;        /**< Magic number | 魔法数字*/
    INT32 rwCount:8;       /**< Times of locking the rwlock, rwCount > 0 when rwkick is read mode, rwCount < 0
                                when the rwlock is write mode, rwCount = 0 when the lock is free. 
                                大于0时为读模式 , 小于0时为写模式 等于0为自由模式
                                rwCount为读锁和写锁的数量,注意它们并不是此消彼长的行为模式*/
    VOID *writeOwner;      /**< The current write thread that is locking the rwlock | 拥有写权限的任务*/
    LOS_DL_LIST readList;  /**< Read waiting list | 等待读操作的任务链表*/
    LOS_DL_LIST writeList; /**< Write waiting list | 等待写操作的任务链表*/
} LosRwlock;
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UINT32 LOS_RwlockInit(LosRwlock *rwlock)
{
    UINT32 intSave;
    if (rwlock == NULL) {
        return LOS_EINVAL;
    }
    SCHEDULER_LOCK(intSave);
    if ((rwlock->magic & RWLOCK_COUNT_MASK) == OS_RWLOCK_MAGIC) {
        SCHEDULER_UNLOCK(intSave);
        return LOS_EPERM;
    }
    rwlock->rwCount = 0;
    rwlock->writeOwner = NULL;
    LOS_ListInit(&(rwlock->readList));
    LOS_ListInit(&(rwlock->writeList));
    rwlock->magic = OS_RWLOCK_MAGIC;
    SCHEDULER_UNLOCK(intSave);
    return LOS_OK;
}
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/// 找到合适的位置 将当前任务插入到 读/写锁链表中
LOS_DL_LIST *OsSchedLockPendFindPos(const LosTaskCB *runTask, LOS_DL_LIST *lockList)
{
    LOS_DL_LIST *node = NULL;
    if (LOS_ListEmpty(lockList)) {
        node = lockList;
    } else {
        LosTaskCB *pendedTask1 = OS_TCB_FROM_PENDLIST(LOS_DL_LIST_FIRST(lockList));//找到首任务
        LosTaskCB *pendedTask2 = OS_TCB_FROM_PENDLIST(LOS_DL_LIST_LAST(lockList)); //找到尾任务
        if (pendedTask1->priority > runTask->priority) { //首任务比当前任务优先级低，所以runTask可以排第一
            node = lockList->pstNext;//下一个，注意priority越大 优先级越低
        } else if (pendedTask2->priority <= runTask->priority) {//尾任务比当前任务优先级高，所以runTask排最后
            node = lockList;
        } else {//两头比较没结果，陷入中间比较
            node = OsSchedLockPendFindPosSub(runTask, lockList);//进入子级循环找
        }
    }
    return node;
}
/// 找到合适的位置 将当前任务插入到 读/写锁链表中
STATIC INLINE LOS_DL_LIST *OsSchedLockPendFindPosSub(const LosTaskCB *runTask, const LOS_DL_LIST *lockList)
{
    LosTaskCB *pendedTask = NULL;
    LOS_DL_LIST *node = NULL;
    LOS_DL_LIST_FOR_EACH_ENTRY(pendedTask, lockList, LosTaskCB, pendList) {//遍历阻塞链表
        if (pendedTask->priority < runTask->priority) {//当前优先级更小,位置不宜,继续
            continue;
        } else if (pendedTask->priority > runTask->priority) {//找到合适位置
            node = &pendedTask->pendList;
            break;
        } else {//找到合适位置
            node = pendedTask->pendList.pstNext;
            break;
        }
    }
    return node;
}
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LOS_RwlockRdLock
    OsRwlockRdUnsafe
        OsRwlockRdPendOp
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STATIC UINT32 OsRwlockRdPendOp(LosTaskCB *runTask, LosRwlock *rwlock, UINT32 timeout)
{
    UINT32 ret;
    if (rwlock->rwCount >= 0) {//第一和第二种情况
        if (OsRwlockPriCompare(runTask, &(rwlock->writeList))) {//读优先级低于写优先级,意思就是必须先写再读
            if (rwlock->rwCount == INT8_MAX) {//读锁任务达到上限
                return LOS_EINVAL;
            }
            rwlock->rwCount++;//拿读锁成功
            return LOS_OK;
        }
    }
    if (!timeout) {
        return LOS_EINVAL;
    }
    if (!OsPreemptableInSched()) {//不可抢占时
        return LOS_EDEADLK;
    }
    /* The current task is not allowed to obtain the write lock when it obtains the read lock. 
    | 当前任务在获得读锁时不允许获得写锁 */
    if ((LosTaskCB *)(rwlock->writeOwner) == runTask) { //拥有写锁任务是否为当前任务
        return LOS_EINVAL;
    }
    /*
     * When the rwlock mode is write mode or the priority of the current read task
     * is lower than the first pended write task, current read task will be pended.
     | 当 rwlock 模式为写模式或当前读任务的优先级低于第一个挂起的写任务时，当前读任务将被挂起。
     反正就是写锁任务优先
     */
    LOS_DL_LIST *node = OsSchedLockPendFindPos(runTask, &(rwlock->readList));//找到要挂入的位置
    //例如现有链表内任务优先级为 0 3 8 9 23 当前为 10 时, 返回的是 9 这个节点 
    ret = OsSchedTaskWait(node, timeout, TRUE);//从尾部插入读锁链表 由此变成了 0 3 8 9 10 23
    if (ret == LOS_ERRNO_TSK_TIMEOUT) {
        return LOS_ETIMEDOUT;
    }
    return ret;
}
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STATIC UINT32 OsRwlockWrPendOp(LosTaskCB *runTask, LosRwlock *rwlock, UINT32 timeout)
{
    UINT32 ret;
    /* When the rwlock is free mode, current write task can obtain this rwlock. 
	| 若该锁当前没有任务持有，或者持有该读模式下的锁的任务和申请该锁的任务为同一个任务，则申请成功，可立即获得写模式下的锁。*/
    if (rwlock->rwCount == 0) {
        rwlock->rwCount = -1;
        rwlock->writeOwner = (VOID *)runTask;//直接给当前进程锁
        return LOS_OK;
    }
    /* Current write task can use one rwlock once again if the rwlock owner is it. 
	| 如果 rwlock 拥有者是当前写入任务，则它可以再次使用该锁。*/
    if ((rwlock->rwCount < 0) && ((LosTaskCB *)(rwlock->writeOwner) == runTask)) {
        if (rwlock->rwCount == INT8_MIN) {
            return LOS_EINVAL;
        }
        rwlock->rwCount--;//注意再次拥有算是两把写锁了.
        return LOS_OK;
    }
    if (!timeout) {
        return LOS_EINVAL;
    }
    if (!OsPreemptableInSched()) {
        return LOS_EDEADLK;
    }
    /*
     * When the rwlock is read mode or other write task obtains this rwlock, current
     * write task will be pended. | 当 rwlock 为读模式或其他写任务获得该 rwlock 时，当前的写任务将被挂起。直到读模式下的锁释放
     */
    LOS_DL_LIST *node =  OsSchedLockPendFindPos(runTask, &(rwlock->writeList));//找到要挂入的位置
    ret = OsSchedTaskWait(node, timeout, TRUE);//从尾部插入写锁链表
    if (ret == LOS_ERRNO_TSK_TIMEOUT) {
        ret = LOS_ETIMEDOUT;
    }
    return ret;
}
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/// 释放锁
STATIC UINT32 OsRwlockPostOp(LosRwlock *rwlock, BOOL *needSched)
{
    UINT32 rwlockMode;
    LosTaskCB *resumedTask = NULL;
    UINT16 pendedWriteTaskPri;

    rwlock->rwCount = 0;
    rwlock->writeOwner = NULL;
    rwlockMode = OsRwlockGetMode(&(rwlock->readList), &(rwlock->writeList));//先获取模式
    if (rwlockMode == RWLOCK_NONE_MODE) {//自由模式则正常返回
        return LOS_OK;
    }
    /* In this case, rwlock will wake the first pended write task. | 在这种情况下，rwlock 将唤醒第一个挂起的写任务。 */
    if ((rwlockMode == RWLOCK_WRITE_MODE) || (rwlockMode == RWLOCK_WRITEFIRST_MODE)) {//如果当前是写模式 (有任务在等写锁涅)
        resumedTask = OS_TCB_FROM_PENDLIST(LOS_DL_LIST_FIRST(&(rwlock->writeList)));//获取任务实体
        rwlock->rwCount = -1;//直接干成-1,注意这里并不是 --
        rwlock->writeOwner = (VOID *)resumedTask;//有锁了则唤醒等锁的任务(写模式)
        OsSchedTaskWake(resumedTask);
        if (needSched != NULL) {
            *needSched = TRUE;
        }
        return LOS_OK;
    }
    /* In this case, rwlock will wake the valid pended read task. 在这种情况下，rwlock 将唤醒有效的挂起读取任务。 */
    if (rwlockMode == RWLOCK_READFIRST_MODE) { //如果是读优先模式
        pendedWriteTaskPri = OS_TCB_FROM_PENDLIST(LOS_DL_LIST_FIRST(&(rwlock->writeList)))->priority;//取出写锁任务的最高优先级
    }
    resumedTask = OS_TCB_FROM_PENDLIST(LOS_DL_LIST_FIRST(&(rwlock->readList)));//获取最高优先级读锁任务
    rwlock->rwCount = 1; //直接干成1,因为是释放操作 
    OsSchedTaskWake(resumedTask);//有锁了则唤醒等锁的任务(读模式)
    while (!LOS_ListEmpty(&(rwlock->readList))) {//遍历读链表,目的是要唤醒其他读模式的任务(优先级得要高于pendedWriteTaskPri才行)
        resumedTask = OS_TCB_FROM_PENDLIST(LOS_DL_LIST_FIRST(&(rwlock->readList)));
        if ((rwlockMode == RWLOCK_READFIRST_MODE) && (resumedTask->priority >= pendedWriteTaskPri)) {//低于写模式的优先级
            break;//跳出循环
        }
        if (rwlock->rwCount == INT8_MAX) {
            return EINVAL;
        }
        rwlock->rwCount++;//读锁任务数量增加
        OsSchedTaskWake(resumedTask);//不断唤醒读锁任务,由此实现了允许多个读操作并发,因为在多核情况下resumedTask很大可能
        //与当前任务并不在同一个核上运行, 此处非常的有意思,点赞! @note_good
    }
    if (needSched != NULL) {
        *needSched = TRUE;
    }
    return LOS_OK;
}
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