Semaphore 내부 구현 추적 — Counter·Wait List·ISR-Safe Variant

1
#define xSemaphoreCreateBinary() \
2
    xQueueGenericCreate(1, 0, queueQUEUE_TYPE_BINARY_SEMAPHORE)
3

4
#define xSemaphoreTake(sem, t)   xQueueSemaphoreTake(sem, t)
5
#define xSemaphoreGive(sem)      xQueueGenericSend(sem, NULL, 0, queueSEND_TO_BACK)

1
// Binary semaphore
2
SemaphoreHandle_t = Queue {
3
    .uxLength = 1,           // max count
4
    .uxMessagesWaiting = 0,  // current count
5
    .uxItemSize = 0,
6
    /* ... */
7
};
8

9
// Take (P) — counter--
10
BaseType_t xQueueSemaphoreTake(...) {
11
    portENTER_CRITICAL();
12
    if (uxMessagesWaiting > 0) {
13
        uxMessagesWaiting--;
14
        portEXIT_CRITICAL();
15
        return pdPASS;
16
    }
17
    /* Empty — block */
18
    prvAddCurrentTaskToWaitList(&xTasksWaitingToReceive);
19
    portEXIT_CRITICAL();
20
    block_with_timeout(timeout);
21
}
22

23
// Give (V) — counter++
24
BaseType_t xQueueGenericSend(...) {
25
    portENTER_CRITICAL();
26
    if (uxMessagesWaiting < uxLength) {
27
        uxMessagesWaiting++;
28
        /* Wake highest-priority waiter if any */
29
        if (!list_empty(&xTasksWaitingToReceive)) {
30
            TCB_t *winner = listGET_OWNER_OF_HEAD_ENTRY(&xTasksWaitingToReceive);
31
            xTaskRemoveFromEventList(&xTasksWaitingToReceive);
32
            /* now winner is on Ready list */
33
        }
34
        portEXIT_CRITICAL();
35
        return pdPASS;
36
    }
37
    portEXIT_CRITICAL();
38
    return errQUEUE_FULL;  // Counting sem이 max 도달
39
}

1
SemaphoreHandle_t sem = xSemaphoreCreateCounting(10, 0);
2
//                                                ↑   ↑
3
//                                            max   initial

1
List_t xTasksWaitingToReceive;   // sorted by task priority

1
void prvAddCurrentTaskToWaitList(List_t *pxEventList) {
2
    /* Priority value를 sort key로 사용 (역순) */
3
    listSET_LIST_ITEM_VALUE(
4
        &pxCurrentTCB->xEventListItem,
5
        configMAX_PRIORITIES - pxCurrentTCB->uxPriority   /* invert */
6
    );
7
    vListInsert(pxEventList, &pxCurrentTCB->xEventListItem);
8
}

1
BaseType_t xSemaphoreGiveFromISR(SemaphoreHandle_t sem, BaseType_t *pxHigherPriorityTaskWoken) {
2
    portENTER_CRITICAL_FROM_ISR();   // BASEPRI save·set
3

4
    if (uxMessagesWaiting < uxLength) {
5
        uxMessagesWaiting++;
6

7
        if (!list_empty(&xTasksWaitingToReceive)) {
8
            TCB_t *waiter = peek_head(&xTasksWaitingToReceive);
9
            if (waiter->uxPriority > pxCurrentTCB->uxPriority) {
10
                *pxHigherPriorityTaskWoken = pdTRUE;
11
            }
12
            xTaskRemoveFromEventList(&xTasksWaitingToReceive);
13
        }
14
        portEXIT_CRITICAL_FROM_ISR(savedStatus);
15
        return pdPASS;
16
    }
17
    portEXIT_CRITICAL_FROM_ISR(savedStatus);
18
    return errQUEUE_FULL;
19
}

1
BaseType_t xSemaphoreTake(SemaphoreHandle_t sem, TickType_t xTicksToWait) {
2
    /* ... 위 동일 ... */
3

4
    if (uxMessagesWaiting == 0) {
5
        if (xTicksToWait == 0) return pdFAIL;
6

7
        prvAddCurrentTaskToWaitList(&xTasksWaitingToReceive);
8
        vTaskPlaceOnEventList(&xTasksWaitingToReceive, xTicksToWait);
9
        portEXIT_CRITICAL();
10

11
        portYIELD_WITHIN_API();   // → Scheduler 호출
12

13
        /* 깨어남 — wake 원인 확인 */
14
        if (xTaskCheckForTimeOut(&xTimeOut, &xTicksToWait) != pdFALSE) {
15
            /* Timeout */
16
            uxListRemove(&pxCurrentTCB->xEventListItem);
17
            return pdFAIL;
18
        }
19
        return pdPASS;
20
    }
21
}

1
// 단순 구현 (잘못된)
2
if (!flag) {
3
    wait();        // ← (1)
4
}
5
// ISR이 flag=1로 만들고 signal — wait() 전이라면 lost

1
portENTER_CRITICAL();
2
if (counter == 0) {
3
    add_to_wait_list();
4
    portEXIT_CRITICAL();
5
    block();
6
}

1
sizeof(Queue_t) ≈ 80 byte
2
+ ListItem_t × waiter count
3
+ (Counting sem은 추가 메모리 없음 — 동일 80 byte)

1
struct k_sem {
2
    _wait_q_t wait_q;     // doubly-linked list
3
    uint32_t count;
4
    uint32_t limit;
5
};