Hello ;-) I'm Boqun Feng

// Intrusive part of a queue element
struct queue_entry {
    struct queue_entry *next;
}

typedef struct queue_entry queue_entry_t;

// A queue
struct queue {
    queue_entry_t *head;
    queue_entry_t *tail;
}


// Assume q != NULL
void init_queue(struct queue *q) {
    q->head = NULL;
    q->tail = NULL;
}

// Assume q != NULL && e != NULL
void enqueue(struct queue *q, struct queue_entry *e) {
    e->next = NULL;
    if (queue == NULL)
        return;
//lock_head T2
// T1
    if (q->head == NULL) {
        q->head = e;
    }
//unlock_head
//T1
//lock_tail
//T2
    if (q->tail != NULL) {
        q->tail->next = e;
    }
    q->tail = e;
//unlock_tail
}

struct queue *q = ..;
init_queue(q);

T1:                    T2:
   enqueue(q, e1)
                       enqueue(q, e2);

// Assume q != NULL
struct queue_entry* dequeue_all(struct queue *q) {
//lock_head
    queue_entry_t *ret = q->head;
    q->head = NULL;
//unlock_head
//lock_tail
    q->tail = NULL;
//unlock_tail
    return ret;
}

Example

struct queue q;

init_queue(&q);
// q is []
enqueue(&q, e1);
//q->head = e1;
// q is [e1]
enqueue(&q, e2);
// p = e1
// head->e1->e2->X
// q is [e1, e2]
// q

struct queue_entry *p = dequeue_all(&q);
// ret = head->e1->e2.
// q->X
//  q is []
// p is [e1, e2], i.e. p == e1, e1->next == e2, and e2->next == NULL