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rwqueue.c
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rwqueue.c
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#include "rwqueue.h"
void rw_queue_init(rw_queue_t *q) {
Sem_init(&q->mutex, 0, 1);
q->reading_count = 0;
q->writing_count = 0;
q->head = NULL;
q->tail = NULL;
}
/*
* Helper routine to insert element in queue
* Assume mutually exclusive access
*/
static void enqueue(rw_queue_t *q, rw_token_t *t) {
if (q->tail == NULL) {
q->tail = q->head = t;
} else {
q->tail->next = t;
q->tail = t;
}
t->next = NULL;
}
/*
* Helper routine to retrieve element at head of queue
*/
static rw_token_t *peekqueue(rw_queue_t *q) {
return q->head;
}
/*
* Helper routine to remove first element from queue
*/
static void dequeue(rw_queue_t *q) {
rw_token_t *t = q->head;
if (t == NULL)
return;
q->head = t->next;
if (q->head == NULL)
q->tail = NULL;
}
void rw_queue_request_read(rw_queue_t *q, rw_token_t *t) {
bool block = false;
P(&q->mutex);
if (q->head == NULL && q->writing_count == 0)
/* Can bypass queue */
q->reading_count++;
else {
/* Must add to queue */
t->is_reader = true;
enqueue(q, t);
Sem_init(&t->enable, 0, 0);
block = true;
}
V(&q->mutex);
if (block)
P(&t->enable);
}
void rw_queue_request_write(rw_queue_t *q, rw_token_t *t) {
bool block = false;
P(&q->mutex);
if (q->head == NULL && q->writing_count == 0 && q->reading_count == 0)
/* Can bypass queue */
q->writing_count++;
else {
t->is_reader = false;
enqueue(q, t);
Sem_init(&t->enable, 0, 0);
block = true;
}
V(&q->mutex);
if (block)
P(&t->enable);
}
void rw_queue_release(rw_queue_t *q) {
rw_token_t *t;
P(&q->mutex);
#ifdef DEBUG
printf("Start of release: ");
rw_queue_status(q);
#endif
if (q->writing_count > 0)
q->writing_count--;
else
q->reading_count--;
t = peekqueue(q);
if (!t) {
V(&q->mutex);
return;
}
bool wanttoread = t->is_reader;
if (!wanttoread && q->reading_count == 0) {
q->writing_count++;
V(&t->enable);
dequeue(q);
} else {
while (wanttoread) {
q->reading_count++;
V(&t->enable);
dequeue(q);
t = peekqueue(q);
wanttoread = t && t->is_reader;
}
}
#ifdef DEBUG
printf("End of release: ");
rw_queue_status(q);
#endif
V(&q->mutex);
}
void rw_queue_status(rw_queue_t *q) {
rw_token_t *t;
printf("Readers = %d, Writers = %d, Queue=[", q->reading_count, q->writing_count);
for (t = q->head; t; t = t->next)
putchar(t->is_reader ? 'r' : 'w');
printf("]\n");
}