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/* libcr_ipc/rwmutex.c - Simple read/write mutexes for libcr
*
* Copyright (C) 2024-2025 Luke T. Shumaker <lukeshu@lukeshu.com>
* SPDX-License-Identifier: AGPL-3.0-or-later
*/
#include <libcr/coroutine.h> /* for cid_t, cr_* */
#define IMPLEMENTATION_FOR_LIBCR_IPC_RWMUTEX_H YES
#include <libcr_ipc/rwmutex.h>
struct cr_rwmutex_waiter {
lm_sll_node;
bool is_reader;
cid_t cid;
};
void cr_rwmutex_lock(cr_rwmutex_t *mu) {
assert(mu);
cr_assert_in_coroutine();
struct cr_rwmutex_waiter self = {
.is_reader = false,
.cid = cr_getcid(),
};
lm_sll_push_to_rear(&mu->waiters, &self);
if (mu->waiters.front != &self.lm_sll_node || mu->locked)
cr_pause_and_yield();
assert(mu->waiters.front == &self.lm_sll_node);
/* We now hold the lock (and are mu->waiters.front). */
lm_sll_pop_from_front(&mu->waiters);
assert(mu->nreaders == 0);
mu->locked = true;
mu->unpausing = false;
}
void cr_rwmutex_rlock(cr_rwmutex_t *mu) {
assert(mu);
cr_assert_in_coroutine();
struct cr_rwmutex_waiter self = {
.is_reader = true,
.cid = cr_getcid(),
};
lm_sll_push_to_rear(&mu->waiters, &self);
if (mu->waiters.front != &self.lm_sll_node || (mu->locked && mu->nreaders == 0))
cr_pause_and_yield();
assert(mu->waiters.front == &self.lm_sll_node);
/* We now hold the lock (and are mu->waiters.front). */
lm_sll_pop_from_front(&mu->waiters);
mu->nreaders++;
mu->locked = true;
struct cr_rwmutex_waiter *waiter =
lm_sll_node_cast(struct cr_rwmutex_waiter, mu->waiters.front);
if (waiter && waiter->is_reader) {
assert(mu->unpausing);
cr_unpause(waiter->cid);
} else {
mu->unpausing = false;
}
}
void cr_rwmutex_unlock(cr_rwmutex_t *mu) {
assert(mu);
cr_assert_in_coroutine();
assert(mu->locked);
assert(mu->nreaders == 0);
assert(!mu->unpausing);
if (mu->waiters.front) {
struct cr_rwmutex_waiter *waiter =
lm_sll_node_cast(struct cr_rwmutex_waiter, mu->waiters.front);
mu->unpausing = true;
cr_unpause(waiter->cid);
} else {
mu->locked = false;
}
}
void cr_rwmutex_runlock(cr_rwmutex_t *mu) {
assert(mu);
cr_assert_in_coroutine();
assert(mu->locked);
assert(mu->nreaders > 0);
mu->nreaders--;
if (mu->nreaders == 0 && !mu->unpausing) {
if (mu->waiters.front) {
struct cr_rwmutex_waiter *waiter =
lm_sll_node_cast(struct cr_rwmutex_waiter, mu->waiters.front);
assert(!waiter->is_reader);
mu->unpausing = true;
cr_unpause(waiter->cid);
} else {
mu->locked = false;
}
}
}
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