1 files changed, 100 insertions, 0 deletions
diff --git a/libcr_ipc/rwmutex.c b/libcr_ipc/rwmutex.c
new file mode 100644
index 0000000..4c5da81
--- /dev/null
+++ b/libcr_ipc/rwmutex.c
@@ -0,0 +1,100 @@
+/* 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;
+		}
+	}
+}