lib: Add semaphores

include/zinnia/semaphore.h

@@ -0,0 +1,15 @@
+// SPDX-License-Identifier: ISC
+
+#pragma once
+
+#include <stdint.h>
+
+typedef struct {
+    _Atomic(uint32_t) count;
+} sem_t;
+
+void sem_init(sem_t* sem, uint32_t count);
+bool sem_trywait(sem_t* sem);
+bool sem_timedwait(sem_t* sem, const uint64_t* deadline_ns);
+void sem_wait(sem_t* sem);
+void sem_post(sem_t* sem);

lib/meson.build

@@ -23,6 +23,7 @@ kernel_sources += files(
     'ringbuf.c',
     'runtime.c',
     'rwlock.c',
+    'semaphore.c',
     'spin.c',
     'stdlib.c',
     'string.c',

lib/semaphore.c

@@ -0,0 +1,40 @@
+// SPDX-License-Identifier: ISC
+
+#include <zinnia/atomic.h>
+#include <zinnia/park.h>
+#include <zinnia/semaphore.h>
+
+void sem_init(sem_t* sem, uint32_t count) {
+    atomic_store(&sem->count, count, ATOMIC_RELAXED);
+}
+
+bool sem_trywait(sem_t* sem) {
+    uint32_t count = atomic_load(&sem->count, ATOMIC_RELAXED);
+    while (count) {
+        if (atomic_compare_exchange_weak(&sem->count, &count, count - 1, ATOMIC_ACQUIRE, ATOMIC_RELAXED))
+            return true;
+    }
+    return false;
+}
+
+static bool sem_empty(void* ctx) {
+    sem_t* sem = ctx;
+    return atomic_load(&sem->count, ATOMIC_RELAXED) == 0;
+}
+
+bool sem_timedwait(sem_t* sem, const uint64_t* deadline_ns) {
+    while (!sem_trywait(sem)) {
+        if (park_timeout(sem, sem_empty, nullptr, sem, 0, deadline_ns).status == PARK_TIMED_OUT)
+            return sem_trywait(sem);
+    }
+    return true;
+}
+
+void sem_wait(sem_t* sem) {
+    sem_timedwait(sem, nullptr);
+}
+
+void sem_post(sem_t* sem) {
+    atomic_fetch_add(&sem->count, 1, ATOMIC_RELEASE);
+    unpark_one(sem, nullptr, nullptr);
+}