Add condvars and events via parking lot

lib/event.c

@@ -0,0 +1,51 @@
+// SPDX-License-Identifier: ISC
+
+#include <zinnia/atomic.h>
+#include <zinnia/event.h>
+#include <zinnia/park.h>
+#include <zinnia/percpu.h>
+#include <zinnia/sched.h>
+
+constexpr uint32_t EVENT_SET = (1u << 0);
+constexpr uint32_t EVENT_PARKED = (1u << 1); // At least one task might be parked on the event.
+
+void event_init(struct event* event) {
+    atomic_store(&event->state, 0, ATOMIC_RELAXED);
+}
+
+void event_set(struct event* event) {
+    uint32_t old = atomic_fetch_or(&event->state, EVENT_SET, ATOMIC_RELEASE);
+    if (!(old & EVENT_PARKED))
+        return;
+
+    atomic_fetch_and(&event->state, ~EVENT_PARKED, ATOMIC_RELAXED);
+    unpark_all(event, 0);
+}
+
+void event_reset(struct event* event) {
+    atomic_fetch_and(&event->state, ~EVENT_SET, ATOMIC_RELAXED);
+}
+
+bool event_is_set(struct event* event) {
+    return atomic_load(&event->state, ATOMIC_ACQUIRE) & EVENT_SET;
+}
+
+void event_wait(struct event* event) {
+    uint32_t state = atomic_load(&event->state, ATOMIC_ACQUIRE);
+    while (!(state & EVENT_SET)) {
+        if (!(state & EVENT_PARKED)) {
+            if (!atomic_compare_exchange_weak(
+                    &event->state,
+                    &state,
+                    state | EVENT_PARKED,
+                    ATOMIC_ACQUIRE,
+                    ATOMIC_ACQUIRE
+                ))
+                continue;
+        }
+
+        // Only sleeps if the event is not set, but parked at this point.
+        park_wait(&event->state, EVENT_PARKED);
+        state = atomic_load(&event->state, ATOMIC_ACQUIRE);
+    }
+}