[Zion] Add a semaphore primitive with related syscalls.

zion/object/kernel_object.h

@@ -15,6 +15,7 @@ class KernelObject : public glcr::RefCounted<KernelObject> {
     ENDPOINT = 0x7,
     REPLY_PORT = 0x8,
     MUTEX = 0x9,
+    SEMAPHORE = 0x10,
   };
 
   virtual uint64_t TypeTag() = 0;

zion/object/semaphore.cpp

@@ -0,0 +1,29 @@
+#include "object/semaphore.h"
+
+#include "scheduler/scheduler.h"
+
+glcr::RefPtr<Semaphore> Semaphore::Create() {
+  return glcr::AdoptPtr<Semaphore>(new Semaphore());
+}
+
+// FIXME: We almost certainly have some race conditions
+// between this and unlock where we could end up with
+// a thread in the blocked_threads_ queue while noone is holding the lock.
+void Semaphore::Signal() {
+  __atomic_fetch_add(&lock_, 0x1, __ATOMIC_SEQ_CST);
+  if (blocked_threads_.size() > 0) {
+    auto thread = blocked_threads_.PopFront();
+    thread->SetState(Thread::RUNNABLE);
+    gScheduler->Enqueue(thread);
+  }
+}
+
+void Semaphore::Wait() {
+  while (lock_ == 0) {
+    auto thread = gScheduler->CurrentThread();
+    thread->SetState(Thread::BLOCKED);
+    blocked_threads_.PushBack(thread);
+    gScheduler->Yield();
+  }
+  __atomic_fetch_sub(&lock_, 0x1, __ATOMIC_SEQ_CST);
+}

zion/object/semaphore.h

@@ -0,0 +1,33 @@
+#pragma once
+
+#include <glacier/container/intrusive_list.h>
+#include <glacier/memory/ref_ptr.h>
+
+#include "include/ztypes.h"
+#include "object/kernel_object.h"
+#include "object/thread.h"
+
+class Semaphore;
+
+template <>
+struct KernelObjectTag<Semaphore> {
+  static const uint64_t type = KernelObject::SEMAPHORE;
+};
+
+class Semaphore : public KernelObject {
+ public:
+  uint64_t TypeTag() override { return KernelObject::SEMAPHORE; }
+  static uint64_t DefaultPermissions() {
+    return kZionPerm_Wait | kZionPerm_Signal;
+  }
+
+  static glcr::RefPtr<Semaphore> Create();
+
+  void Wait();
+  void Signal();
+
+ private:
+  uint8_t lock_ = 0;
+
+  glcr::IntrusiveList<Thread> blocked_threads_;
+};