[kernel] Add lock-releasing version of thread::block()

Add a version of thread::block() that takes a lock and releases it after
marking the thread as unready, but before calling the scheduler.

Use this version of block() in the wait_queue.

src/kernel/objects/thread.cpp

@@ -52,6 +52,18 @@ thread::block()
     return m_wake_value;
 }
 
+uint64_t
+thread::block(util::scoped_lock &lock)
+{
+    kassert(current_cpu().thread == this,
+            "unlocking block() called on non-current thread");
+
+    clear_state(state::ready);
+    lock.release();
+    scheduler::get().schedule();
+    return m_wake_value;
+}
+
 j6_status_t
 thread::join()
 {

src/kernel/objects/thread.h

@@ -96,9 +96,14 @@ public:
     inline void set_priority(uint8_t p) { if (!constant()) m_tcb.priority = p; }
 
     /// Block this thread, waiting for a value
-    /// \returns  The value passed to wake()
+    /// \returns   The value passed to wake()
     uint64_t block();
 
+    /// Block this thread, waiting for a value
+    /// \arg held  A held lock to unlock when blocking
+    /// \returns   The value passed to wake()
+    uint64_t block(util::scoped_lock &held);
+
     /// Block the calling thread until this thread exits
     j6_status_t join();
 

src/kernel/wait_queue.cpp

@@ -6,6 +6,7 @@ wait_queue::~wait_queue() { clear(); }
 void
 wait_queue::add_thread(obj::thread *t)
 {
+    kassert(t, "Adding a null thread to the wait queue");
     util::scoped_lock lock {m_lock};
     t->handle_retain();
     m_threads.push_back(t);
@@ -15,8 +16,12 @@ void
 wait_queue::wait()
 {
     obj::thread &current = obj::thread::current();
-    add_thread(&current);
-    current.block();
+
+    util::scoped_lock lock {m_lock};
+    current.handle_retain();
+    m_threads.push_back(&current);
+
+    current.block(lock);
 }
 
 void
@@ -54,9 +59,10 @@ void
 wait_queue::clear(uint64_t value)
 {
     util::scoped_lock lock {m_lock};
-    for (auto *t : m_threads) {
+    while (!m_threads.empty()) {
+        obj::thread *t = pop_next_unlocked();
+        kassert(t, "Null thread in the wait queue");
         if (!t->exited()) t->wake(value);
         t->handle_release();
     }
-    m_threads.clear();
 }