[util] Add release() and reaquire() to scoped_lock

Added methods for releasing the lock held in a scoped_lock early, as
well as reacquiring it after. Useful when, eg a thread is about to block
and should not be holding the lock while blocked.

src/libraries/util/include/util/spinlock.h

@@ -34,18 +34,37 @@ class scoped_lock
 public:
     inline scoped_lock(
             spinlock &lock,
-            const char *where = __builtin_FUNCTION()
-            ) : m_lock(lock), m_waiter {false, nullptr, where} {
+            const char *where = __builtin_FUNCTION()) :
+        m_lock(lock),
+        m_waiter {false, nullptr, where},
+        m_is_locked {false}
+    {
         m_lock.acquire(&m_waiter);
+        m_is_locked = true;
     }
 
-    inline ~scoped_lock() {
-        m_lock.release(&m_waiter);
+    inline ~scoped_lock() { release(); }
+
+    /// Re-acquire the previously-held lock
+    inline void reacquire() {
+        if (!m_is_locked) {
+            m_lock.acquire(&m_waiter);
+            m_is_locked = true;
+        }
+    }
+
+    /// Release the held lock
+    inline void release() {
+        if (m_is_locked) {
+            m_lock.release(&m_waiter);
+            m_is_locked = false;
+        }
     }
 
 private:
     spinlock &m_lock;
     spinlock::waiter m_waiter;
+    bool m_is_locked;
 };
 
 /// Scoped lock that owns a spinlock::waiter and calls
@@ -57,18 +76,18 @@ public:
             spinlock &lock,
             const char *where = __builtin_FUNCTION()
             ) : m_lock(lock), m_waiter {false, nullptr, where} {
-        m_locked = m_lock.try_acquire(&m_waiter);
+        m_is_locked = m_lock.try_acquire(&m_waiter);
     }
 
     inline ~scoped_trylock() {
-        if (m_locked)
+        if (m_is_locked)
             m_lock.release(&m_waiter);
     }
 
-    inline bool locked() const { return m_locked; }
+    inline bool locked() const { return m_is_locked; }
 
 private:
-    bool m_locked;
+    bool m_is_locked;
     spinlock &m_lock;
     spinlock::waiter m_waiter;
 };