Bug 1601024 - ThreadSafeWeakReference::tryDetach must acquire its lock r=froydnj

Differential Revision: https://phabricator.services.mozilla.com/D60037

--HG--
extra : moz-landing-system : lando

mfbt/ThreadSafeWeakPtr.h

@@ -111,10 +111,16 @@ class ReadWriteSpinLock {
   // Decrement the counter to remove a read lock.
   void readUnlock() { mCounter--; }
 
-  // Try to acquire the write lock, but only if there are no readers.
-  // If successful, sets the counter to a negative value.
-  bool tryWriteLock() {
-    return mCounter.compareExchange(0, std::numeric_limits<CounterType>::min());
+  // Spins until a write lock is acquired. This can only occur if there are no
+  // readers or writers. Once it is acquired, the counter is set to a negative
+  // value.
+  void writeLock() {
+    while (true) {
+      if (mCounter.compareExchange(0,
+                                   std::numeric_limits<CounterType>::min())) {
+        return;
+      }
+    }
   }
 
   // Reset the counter to 0.
@@ -169,13 +175,12 @@ class ThreadSafeWeakReference
   // to check the refcount and verify that this is the last reference to
   // the tracked object, so the weak reference can be safely detached.
   void tryDetach(const SupportsThreadSafeWeakPtr<T>* aOwner) {
-    if (mLock.tryWriteLock()) {
+    mLock.writeLock();
     if (aOwner->hasOneRef()) {
       mPtr = nullptr;
     }
     mLock.writeUnlock();
   }
-  }
 
   ReadWriteSpinLock mLock;
   Atomic<T*> mPtr;