QtxAcqHistTracker: Add spinlock and use it

smocore/qutexAcquisitionHistoryTracker.cpp

@@ -66,7 +66,6 @@ bool QutexAcquisitionHistoryTracker
 ::heuristicallyTraceContinuationHistoryForGridlockOn(
 	Qutex &firstFailedQutex,
 	std::shared_ptr<AsynchronousContinuationChainLink>& currentContinuation)
-const
 {
 	/**	HEURISTIC APPROACH:
 	 * Due to the computational complexity of full circularity detection,
@@ -82,6 +81,28 @@ const
 	 * explanation.
 	 */
 
+	/**	NOTICE:
+	 * Generally we should have all global data structures owned by a single
+	 * ComponentThread; and qutexes really should only be used to serialize
+	 * async sequences being enqueued on the same ComponentThread. But this
+	 * doesn't prevent multiple CPUs from trying to add/remove entries to/from
+	 * the acquisition history at the same time. Why? The acquisition history
+	 * isn't per-CPU, it's global.
+	 *
+	 * The problem with using a SpinLock here is that if the STL uses mutexes
+	 * internally to lock containers, we could end up in a situation where
+	 * spinning waiters will be busy-spinning while the owner is sleeping?
+	 *
+	 * But this should not happen since the nature of the order of operations is
+	 * that the spinlock ensures that only one CPU at a time can be
+	 * adding/removing entries; and thus everytime an method is called on the
+	 * unordered_map, the caller will always succeed at acquiring the underlying
+	 * STL mutex.
+	 *
+	 * So it should be safe to use a SpinLock here.
+	 */
+	acquisitionHistoryLock.acquire();
+
 	// Iterate through all entries in the acquisition history
 	for (const auto& entry : acquisitionHistory) {
 		const auto& continuation = entry.first;
@@ -104,18 +125,20 @@ const
 			/* Found firstFailedQutex in another continuation's held locks
 			 * This indicates a potential gridlock
 			 */
-			if (&heldLock.get() == &firstFailedQutex) {
+			if (&heldLock.get() == &firstFailedQutex)
+			{
+				acquisitionHistoryLock.release();
 				return true;
 			}
 		}
 	}
 
+	acquisitionHistoryLock.release();
 	return false;
 }
 
 bool QutexAcquisitionHistoryTracker
 ::completelyTraceContinuationHistoryForGridlockOn(Qutex &firstFailedQutex)
-const
 {
 	/** ALGORITHMICALLY COMPLETE VERSION:
 	 * This function is intended to implement the algorithmically complete
@@ -128,6 +151,9 @@ const
 	 * explanation.
 	 */
 
+	// acquisitionHistoryLock.acquire();
+	// TODO: Implement full circularity detection algorithm
+	// acquisitionHistoryLock.release();
 	return false;
 }