Docs: update qutex algo plan

docs/design/qutexes.md

@@ -257,6 +257,25 @@ class LockSet
 	std::vector<LockUsageDesc> locks;
 }
 
+bool LockerAndInvoker::operator==(const LockerAndInvoker &other)
+{
+	/* Compare by the address of the continuation objects. Why?
+	 * Because there's no guarantee that the lockvoker object that was
+	 * passed in by the io_service invocation is the same object as that
+	 * which is in the qutexQs. Especially because we make_shared() a
+	 * copy when registerInQutexQueues()ing.
+	 *
+	 * Generally when we "wake" a lockvoker by enqueuing it, boost's
+	 * io_service::post will copy the lockvoker object.
+	 */
+	return &this->serializedContinuation == &other.serializedContinuation;
+}
+
+bool LockerAndInvoker::operator !=(const LockerAndInvoker &other)
+{
+	return &this->serializedContinuation != &other.serializedContinuation;
+}
+
 class Qutex
 {
 public:
@@ -280,17 +299,27 @@ public:
 		return it;
 	}
 
-	void unregister(LockerAndInvokerList::iterator it)
+	void unregister(LockerAndInvokerList::iterator it, bool shouldLock=1)
 	{
-		lock.acquire();
-		queue.erase(it);
-		lock.release();
+		if (shouldLock)
+		{
+			lock.acquire();
+			queue.erase(it);
+			lock.release();
+		}
+		else{
+			queue.erase(it);
+		}
 	}
 
-	bool tryAcquire(LockerAndInvoker &tryingLockvoker, int nRequiredLocks)
+	bool tryAcquire(LockerAndInvoker &tryingLockvoker)
 	{
+		const nRequiredLocks = tryingLockvoker.serializedContinuation
+			.requiredLocks.size();
+
 		lock.acquire();
-		qNItems = queue.size();
+
+		const qNItems = queue.size();
 
 		if (qNItems < 1) {
 			lock.release();
@@ -304,17 +333,11 @@ public:
 			throw;
 		}
 
-		if (isOwned) {
+		if (!!currentOwner) {
 			lock.release();
 			return false;
 		}
 
-		if (nRequiredLocks == 1) {
-			isOwned.store(true);
-			lock.release();
-			return true;
-		}
-
 		/**	EXPLANATION:
 		 * From here:
 		 * if qNItems == 1 the we are the only one in the ticketQ and we have
@@ -340,17 +363,31 @@ public:
 		const int nRearItemsToScan = qNItems / nRequiredLocks;
 
 		if (qNItems == 1 || nRearItemsToScan < 1) {
-			isOwned.store(true);
+			currOwner = tryingLockvoker;
 			lock.release();
 			return true;
 		}
 
+		/**	EXPLANATION:
+		 * For lockvokers that only have 1 requiredLock, they must be at the
+		 * front of the queue to successfully acquire.
+		 */
+		if (nRequiredLocks == 1)
+		{
+			if (tryingLockvoker == &queue.front())
+			{
+				currOwner = tryingLockvoker;
+				lock.release();
+				return true;
+			}
+		}
+
 		auto rIt = queue.rbegin();
 		auto rEndIt = queue.rend();
 		bool foundInRear = false;
 		for (int i=0; i<nRearItemsToScan && rIt != rEndIt; rIt++, i++)
 		{
-			if (&(*rIt) != &tryingLockvoker) { continue; }
+			if (*rIt != tryingLockvoker) { continue; }
 
 			foundInRear == true;
 			break;
@@ -364,7 +401,7 @@ public:
 		/* Not found in rear: this means the item is in the top X%. That means
 		 * it should be allowed to claim the lock.
 		 */
-		isOwned.store(true);
+		currOwner = tryingLockvoker;
 		lock.release();
 		return true;
 	}
@@ -445,6 +482,8 @@ public:
 			 */
 		}
 
+		currOwner.release();
+
 		LockerAndInvoker &newFront = queue.front();
 
 		lock.release();
@@ -452,7 +491,7 @@ public:
 		wakeUp(newFront);
 	}
 
-	void release(LockerAndInvoker &prevOwner)
+	void release()
 	{
 		lock.acquire();
 
@@ -460,8 +499,13 @@ public:
 		LockerAndInvoker	&oldFront = queue.front();
 #endif
 
-		unregister(prevOwner.serializedContinuation.requiredLocks.getDesc(
-			*this).second);
+		/* Get the saved iterator and use it to unregister.
+		 * Don't acquire lock because we already acquired it in this function.
+		 */
+		unregister(currOwner->serializedContinuation.requiredLocks
+			.getLockUsageDesc(*this).second, false);
+
+		currOwner.release();
 
 		/** NOTE:
 		 * I am not sure whether we should only wake up the front item if
@@ -514,7 +558,7 @@ public:
 
 public:
 	SpinLock					lock;
-	std::atomic<bool>			isOwned;
+	std::shared_ptr<LockerAndInvoker> currOwner;
 	LockerAndInvokerList		queue;
 };
 ```