Remove mutex from SmallStack/StationIDStack.

It is only ever used from the main thread.

src/core/smallstack_type.hpp

@@ -13,7 +13,6 @@
 #define SMALLSTACK_TYPE_HPP
 
 #include "smallvec_type.hpp"
-#include "../thread/thread.h"
 
 /**
  * A simplified pool which stores values instead of pointers and doesn't
@@ -23,15 +22,7 @@
 template<typename Titem, typename Tindex, Tindex Tgrowth_step, Tindex Tmax_size>
 class SimplePool {
 public:
-	inline SimplePool() : first_unused(0), first_free(0), mutex(ThreadMutex::New()) {}
+	inline SimplePool() : first_unused(0), first_free(0) {}
-	inline ~SimplePool() { delete this->mutex; }
-
-	/**
-	 * Get the mutex. We don't lock the mutex in the pool methods as the
-	 * SmallStack isn't necessarily in a consistent state after each method.
-	 * @return Mutex.
-	 */
-	inline ThreadMutex *GetMutex() { return this->mutex; }
 
 	/**
 	 * Get the item at position index.
@@ -86,7 +77,6 @@ private:
 	Tindex first_unused;
 	Tindex first_free;
 
-	ThreadMutex *mutex;
 	SmallVector<SimplePoolPoolItem, Tgrowth_step> data;
 };
 
@@ -124,10 +114,8 @@ struct SmallStackItem {
  * 5. You can choose your own index type, so that you can align it with your
  *    value type. E.G. value types of 16 bits length like to be combined with
  *    index types of the same length.
- * 6. All accesses to the underlying pool are guarded by a mutex and atomic in
+ * 6. This data structure is only ever used from the main thread, so
- *    the sense that the mutex stays locked until the pool has reacquired a
+ *    accesses to the underlying pool are not guarded by locks.
- *    consistent state. This means that even though a common data structure is
- *    used the SmallStack is still reentrant.
  * @tparam Titem Value type to be used.
  * @tparam Tindex Index type to use for the pool.
  * @tparam Tinvalid Invalid item to keep at the bottom of each stack.
@@ -195,7 +183,6 @@ public:
 	inline void Push(const Titem &item)
 	{
 		if (this->value != Tinvalid) {
-			ThreadMutexLocker lock(_pool.GetMutex());
 			Tindex new_item = _pool.Create();
 			if (new_item != Tmax_size) {
 				PooledSmallStack &pushed = _pool.Get(new_item);
@@ -218,17 +205,13 @@ public:
 		if (this->next == Tmax_size) {
 			this->value = Tinvalid;
 		} else {
-			ThreadMutexLocker lock(_pool.GetMutex());
 			PooledSmallStack &popped = _pool.Get(this->next);
 			this->value = popped.value;
 			if (popped.branch_count == 0) {
 				_pool.Destroy(this->next);
 			} else {
 				--popped.branch_count;
-				/* We can't use Branch() here as we already have the mutex.*/
+				this->Branch();
-				if (popped.next != Tmax_size) {
-					++(_pool.Get(popped.next).branch_count);
-				}
 			}
 			/* Accessing popped here is no problem as the pool will only set
 			 * the validity flag, not actually delete the item, on Destroy().
@@ -257,7 +240,6 @@ public:
 	{
 		if (item == Tinvalid || item == this->value) return true;
 		if (this->next != Tmax_size) {
-			ThreadMutexLocker lock(_pool.GetMutex());
 			const SmallStack *in_list = this;
 			do {
 				in_list = static_cast<const SmallStack *>(
@@ -277,7 +259,6 @@ protected:
 	inline void Branch()
 	{
 		if (this->next != Tmax_size) {
-			ThreadMutexLocker lock(_pool.GetMutex());
 			++(_pool.Get(this->next).branch_count);
 		}
 	}