kernel/slab: Use FIFO queuing for PARANOID_KERNEL_FREE only.

FIFO does seem to be a slight performance hit, and the security benefits
are probably not too large in the end. So just use LIFO otherwise.

src/system/kernel/slab/ObjectDepot.cpp

@@ -357,10 +357,10 @@ object_depot_make_empty(object_depot* depot, uint32 flags)
 	// detach the depot's full and empty magazines
 
 	slab_queue fullMagazines = depot->full;
-	depot->full.head = depot->full.tail = NULL;
+	depot->full.Init();
 
 	slab_queue emptyMagazines = depot->empty;
-	depot->empty.head = depot->empty.tail = NULL;
+	depot->empty.Init();
 
 	writeLocker.Unlock();
 

src/system/kernel/slab/slab_queue.h

@@ -8,11 +8,14 @@
 
 #include <stddef.h>
 
+#include "kernel_debug_config.h"
+
 
 struct slab_queue_link {
 	slab_queue_link* next;
 };
 
+#if PARANOID_KERNEL_FREE
 struct slab_queue {
 	slab_queue_link* head;
 	slab_queue_link* tail;
@@ -44,6 +47,29 @@ struct slab_queue {
 		return item;
 	}
 };
+#else /* LIFO queue */
+struct slab_queue {
+	slab_queue_link* head;
+
+	void Init()
+	{
+		head = NULL;
+	}
+
+	void Push(slab_queue_link* item)
+	{
+		item->next = head;
+		head = item;
+	}
+
+	slab_queue_link* Pop()
+	{
+		slab_queue_link* item = head;
+		head = item->next;
+		return item;
+	}
+};
+#endif
 
 
 #endif	// SLAB_QUEUE_H