replace full locks with stamped locks

src/main/java/me/cortex/voxy/common/world/ActiveSectionTracker.java

@@ -9,6 +9,7 @@ import org.jetbrains.annotations.Nullable;
 import java.lang.invoke.VarHandle;
 import java.util.Arrays;
 import java.util.concurrent.locks.ReentrantLock;
+import java.util.concurrent.locks.StampedLock;
 
 public class ActiveSectionTracker {
 
@@ -18,7 +19,7 @@ public class ActiveSectionTracker {
     //Loaded section world cache, TODO: get rid of VolatileHolder and use something more sane
 
     private final Long2ObjectOpenHashMap<VolatileHolder<WorldSection>>[] loadedSectionCache;
-    private final ReentrantLock[] locks;//TODO: replace with StampedLocks
+    private final StampedLock[] locks;
     private final SectionLoader loader;
 
     private final int lruSize;
@@ -37,11 +38,11 @@ public class ActiveSectionTracker {
         this.loader = loader;
         this.loadedSectionCache = new Long2ObjectOpenHashMap[1<<numSlicesBits];
         this.lruSecondaryCache = new Long2ObjectLinkedOpenHashMap<>(cacheSize);
-        this.locks = new ReentrantLock[1<<numSlicesBits];
+        this.locks = new StampedLock[1<<numSlicesBits];
         this.lruSize = cacheSize;
         for (int i = 0; i < this.loadedSectionCache.length; i++) {
             this.loadedSectionCache[i] = new Long2ObjectOpenHashMap<>(1024);
-            this.locks[i] = new ReentrantLock();
+            this.locks[i] = new StampedLock();
         }
     }
 
@@ -55,26 +56,37 @@ public class ActiveSectionTracker {
         final var lock = this.locks[index];
         VolatileHolder<WorldSection> holder = null;
         boolean isLoader = false;
-        WorldSection section;
+        WorldSection section = null;
 
-        lock.lock();
         {
-            VarHandle.fullFence();
+            long stamp = lock.readLock();
             holder = cache.get(key);
-            if (holder == null) {
+            if (holder != null) {//Return already loaded entry
+                section = holder.obj;
+                if (section != null) {
+                    section.acquire();
+                    lock.unlockRead(stamp);
+                    return section;
+                }
+                lock.unlockRead(stamp);
+            } else {//Try to create holder
                 holder = new VolatileHolder<>();
-                cache.put(key, holder);
+                long ws = lock.tryConvertToWriteLock(stamp);
-                isLoader = true;
+                if (ws == 0) {//Failed to convert, unlock read and get write
+                    lock.unlockRead(stamp);
+                    stamp = lock.writeLock();
+                } else {
+                    stamp = ws;
+                }
+                var eHolder = cache.putIfAbsent(key, holder);//We put if absent because on failure to convert to write, it leaves race condition
+                lock.unlockWrite(stamp);
+                if (eHolder == null) {//We are the loader
+                    isLoader = true;
+                } else {
+                    holder = eHolder;
+                }
             }
-            section = holder.obj;
-            if (section != null) {
-                section.acquire();
-                lock.unlock();
-                return section;
-            }
-            VarHandle.fullFence();
         }
-        lock.unlock();
 
         if (isLoader) {
             synchronized (this.lruSecondaryCache) {
@@ -128,6 +140,8 @@ public class ActiveSectionTracker {
                 }
             }
             //lock.unlock();
+
+            //We failed everything, try get it again
             return this.acquire(key, nullOnEmpty);
         }
     }
@@ -137,9 +151,8 @@ public class ActiveSectionTracker {
         final var cache = this.loadedSectionCache[index];
         WorldSection sec = null;
         final var lock = this.locks[index];
-        lock.lock();
+        long stamp = lock.writeLock();
         {
-            VarHandle.fullFence();
             if (section.trySetFreed()) {
                 var cached = cache.remove(section.key);
                 var obj = cached.obj;
@@ -151,9 +164,8 @@ public class ActiveSectionTracker {
                 }
                 sec = section;
             }
-            VarHandle.fullFence();
         }
-        lock.unlock();
+        lock.unlockWrite(stamp);
 
         if (sec != null) {
             WorldSection a;

src/main/java/me/cortex/voxy/common/world/WorldEngine.java

@@ -51,7 +51,7 @@ public class WorldEngine {
         this.storage = storage;
         this.mapper = new Mapper(this.storage);
         //5 cache size bits means that the section tracker has 32 separate maps that it uses
-        this.sectionTracker = new ActiveSectionTracker(10, storage::loadSection, 2048, this);
+        this.sectionTracker = new ActiveSectionTracker(6, storage::loadSection, 2048, this);
     }
 
     public WorldSection acquireIfExists(int lvl, int x, int y, int z) {

src/main/java/me/cortex/voxy/common/world/WorldSection.java

@@ -119,10 +119,8 @@ public final class WorldSection {
 
     public int acquire(int count) {
         int state =((int)  ATOMIC_STATE_HANDLE.getAndAdd(this, count<<1)) + (count<<1);
-        if (VERIFY_WORLD_SECTION_EXECUTION) {
+        if ((state & 1) == 0) {
-            if ((state & 1) == 0) {
+            throw new IllegalStateException("Tried to acquire unloaded section");
-                throw new IllegalStateException("Tried to acquire unloaded section");
-            }
         }
         return state>>1;
     }
@@ -134,13 +132,11 @@ public final class WorldSection {
     //TODO: add the ability to hint to the tracker that yes the section is unloaded, try to cache it in a secondary cache since it will be reused/needed later
     public int release() {
         int state = ((int) ATOMIC_STATE_HANDLE.getAndAdd(this, -2)) - 2;
-        if (VERIFY_WORLD_SECTION_EXECUTION) {
+        if (state < 1) {
-            if (state < 1) {
+            throw new IllegalStateException("Section got into an invalid state");
-                throw new IllegalStateException("Section got into an invalid state");
+        }
-            }
+        if ((state & 1) == 0) {
-            if ((state & 1) == 0) {
+            throw new IllegalStateException("Tried releasing a freed section");
-                throw new IllegalStateException("Tried releasing a freed section");
-            }
         }
         if ((state>>1)==0) {
             if (this.tracker != null) {
@@ -159,10 +155,8 @@ public final class WorldSection {
     //Returns true on success, false on failure
     boolean trySetFreed() {
         int witness = (int) ATOMIC_STATE_HANDLE.compareAndExchange(this, 1, 0);
-        if (VERIFY_WORLD_SECTION_EXECUTION) {
+        if ((witness & 1) == 0 && witness != 0) {
-            if ((witness & 1) == 0 && witness != 0) {
+            throw new IllegalStateException("Section marked as free but has refs");
-                throw new IllegalStateException("Section marked as free but has refs");
-            }
         }
         return witness == 1;
     }