diff --git a/paper-server/src/main/java/org/bukkit/craftbukkit/util/LongHashset.java b/paper-server/src/main/java/org/bukkit/craftbukkit/util/LongHashset.java
index 2f8c2ccb3b..4422ec0797 100644
--- a/paper-server/src/main/java/org/bukkit/craftbukkit/util/LongHashset.java
+++ b/paper-server/src/main/java/org/bukkit/craftbukkit/util/LongHashset.java
@@ -1,14 +1,27 @@
 package org.bukkit.craftbukkit.util;
-import static org.bukkit.craftbukkit.util.Java15Compat.Arrays_copyOf;
 
-public class LongHashset extends LongHash {
+import static org.bukkit.craftbukkit.util.Java15Compat.Arrays_copyOf;
+import java.util.concurrent.locks.ReentrantReadWriteLock;
+import java.util.concurrent.locks.ReentrantReadWriteLock.ReadLock;
+import java.util.concurrent.locks.ReentrantReadWriteLock.WriteLock;
+
+public class LongHashset extends LongHash
+{
     long values[][][] = new long[256][][];
     int count = 0;
-    
+    ReentrantReadWriteLock rwl = new ReentrantReadWriteLock();
+    ReadLock rl = rwl.readLock();
+    WriteLock wl = rwl.writeLock();
+
     public boolean isEmpty() {
-        return count == 0;
+        rl.lock();
+        try {
+            return count == 0;
+        } finally {
+            rl.unlock();
+        }
     }
-    
+
     public void add(int msw, int lsw) {
         add(toLong(msw, lsw));
     }
@@ -16,90 +29,142 @@ public class LongHashset extends LongHash {
     public void add(long key) {
         int mainIdx = (int) (key & 255);
         int outerIdx = (int) ((key >> 32) & 255);
-        long outer[][] = values[mainIdx], inner[];
-        if(outer == null) values[mainIdx] = outer = new long[256][];
-        inner = outer[outerIdx];
-        if(inner == null) {
-            outer[outerIdx] = inner = new long[1];
-            inner[0] = key;
-            count++;
-        }
-        else {
-            int i;
-            for(i = 0; i < inner.length; i++) {
-                if(inner[i] == key) {
-                    return;
+        rl.lock();
+        try {
+            wl.lock();
+            try {
+                long outer[][] = values[mainIdx], inner[];
+                if (outer == null)
+                    values[mainIdx] = outer = new long[256][];
+                inner = outer[outerIdx];
+                if (inner == null) {
+                    synchronized (this) {
+                        outer[outerIdx] = inner = new long[1];
+                        inner[0] = key;
+                        count++;
+                    }
+                } else {
+                    int i;
+                    for (i = 0; i < inner.length; i++) {
+                        if (inner[i] == key) {
+                            return;
+                        }
+                    }
+                    inner = Arrays_copyOf(inner, i + 1);
+                    outer[outerIdx] = inner;
+                    inner[i] = key;
+                    count++;
                 }
+            } finally {
+                wl.unlock();
             }
-            outer[outerIdx] = inner = Arrays_copyOf(inner, i+1);
-            inner[i] = key;
-            count++;
+        } finally {
+            rl.unlock();
         }
     }
-    
+
     public boolean containsKey(long key) {
         int mainIdx = (int) (key & 255);
         int outerIdx = (int) ((key >> 32) & 255);
-        long outer[][] = values[mainIdx], inner[];
-        if(outer == null) return false;
-        inner = outer[outerIdx];
-        if(inner == null) return false;
-        else {
-            for(long entry : inner) {
-                if(entry == key) return true;
-            }
-            return false;
-        }
-    }
-    
-    public void remove(long key) {
-        long[][] outer = this.values[(int) (key & 255)];
-        if (outer == null) return;
-
-        long[] inner = outer[(int) ((key >> 32) & 255)];
-        if (inner == null) return;
-        
-        int max = inner.length - 1;
-        for(int i = 0; i <= max; i++) {
-            if(inner[i] == key) {
-                count--;
-                if(i != max) {
-                    inner[i] = inner[max];
+        rl.lock();
+        try {
+            long outer[][] = values[mainIdx], inner[];
+            if (outer == null)
+                return false;
+            inner = outer[outerIdx];
+            if (inner == null)
+                return false;
+            else {
+                for (long entry : inner) {
+                    if (entry == key)
+                        return true;
                 }
-                outer[(int) ((key >> 32) & 255)] = (max == 0 ? null : Arrays_copyOf(inner, max));
-                return;
+                return false;
             }
+        } finally {
+            rl.unlock();
         }
     }
-    
-    public long popFirst() {
-        for(long[][] outer : values) {
-            if(outer == null) continue;
-            for(int i = 0; i < outer.length ; i++) {
-                long[] inner = outer[i];
-                if(inner == null || inner.length == 0) continue;
-                count--;
-                long ret = inner[inner.length - 1];
-                outer[i] = Arrays_copyOf(inner, inner.length - 1);
-                return ret;
-                
+
+    public void remove(long key) {
+        rl.lock();
+        try {
+            long[][] outer = this.values[(int) (key & 255)];
+            if (outer == null)
+                return;
+
+            long[] inner = outer[(int) ((key >> 32) & 255)];
+            if (inner == null)
+                return;
+
+            int max = inner.length - 1;
+            for (int i = 0; i <= max; i++) {
+                if (inner[i] == key) {
+                    wl.lock();
+                    try {
+                        count--;
+                        if (i != max) {
+                            inner[i] = inner[max];
+                        }
+                        outer[(int) ((key >> 32) & 255)] = (max == 0 ? null : Arrays_copyOf(inner, max));
+                    } finally {
+                        wl.unlock();
+                    }
+                    return;
+                }
             }
+        } finally {
+            rl.unlock();
+        }
+    }
+
+    public long popFirst() {
+        rl.lock();
+        try {
+            for (long[][] outer : values) {
+                if (outer == null)
+                    continue;
+                for (int i = 0; i < outer.length; i++) {
+                    long[] inner = outer[i];
+                    if (inner == null || inner.length == 0)
+                        continue;
+                    wl.lock();
+                    try {
+                        count--;
+                        long ret = inner[inner.length - 1];
+                        outer[i] = Arrays_copyOf(inner, inner.length - 1);
+                        return ret;
+                    } finally {
+                        wl.unlock();
+                    }
+
+                }
+            }
+        } finally {
+            rl.unlock();
         }
         return 0;
     }
-    
+
     public long[] keys() {
         int index = 0;
-        long ret[] = new long[count];
-        for(long[][] outer : values) {
-            if(outer == null) continue;
-            for(long[] inner : outer) {
-                if(inner == null) continue;
-                for(long entry : inner) {
-                    ret[index++] = entry;
+        rl.lock();
+        try {
+            long ret[] = new long[count];
+            for (long[][] outer : values) {
+                if (outer == null)
+                    continue;
+                for (long[] inner : outer) {
+                    if (inner == null)
+                        continue;
+                    for (long entry : inner) {
+                        ret[index++] = entry;
+                    }
                 }
             }
+            return ret;
+        } finally {
+            rl.unlock();
         }
-        return ret;
     }
 }
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