chore: remove unnecessary thread locks and fix possible struct copy issue

README.md

@@ -108,5 +108,16 @@ Console.WriteLine(*ptr);
 * **Limitations:** Not available in WASM environments because it requires virtual memory OS APIs.
 * **When to use:** Better when you need to continuous blocks of very large memory without worrying about re-allocations or segment limits, and you are on a supported platform (Windows/Linux/macOS).
 
+## Thread Safety
+
+> [!IMPORTANT]
+> **SharpArena is strictly NOT thread-safe.**
+
+To achieve maximum performance and zero overhead on the hot path, `ArenaAllocator` and its associated collections do not use any synchronization primitives (locks, interlocked operations, or volatile reads).
+
+- **One Arena Per Thread:** You should create a separate `ArenaAllocator` instance for each thread or use a `[ThreadStatic]` field.
+- **No Concurrent Access:** Do not call `Alloc`, `Reset`, or `Dispose` concurrently from multiple threads on the same instance.
+- **Single-Threaded Collections:** Collections like `ArenaList<T>` and `ArenaString` are intended to be used within the same thread that owns the arena.
+
 ## Benchmarks
 See `bench/ArenaBench.md` for performance numbers compared to NativeMemory and Varena.

src/SharpArena/Allocators/ArenaAllocator.cs

@@ -21,9 +21,6 @@ public unsafe class ArenaAllocator : IDisposable
     private bool _disposed;
     private int _generation = 0;
 
-    private readonly object _disposeLock = new();
-    private int _activeAllocations;
-    private int _resetInProgress;
     private nuint _peakBytes;
 
     /// <summary>
@@ -81,85 +78,60 @@ public ArenaAllocator(
     /// <returns>A pointer to the allocated memory.</returns>
     public void* Alloc(nuint size, nuint align = 8)
     {
-        SpinWait spinner = default;
+        if (_disposed)
+        {
+            throw new ObjectDisposedException(nameof(ArenaAllocator));
+        }
+
+        if (size == 0)
+        {
+            return null;
+        }
+
+        var seg = _current;
+        if (seg is null)
+        {
+            throw new ObjectDisposedException(nameof(ArenaAllocator));
+        }
+
+        align = AlignUp(align, (nuint)IntPtr.Size);
+        if (seg->TryAlloc(size, align, out var ptr))
+        {
+            return ptr;
+        }
+
         while (true)
         {
-            if (Volatile.Read(ref _disposed))
+            if (_disposed)
             {
                 throw new ObjectDisposedException(nameof(ArenaAllocator));
             }
 
-            Interlocked.Increment(ref _activeAllocations);
-            if (Volatile.Read(ref _disposed))
-            {
-                Interlocked.Decrement(ref _activeAllocations);
-                throw new ObjectDisposedException(nameof(ArenaAllocator));
-            }
+            var nextSize = NextSegmentSize(seg->Size, size);
 
-            if (Volatile.Read(ref _resetInProgress) != 0)
+            if (nextSize < size)
             {
-                Interlocked.Decrement(ref _activeAllocations);
-                spinner.SpinOnce();
-                continue;
+                nextSize = AlignUp(size, DefaultPageSize);
             }
 
-            try
+            if (nextSize > _maxSegmentSize)
             {
-                if (size == 0)
-                {
-                    return null;
-                }
+                nextSize = AlignUp(size, DefaultPageSize); // fallback if request > maxSegmentSize
+            }
 
-                var seg = _current;
-                if (seg is null)
-                {
-                    throw new ObjectDisposedException(nameof(ArenaAllocator));
-                }
+            var newSeg = AllocateSegment(nextSize);
+            seg->Next = newSeg;
+            _current = newSeg;
+            seg = newSeg;
 
-                align = AlignUp(align, (nuint)IntPtr.Size);
-                if (seg->TryAlloc(size, align, out var ptr))
-                {
-                    return ptr;
-                }
-
-                while (true)
-                {
-                    if (Volatile.Read(ref _disposed))
-                    {
-                        throw new ObjectDisposedException(nameof(ArenaAllocator));
-                    }
-
-                    var nextSize = NextSegmentSize(seg->Size, size);
-
-                    if (nextSize < size)
-                    {
-                        nextSize = AlignUp(size, DefaultPageSize);
-                    }
-
-                    if (nextSize > _maxSegmentSize)
-                    {
-                        nextSize = AlignUp(size, DefaultPageSize); // fallback if request > maxSegmentSize
-                    }
-
-                    var newSeg = AllocateSegment(nextSize);
-                    seg->Next = newSeg;
-                    _current = newSeg;
-                    seg = newSeg;
-
-                    if (seg->TryAlloc(size, align, out ptr))
-                    {
-                        return ptr;
-                    }
-
-                    if (nextSize == size)
-                    {
-                        throw new OutOfMemoryException("Failed to allocate memory in arena; request too large.");
-                    }
-                }
+            if (seg->TryAlloc(size, align, out ptr))
+            {
+                return ptr;
             }
-            finally
+
+            if (nextSize == size)
             {
-                Interlocked.Decrement(ref _activeAllocations);
+                throw new OutOfMemoryException("Failed to allocate memory in arena; request too large.");
             }
         }
     }
@@ -251,85 +223,56 @@ private static nuint RoundUpToPowerOfTwo(nuint x)
     /// </remarks>
     public void Reset()
     {
-        lock (_disposeLock)
+        if (_disposed)
         {
-            if (_disposed)
-            {
-                throw new ObjectDisposedException(nameof(ArenaAllocator));
-            }
-
-            Volatile.Write(ref _resetInProgress, 1);
-            try
-            {
-                SpinWait spinner = default;
-                while (Volatile.Read(ref _activeAllocations) != 0)
-                {
-                    spinner.SpinOnce();
-                }
-
-                var currentAllocated = AllocatedBytes;
-                if (currentAllocated > _peakBytes)
-                {
-                    _peakBytes = currentAllocated;
-                }
+            throw new ObjectDisposedException(nameof(ArenaAllocator));
+        }
 
-                for (var seg = _first; seg != null; seg = seg->Next)
-                {
-                    seg->Offset = 0;
-                }
+        var currentAllocated = AllocatedBytes;
+        if (currentAllocated > _peakBytes)
+        {
+            _peakBytes = currentAllocated;
+        }
 
-                _current = _first;
-                Interlocked.Increment(ref _generation);
-            }
-            finally
-            {
-                Volatile.Write(ref _resetInProgress, 0);
-            }
+        for (var seg = _first; seg != null; seg = seg->Next)
+        {
+            seg->Offset = 0;
         }
+
+        _current = _first;
+        _generation++;
     }
 
     /// <summary>
     /// Disposes the arena, freeing all allocated unmanaged memory.
     /// </summary>
     public void Dispose()
     {
-        lock (_disposeLock)
-        {
-            Dispose(true);
-        }
+        Dispose(true);
     }
 
     private void Dispose(bool isDisposing)
     {
-        lock (_disposeLock)
+        if (_disposed)
         {
-            if (_disposed)
-            {
-                return;
-            }
-
-            _disposed = true;
+            return;
+        }
 
-            SpinWait spinner = default;
-            while (Volatile.Read(ref _activeAllocations) != 0)
-            {
-                spinner.SpinOnce();
-            }
+        _disposed = true;
 
-            var head = _first;
-            _first = _current = null;
+        var head = _first;
+        _first = _current = null;
 
-            while (head != null)
-            {
-                var next = head->Next;
-                NativeAllocator.Free(head, _backend);
-                head = next;
-            }
+        while (head != null)
+        {
+            var next = head->Next;
+            NativeAllocator.Free(head, _backend);
+            head = next;
+        }
 
-            if (isDisposing)
-            {
-                GC.SuppressFinalize(this);
-            }
+        if (isDisposing)
+        {
+            GC.SuppressFinalize(this);
         }
     }
 
@@ -338,9 +281,6 @@ private void Dispose(bool isDisposing)
     /// </summary>
     ~ArenaAllocator()
     {
-        lock (_disposeLock)
-        {
-            Dispose(false);
-        }
+        Dispose(false);
     }
 }

src/SharpArena/Collections/ArenaBlock.cs

@@ -36,6 +36,27 @@ public unsafe struct ArenaBlock<T>
     public ArenaBlock<T>* Next;
 }
 
+/// <summary>
+/// Metadata describing the shared state of an <see cref="ArenaBlockList{T}"/>.
+/// </summary>
+[StructLayout(LayoutKind.Sequential)]
+public unsafe struct ArenaBlockListHeader<T>
+    where T : unmanaged
+{
+    /// <summary>
+    /// Gets or sets the total number of elements stored across all blocks.
+    /// </summary>
+    public nuint TotalCount;
+    /// <summary>
+    /// Gets or sets the total allocated capacity across all blocks.
+    /// </summary>
+    public nuint TotalCapacity;
+    /// <summary>
+    /// Gets or sets the pointer to the current block where the next addition will occur.
+    /// </summary>
+    public ArenaBlock<T>* CurrentBlock;
+}
+
 /// <summary>
 /// Provides a growable sequence of arena-backed blocks that stores unmanaged values without GC allocations.
 /// </summary>
@@ -50,11 +71,7 @@ public unsafe struct ArenaBlockList<T> : IEnumerable<T>
     private readonly ArenaAllocator _arena;
     private readonly int _generation; //generation snapshot
     private readonly ArenaBlock<T>* _head;
-    private ArenaBlock<T>* _current;
-
-    // Cache total count and capacity to avoid O(N) traversal
-    private nuint _count;
-    private nuint _capacity;
+    private ArenaBlockListHeader<T>* _header;
 
     /// <summary>
     /// Initializes a new instance of the <see cref="ArenaBlockList{T}"/> struct.
@@ -64,10 +81,15 @@ public unsafe struct ArenaBlockList<T> : IEnumerable<T>
     public ArenaBlockList(ArenaAllocator arena, nuint blockSize = DefaultBlockSize)
     {
         _arena = arena;
-        _head = _current = CreateBlock(arena, blockSize);
         _generation = arena.CurrentGeneration;
-        _count = 0;
-        _capacity = blockSize;
+
+        var firstBlock = CreateBlock(arena, blockSize);
+        _head = firstBlock;
+
+        _header = (ArenaBlockListHeader<T>*)arena.Alloc((nuint)sizeof(ArenaBlockListHeader<T>), align: (nuint)IntPtr.Size);
+        _header->TotalCount = 0;
+        _header->TotalCapacity = blockSize;
+        _header->CurrentBlock = firstBlock;
     }
 
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
@@ -113,7 +135,7 @@ public nuint Count
         get
         {
             CheckAliveThrowIfNot();
-            return _count;
+            return _header->TotalCount;
         }
     }
 
@@ -125,7 +147,7 @@ public nuint Capacity
         get
         {
             CheckAliveThrowIfNot();
-            return _capacity;
+            return _header->TotalCapacity;
         }
     }
 
@@ -140,8 +162,8 @@ public void Reset()
             b->Count = 0;
         }
 
-        _current = _head;
-        _count = 0;
+        _header->CurrentBlock = _head;
+        _header->TotalCount = 0;
     }
 
     /// <summary>
@@ -151,17 +173,19 @@ public void Reset()
     public void Add(in T value)
     {
         CheckAliveThrowIfNot();
-        if (_current->Count >= _current->Capacity)
+        var current = _header->CurrentBlock;
+        if (current->Count >= current->Capacity)
         {
-            var nextCapacity = _current->Capacity * 2;
+            var nextCapacity = current->Capacity * 2;
             var newBlock = CreateBlock(_arena, nextCapacity);
-            _current->Next = newBlock;
-            _current = newBlock;
-            _capacity += nextCapacity;
+            current->Next = newBlock;
+            _header->CurrentBlock = newBlock;
+            _header->TotalCapacity += nextCapacity;
+            current = newBlock;
         }
 
-        _current->Data[_current->Count++] = value;
-        _count++;
+        current->Data[current->Count++] = value;
+        _header->TotalCount++;
     }
 
     /// <summary>