SequenceLock: Newly added

What can I say? It's a very portable primitive that allows us to
create many RCU-type data structures with many readers and one
writer, locklessly.

include/user/sequenceLock.h

@@ -0,0 +1,68 @@
+#ifndef _SEQUENCE_LOCK_H
+#define _SEQUENCE_LOCK_H
+
+#include <atomic>
+#include <optional>
+
+namespace smo {
+
+/**
+ * @brief Sequence lock synchronization primitive
+ *
+ * A reader-writer synchronization primitive where writers increment the
+ * sequence number (odd = writing in progress, even = stable) and readers
+ * check the sequence number to detect concurrent modifications.
+ */
+class SequenceLock
+{
+public:
+	SequenceLock()
+	: sequenceNo(0)
+	{}
+
+	/* Atomically increments sequenceNo and issues a release barrier.
+	 * Makes the sequence number odd, indicating a write is in progress.
+	 */
+	void writeAcquire()
+		{ sequenceNo.fetch_add(1, std::memory_order_release); }
+
+	/* Atomically increments sequenceNo and issues a release barrier.
+	 * Makes the sequence number even again, indicating write is complete.
+	 */
+	void writeRelease()
+		{ sequenceNo.fetch_add(1, std::memory_order_release); }
+
+	/* Issues an acquire barrier and checks if the sequence number is even
+	 * (stable state). If odd (writer active), returns nullopt. Otherwise
+	 * returns the sequence number.
+	 *
+	 * @return std::nullopt if writer is active, otherwise the sequence number
+	 */
+	std::optional<size_t> readAcquire()
+	{
+		size_t seq = sequenceNo.load(std::memory_order_acquire);
+		if (seq & 1) {
+			return std::nullopt;
+		}
+		return seq;
+	}
+
+	/* Issues an acquire barrier and checks if the sequence number matches
+	 * the original value from readAcquire(). If equal, the read was consistent.
+	 *
+	 * @param originalSequenceNo The sequence number obtained from readAcquire()
+	 * @return true if read was consistent, false if writer modified during read
+	 */
+	bool readRelease(size_t originalSequenceNo)
+	{
+		size_t seq = sequenceNo.load(std::memory_order_acquire);
+		return seq == originalSequenceNo;
+	}
+
+private:
+	std::atomic<size_t> sequenceNo;
+};
+
+} // namespace smo
+
+#endif // _SEQUENCE_LOCK_H