gcs_tagged_lock.h

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#ifndef GCS_TAGGED_LOCK_INCLUDED
#define GCS_TAGGED_LOCK_INCLUDED
 
#include <atomic>   // std::atomic
#include <cstdint>  // std::uint64_t
 
/**
 * @brief The Gcs_tagged_lock class
 * Implements a tagged lock for optimistic read-side sections.
 *
 * In a nutshell, the tagged lock is a read-write spin lock which offers the
 * following API:
 *
 *     try_lock() -> bool
 *     unlock()
 *     optimistic_read() -> tag
 *     validate_optimistic_read(tag) -> bool
 *
 * For the write-side section, one uses it as a typical spin lock, e.g.:
 *
 *     do:
 *       lock_acquired := try_lock()
 *     while (not lock_acquired)
 *     write-side section
 *     unlock()
 *
 * For the read-side section, one can use it as follows:
 *
 *     done := false
 *     while (not done):
 *       tag := optimistic_read()
 *       unsynchronised read-side section
 *       done := validate_optimistic_read(tag)
 *       if (not done):
 *         rollback unsynchronized read-side section
 *
 * The idea is to allow an optimistic read-side section that does not perform
 * any memory stores.
 * This is in contrast with a typical read-write lock, where the read side
 * performs some memory stores to account for the reader, e.g. keeping a reader
 * counter.
 * The trade off is that:
 *
 *   a. the execution of the read-side of a tagged lock may be concurrent with
 *      the write-side section if meanwhile the tagged lock is acquired
 *   b. the read-side of a tagged lock may fail if meanwhile the tagged lock is
 *      acquired, in which case one may want to rollback the effects of the
 *      failed read-side section
 *
 * The tagged lock is implemented over a single atomic 64-bit word with the
 * following bit layout:
 *
 *     bit #    64  63  62        3   2   1
 *            +---+---+---+-...-+---+---+---+
 *            |   |   |   |     |   |   |   |
 *            +---+---+---+-...-+---+---+---+
 *             \__________  ___________/ \ /
 *                        \/              v
 *                        tag            locked?
 */
class Gcs_tagged_lock {
 public:
  using Tag = std::uint64_t;
 
  Gcs_tagged_lock() noexcept;
  ~Gcs_tagged_lock();
 
  /**
   * Starts an optimistic read-side section.
   * @returns the tag associated with the optimistic execution.
   */
  Tag optimistic_read() const;
 
  /**
   * Validates an optimistic read-side section.
   * @param tag The tag returned by the corresponding @c optimistic_read
   * @returns true if the optimistic read-side was atomically executed while the
   * lock was free, false otherwise
   */
  bool validate_optimistic_read(Tag const &tag) const;
 
  /**
   * Attempts to start a write-side section, i.e. acquire the lock.
   * @returns true if the write-side section was successfully started, i.e. we
   * acquired the lock, false otherwise
   */
  bool try_lock();
 
  /**
   * Finishes the write-side section, i.e. releases the lock.
   */
  void unlock();
 
  /**
   * Checks whether the lock is currently acquired.
   * @returns true if the lock is currently acquired, false otherwise
   */
  bool is_locked() const;
 
 private:
  /*
   * Atomically loads the underlying lock word.
   */
  std::uint64_t get_lock_word(
      std::memory_order order = std::memory_order_acquire) const;
 
  /* The underlying lock word. */
  std::atomic<std::uint64_t> m_lock_word;
};
 
#endif  // GCS_TAGGED_LOCK_INCLUDED