ib0mutex.h

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/** @file include/ib0mutex.h
 Policy based mutexes.
 
 Created 2013-03-26 Sunny Bains.
 ***********************************************************************/
 
#ifndef ib0mutex_h
#define ib0mutex_h
 
#include "os0atomic.h"
#include "os0event.h"
#include "sync0policy.h"
#include "ut0rnd.h"
#include "ut0ut.h"
 
#include <atomic>
 
/** OS mutex for tracking lock/unlock for debugging */
template <template <typename> class Policy = NoPolicy>
struct OSTrackMutex {
  typedef Policy<OSTrackMutex> MutexPolicy;
 
  explicit OSTrackMutex(bool destroy_mutex_at_exit = true) UNIV_NOTHROW {
    ut_d(m_freed = true);
    ut_d(m_locked = false);
    ut_d(m_destroy_at_exit = destroy_mutex_at_exit);
  }
 
  ~OSTrackMutex() UNIV_NOTHROW { ut_ad(!m_destroy_at_exit || !m_locked); }
 
  /** Initialise the mutex.
  @param[in]    id              Mutex ID
  @param[in]    filename        File where mutex was created
  @param[in]    line            Line in filename */
  void init(latch_id_t id, const char *filename, uint32_t line) UNIV_NOTHROW {
    ut_ad(m_freed);
    ut_ad(!m_locked);
 
    m_mutex.init();
 
    ut_d(m_freed = false);
 
    m_policy.init(*this, id, filename, line);
  }
 
  /** Destroy the mutex */
  void destroy() UNIV_NOTHROW {
    ut_ad(!m_locked);
    ut_ad(innodb_calling_exit || !m_freed);
 
    m_mutex.destroy();
 
    ut_d(m_freed = true);
 
    m_policy.destroy();
  }
 
  /** Release the mutex. */
  void exit() UNIV_NOTHROW {
    ut_ad(m_locked);
    ut_d(m_locked = false);
    ut_ad(innodb_calling_exit || !m_freed);
 
    m_mutex.exit();
  }
 
  /** Acquire the mutex.
  @param[in]    max_spins       max number of spins
  @param[in]    max_delay       max delay per spin
  @param[in]    filename        from where called
  @param[in]    line            within filename */
  void enter(uint32_t max_spins [[maybe_unused]],
             uint32_t max_delay [[maybe_unused]],
             const char *filename [[maybe_unused]],
             uint32_t line [[maybe_unused]]) UNIV_NOTHROW {
    ut_ad(innodb_calling_exit || !m_freed);
 
    m_mutex.enter();
 
    ut_ad(!m_locked);
    ut_d(m_locked = true);
  }
 
  void lock() { enter(); }
  void unlock() { exit(); }
 
  /** @return true if locking succeeded */
  bool try_lock() UNIV_NOTHROW {
    ut_ad(innodb_calling_exit || !m_freed);
 
    bool locked = m_mutex.try_lock();
 
    if (locked) {
      ut_ad(!m_locked);
      ut_d(m_locked = locked);
    }
 
    return (locked);
  }
 
#ifdef UNIV_DEBUG
  /** @return true if the thread owns the mutex. */
  bool is_owned() const UNIV_NOTHROW {
    return (m_locked && m_policy.is_owned());
  }
#endif /* UNIV_DEBUG */
 
  /** @return non-const version of the policy */
  MutexPolicy &policy() UNIV_NOTHROW { return (m_policy); }
 
  /** @return the const version of the policy */
  const MutexPolicy &policy() const UNIV_NOTHROW { return (m_policy); }
 
 private:
#ifdef UNIV_DEBUG
  /** true if the mutex has not be initialized */
  bool m_freed;
 
  /** true if the mutex has been locked. */
  bool m_locked;
 
  /** Do/Dont destroy mutex at exit */
  bool m_destroy_at_exit;
#endif /* UNIV_DEBUG */
 
  /** OS Mutex instance */
  OSMutex m_mutex;
 
  /** Policy data */
  MutexPolicy m_policy;
};
 
#ifdef HAVE_IB_LINUX_FUTEX
 
#include <linux/futex.h>
#include <sys/syscall.h>
 
/** Mutex implementation that used the Linux futex. */
template <template <typename> class Policy = NoPolicy>
struct TTASFutexMutex {
  typedef Policy<TTASFutexMutex> MutexPolicy;
 
  /** The type of second argument to syscall(SYS_futex, uint32_t *uaddr,...)*/
  using futex_word_t = uint32_t;
  /** Mutex states. */
  enum class mutex_state_t : futex_word_t {
    /** Mutex is free */
    UNLOCKED = 0,
 
    /** Mutex is acquired by some thread. */
    LOCKED = 1,
 
    /** Mutex is contended and there are threads waiting on the lock. */
    LOCKED_WITH_WAITERS = 2
  };
  using lock_word_t = mutex_state_t;
 
  TTASFutexMutex() UNIV_NOTHROW : m_lock_word(mutex_state_t::UNLOCKED) {
    /* The futex API operates on uint32_t futex words, aligned to 4 byte
    boundaries. OTOH we want the luxury of accessing it via std::atomic<>.
    Thus we need to verify that std::atomic doesn't add any extra fluff,
    and is properly aligned */
    using m_lock_word_t = decltype(m_lock_word);
    static_assert(m_lock_word_t::is_always_lock_free);
    static_assert(sizeof(m_lock_word_t) == sizeof(futex_word_t));
#ifdef UNIV_DEBUG
    const auto addr = reinterpret_cast<std::uintptr_t>(&m_lock_word);
    ut_a(addr % alignof(m_lock_word_t) == 0);
    ut_a(addr % 4 == 0);
#endif
  }
 
  ~TTASFutexMutex() { ut_a(m_lock_word == mutex_state_t::UNLOCKED); }
 
  /** Called when the mutex is "created". Note: Not from the constructor
  but when the mutex is initialised.
  @param[in]    id              Mutex ID
  @param[in]    filename        File where mutex was created
  @param[in]    line            Line in filename */
  void init(latch_id_t id, const char *filename, uint32_t line) UNIV_NOTHROW {
    ut_a(m_lock_word == mutex_state_t::UNLOCKED);
    m_policy.init(*this, id, filename, line);
  }
 
  /** Destroy the mutex. */
  void destroy() UNIV_NOTHROW {
    /* The destructor can be called at shutdown. */
    ut_a(m_lock_word == mutex_state_t::UNLOCKED);
    m_policy.destroy();
  }
 
  /** Acquire the mutex.
  @param[in]    max_spins       max number of spins
  @param[in]    max_delay       max delay per spin
  @param[in]    filename        from where called
  @param[in]    line            within filename */
  void enter(uint32_t max_spins, uint32_t max_delay, const char *filename,
             uint32_t line) UNIV_NOTHROW {
    uint32_t n_spins;
    lock_word_t lock = ttas(max_spins, max_delay, n_spins);
 
    /* If there were no waiters when this thread tried
    to acquire the mutex then set the waiters flag now.
    Additionally, when this thread set the waiters flag it is
    possible that the mutex had already been released
    by then. In this case the thread can assume it
    was granted the mutex. */
 
    const uint32_t n_waits = (lock == mutex_state_t::LOCKED_WITH_WAITERS ||
                              (lock == mutex_state_t::LOCKED && !set_waiters()))
                                 ? wait()
                                 : 0;
 
    m_policy.add(n_spins, n_waits);
  }
 
  /** Release the mutex. */
  void exit() UNIV_NOTHROW {
    /* If there are threads waiting then we have to wake
    them up. Reset the lock state to unlocked so that waiting
    threads can test for success. */
 
    std::atomic_thread_fence(std::memory_order_acquire);
 
    if (state() == mutex_state_t::LOCKED_WITH_WAITERS) {
      m_lock_word = mutex_state_t::UNLOCKED;
 
    } else if (unlock() == mutex_state_t::LOCKED) {
      /* No threads waiting, no need to signal a wakeup. */
      return;
    }
 
    signal();
  }
 
  /** Try and lock the mutex.
  @return the old state of the mutex */
  lock_word_t trylock() UNIV_NOTHROW {
    lock_word_t unlocked = mutex_state_t::UNLOCKED;
    m_lock_word.compare_exchange_strong(unlocked, mutex_state_t::LOCKED);
    return unlocked;
  }
 
  /** Try and lock the mutex.
  @return true if successful */
  bool try_lock() UNIV_NOTHROW {
    lock_word_t unlocked = mutex_state_t::UNLOCKED;
    return m_lock_word.compare_exchange_strong(unlocked, mutex_state_t::LOCKED);
  }
 
  /** @return true if mutex is unlocked */
  bool is_locked() const UNIV_NOTHROW {
    return (state() != mutex_state_t::UNLOCKED);
  }
 
#ifdef UNIV_DEBUG
  /** @return true if the thread owns the mutex. */
  bool is_owned() const UNIV_NOTHROW {
    return (is_locked() && m_policy.is_owned());
  }
#endif /* UNIV_DEBUG */
 
  /** @return non-const version of the policy */
  MutexPolicy &policy() UNIV_NOTHROW { return (m_policy); }
 
  /** @return const version of the policy */
  const MutexPolicy &policy() const UNIV_NOTHROW { return (m_policy); }
 
 private:
  /** @return the lock state. */
  lock_word_t state() const UNIV_NOTHROW { return (m_lock_word); }
 
  /** Release the mutex.
  @return the old state of the mutex */
  lock_word_t unlock() UNIV_NOTHROW {
    return m_lock_word.exchange(mutex_state_t::UNLOCKED);
  }
 
  /** Note that there are threads waiting and need to be woken up.
  @return true if state was mutex_state_t::UNLOCKED (ie. granted) */
  bool set_waiters() UNIV_NOTHROW {
    return m_lock_word.exchange(mutex_state_t::LOCKED_WITH_WAITERS) ==
           mutex_state_t::UNLOCKED;
  }
 
  /** Wait if the lock is contended.
  @return the number of waits */
  uint32_t wait() UNIV_NOTHROW {
    uint32_t n_waits = 0;
 
    /* Use FUTEX_WAIT_PRIVATE because our mutexes are
    not shared between processes. */
 
    do {
      ++n_waits;
 
      syscall(SYS_futex, &m_lock_word, FUTEX_WAIT_PRIVATE,
              mutex_state_t::LOCKED_WITH_WAITERS, 0, 0, 0);
 
      // Since we are retrying the operation the return
      // value doesn't matter.
 
    } while (!set_waiters());
 
    return (n_waits);
  }
 
  /** Wakeup a waiting thread */
  void signal() UNIV_NOTHROW {
    syscall(SYS_futex, &m_lock_word, FUTEX_WAKE_PRIVATE, 1, 0, 0, 0);
  }
 
  /** Poll waiting for mutex to be unlocked.
  @param[in]    max_spins       max spins
  @param[in]    max_delay       max delay per spin
  @param[out]   n_spins         retries before acquire
  @return value of lock word before locking. */
  lock_word_t ttas(uint32_t max_spins, uint32_t max_delay,
                   uint32_t &n_spins) UNIV_NOTHROW {
    std::atomic_thread_fence(std::memory_order_acquire);
 
    for (n_spins = 0; n_spins < max_spins; ++n_spins) {
      if (!is_locked()) {
        lock_word_t lock = trylock();
 
        if (lock == mutex_state_t::UNLOCKED) {
          /* Lock successful */
          return (lock);
        }
      }
 
      ut_delay(ut::random_from_interval_fast(0, max_delay));
    }
 
    return (trylock());
  }
 
 private:
  /** Policy data */
  MutexPolicy m_policy;
 
  alignas(4) std::atomic<lock_word_t> m_lock_word;
};
 
#endif /* HAVE_IB_LINUX_FUTEX */
 
template <template <typename> class Policy = NoPolicy>
struct TTASEventMutex {
  typedef Policy<TTASEventMutex> MutexPolicy;
 
  TTASEventMutex() UNIV_NOTHROW {
    /* Check that m_owner is aligned. */
    using m_owner_t = decltype(m_owner);
    ut_ad(reinterpret_cast<std::uintptr_t>(&m_owner) % alignof(m_owner_t) == 0);
    static_assert(m_owner_t::is_always_lock_free);
  }
 
  ~TTASEventMutex() UNIV_NOTHROW { ut_ad(!is_locked()); }
 
  /** If the lock is locked, returns the current owner of the lock, otherwise
  returns the default std::thread::id{} */
  std::thread::id peek_owner() const UNIV_NOTHROW { return m_owner.load(); }
 
  /** Called when the mutex is "created". Note: Not from the constructor
  but when the mutex is initialised.
  @param[in]    id              Mutex ID
  @param[in]    filename        File where mutex was created
  @param[in]    line            Line in filename */
  void init(latch_id_t id, const char *filename, uint32_t line) UNIV_NOTHROW {
    ut_a(m_event == nullptr);
    ut_a(!is_locked());
 
    m_event = os_event_create();
 
    m_policy.init(*this, id, filename, line);
  }
 
  /** This is the real destructor. This mutex can be created in BSS and
  its destructor will be called on exit(). We can't call
  os_event_destroy() at that stage. */
  void destroy() UNIV_NOTHROW {
    ut_ad(!is_locked());
 
    /* We have to free the event before InnoDB shuts down. */
    os_event_destroy(m_event);
    m_event = nullptr;
 
    m_policy.destroy();
  }
 
  /** Try and lock the mutex. Note: POSIX returns 0 on success.
  @return true on success */
  bool try_lock() UNIV_NOTHROW {
    auto expected = std::thread::id{};
    return m_owner.compare_exchange_strong(expected,
                                           std::this_thread::get_id());
  }
 
  /** Release the mutex. */
  void exit() UNIV_NOTHROW {
    m_owner.store(std::thread::id{});
 
    if (m_waiters.load()) {
      signal();
    }
  }
 
  /** Acquire the mutex.
  @param[in]    max_spins       max number of spins
  @param[in]    max_delay       max delay per spin
  @param[in]    filename        from where called
  @param[in]    line            within filename */
  void enter(uint32_t max_spins, uint32_t max_delay, const char *filename,
             uint32_t line) UNIV_NOTHROW {
    if (!try_lock()) {
      spin_and_try_lock(max_spins, max_delay, filename, line);
    }
  }
 
  /** The event that the mutex will wait in sync0arr.cc
  @return even instance */
  os_event_t event() UNIV_NOTHROW { return (m_event); }
 
  /** @return true if locked by some thread */
  bool is_locked() const UNIV_NOTHROW {
    return peek_owner() != std::thread::id{};
  }
 
  /** @return true if the calling thread owns the mutex. */
  bool is_owned() const UNIV_NOTHROW {
    return peek_owner() == std::this_thread::get_id();
  }
 
  /** @return non-const version of the policy */
  MutexPolicy &policy() UNIV_NOTHROW { return (m_policy); }
 
  /** @return const version of the policy */
  const MutexPolicy &policy() const UNIV_NOTHROW { return (m_policy); }
 
 private:
  /** Wait in the sync array.
  @param[in]    filename        from where it was called
  @param[in]    line            line number in file
  @param[in]    spin            retry this many times again
  @return true if the mutex acquisition was successful. */
  bool wait(const char *filename, uint32_t line, uint32_t spin) UNIV_NOTHROW;
 
  /** Spin and wait for the mutex to become free.
  @param[in]    max_spins       max spins
  @param[in]    max_delay       max delay per spin
  @param[in,out]        n_spins         spin start index
  @return true if unlocked */
  bool is_free(uint32_t max_spins, uint32_t max_delay,
               uint32_t &n_spins) const UNIV_NOTHROW {
    ut_ad(n_spins <= max_spins);
 
    /* Spin waiting for the lock word to become zero. Note
    that we do not have to assume that the read access to
    the lock word is atomic, as the actual locking is always
    committed with atomic test-and-set. In reality, however,
    all processors probably have an atomic read of a memory word. */
 
    do {
      if (!is_locked()) {
        return (true);
      }
 
      ut_delay(ut::random_from_interval_fast(0, max_delay));
 
      ++n_spins;
 
    } while (n_spins < max_spins);
 
    return (false);
  }
 
  /** Spin while trying to acquire the mutex
  @param[in]    max_spins       max number of spins
  @param[in]    max_delay       max delay per spin
  @param[in]    filename        from where called
  @param[in]    line            within filename */
  void spin_and_try_lock(uint32_t max_spins, uint32_t max_delay,
                         const char *filename, uint32_t line) UNIV_NOTHROW {
    uint32_t n_spins = 0;
    uint32_t n_waits = 0;
    const uint32_t step = max_spins;
 
    for (;;) {
      /* If the lock was free then try and acquire it. */
 
      if (is_free(max_spins, max_delay, n_spins)) {
        if (try_lock()) {
          break;
        } else {
          continue;
        }
 
      } else {
        max_spins = n_spins + step;
      }
 
      ++n_waits;
 
      std::this_thread::yield();
 
      /* The 4 below is a heuristic that has existed for a
      very long time now. It is unclear if changing this
      value will make a difference.
 
      NOTE: There is a delay that happens before the retry,
      finding a free slot in the sync arary and the yield
      above. Otherwise we could have simply done the extra
      spin above. */
 
      if (wait(filename, line, 4)) {
        n_spins += 4;
 
        break;
      }
    }
 
    /* Waits and yields will be the same number in our
    mutex design */
 
    m_policy.add(n_spins, n_waits);
  }
 
  /** Note that there are threads waiting on the mutex */
  void set_waiters() UNIV_NOTHROW { m_waiters.store(true); }
 
  /** Note that there are no threads waiting on the mutex */
  void clear_waiters() UNIV_NOTHROW { m_waiters.store(false); }
 
  /** Wakeup any waiting thread(s). */
  void signal() UNIV_NOTHROW;
 
 private:
  /** Disable copying */
  TTASEventMutex(TTASEventMutex &&) = delete;
  TTASEventMutex(const TTASEventMutex &) = delete;
  TTASEventMutex &operator=(TTASEventMutex &&) = delete;
  TTASEventMutex &operator=(const TTASEventMutex &) = delete;
 
  /** Set to owner's thread's id when locked, and reset to the default
  std::thread::id{} when unlocked. */
  std::atomic<std::thread::id> m_owner{std::thread::id{}};
 
  /** Used by sync0arr.cc for the wait queue */
  os_event_t m_event{};
 
  /** Policy data */
  MutexPolicy m_policy;
 
  /** true if there are (or may be) threads waiting
  in the global wait array for this mutex to be released. */
  std::atomic_bool m_waiters{false};
};
 
/** Mutex interface for all policy mutexes. This class handles the interfacing
with the Performance Schema instrumentation. */
template <typename MutexImpl>
struct PolicyMutex {
  typedef MutexImpl MutexType;
  typedef typename MutexImpl::MutexPolicy Policy;
 
  PolicyMutex() UNIV_NOTHROW : m_impl() {
#ifdef UNIV_PFS_MUTEX
    m_ptr = nullptr;
#endif /* UNIV_PFS_MUTEX */
  }
 
  ~PolicyMutex() = default;
 
  /** @return non-const version of the policy */
  Policy &policy() UNIV_NOTHROW { return (m_impl.policy()); }
 
  /** @return const version of the policy */
  const Policy &policy() const UNIV_NOTHROW { return (m_impl.policy()); }
 
  /** Release the mutex. */
  void exit() UNIV_NOTHROW {
#ifdef UNIV_PFS_MUTEX
    pfs_exit();
#endif /* UNIV_PFS_MUTEX */
 
    policy().release(m_impl);
 
    m_impl.exit();
  }
 
  /** Acquire the mutex.
  @param n_spins        max number of spins
  @param n_delay        max delay per spin
  @param name   filename where locked
  @param line   line number where locked */
  void enter(uint32_t n_spins, uint32_t n_delay, const char *name,
             uint32_t line) UNIV_NOTHROW {
#ifdef UNIV_PFS_MUTEX
    /* Note: locker is really an alias for state. That's why
    it has to be in the same scope during pfs_end(). */
 
    PSI_mutex_locker_state state;
    PSI_mutex_locker *locker;
 
    locker = pfs_begin_lock(&state, name, line);
#endif /* UNIV_PFS_MUTEX */
 
    policy().enter(m_impl, name, line);
 
    m_impl.enter(n_spins, n_delay, name, line);
 
    policy().locked(m_impl, name, line);
#ifdef UNIV_PFS_MUTEX
    pfs_end(locker, 0);
#endif /* UNIV_PFS_MUTEX */
  }
 
  /** Try and lock the mutex, return 0 on SUCCESS and 1 otherwise.
  @param name   filename where locked
  @param line   line number where locked */
  int trylock(const char *name, uint32_t line) UNIV_NOTHROW {
#ifdef UNIV_PFS_MUTEX
    /* Note: locker is really an alias for state. That's why
    it has to be in the same scope during pfs_end(). */
 
    PSI_mutex_locker_state state;
    PSI_mutex_locker *locker;
 
    locker = pfs_begin_trylock(&state, name, line);
#endif /* UNIV_PFS_MUTEX */
 
    /* There is a subtlety here, we check the mutex ordering
    after locking here. This is only done to avoid add and
    then remove if the trylock was unsuccessful. */
 
    int ret = m_impl.try_lock() ? 0 : 1;
 
    if (ret == 0) {
      policy().enter(m_impl, name, line);
 
      policy().locked(m_impl, name, line);
    }
 
#ifdef UNIV_PFS_MUTEX
    pfs_end(locker, ret);
#endif /* UNIV_PFS_MUTEX */
 
    return (ret);
  }
 
#ifdef UNIV_DEBUG
  /** @return true if the thread owns the mutex. */
  bool is_owned() const UNIV_NOTHROW { return (m_impl.is_owned()); }
#endif /* UNIV_DEBUG */
 
  /**
  Initialise the mutex.
 
  @param[in]    id              Mutex ID
  @param[in]    filename        file where created
  @param[in]    line            line number in file where created */
  void init(latch_id_t id, const char *filename, uint32_t line) UNIV_NOTHROW {
#ifdef UNIV_PFS_MUTEX
    pfs_add(sync_latch_get_pfs_key(id));
#endif /* UNIV_PFS_MUTEX */
 
    m_impl.init(id, filename, line);
  }
 
  /** Free resources (if any) */
  void destroy() UNIV_NOTHROW {
#ifdef UNIV_PFS_MUTEX
    pfs_del();
#endif /* UNIV_PFS_MUTEX */
    m_impl.destroy();
  }
 
  /** Required for os_event_t */
  operator sys_mutex_t *() UNIV_NOTHROW {
    return (m_impl.operator sys_mutex_t *());
  }
 
#ifdef UNIV_PFS_MUTEX
  /** Performance schema monitoring - register mutex with PFS.
 
  Note: This is public only because we want to get around an issue
  with registering a subset of buffer pool pages with PFS when
  PFS_GROUP_BUFFER_SYNC is defined. Therefore this has to then
  be called by external code (see buf0buf.cc).
 
  @param key - Performance Schema key. */
  void pfs_add(mysql_pfs_key_t key) UNIV_NOTHROW {
    ut_ad(m_ptr == nullptr);
    m_ptr = PSI_MUTEX_CALL(init_mutex)(key.m_value, this);
  }
 
 private:
  /** Performance schema monitoring.
  @param state - PFS locker state
  @param name - file name where locked
  @param line - line number in file where locked */
  PSI_mutex_locker *pfs_begin_lock(PSI_mutex_locker_state *state,
                                   const char *name,
                                   uint32_t line) UNIV_NOTHROW {
    if (m_ptr != nullptr) {
      if (m_ptr->m_enabled) {
        return (PSI_MUTEX_CALL(start_mutex_wait)(state, m_ptr, PSI_MUTEX_LOCK,
                                                 name, (uint)line));
      }
    }
 
    return (nullptr);
  }
 
  /** Performance schema monitoring.
  @param state - PFS locker state
  @param name - file name where locked
  @param line - line number in file where locked */
  PSI_mutex_locker *pfs_begin_trylock(PSI_mutex_locker_state *state,
                                      const char *name,
                                      uint32_t line) UNIV_NOTHROW {
    if (m_ptr != nullptr) {
      if (m_ptr->m_enabled) {
        return (PSI_MUTEX_CALL(start_mutex_wait)(
            state, m_ptr, PSI_MUTEX_TRYLOCK, name, (uint)line));
      }
    }
 
    return (nullptr);
  }
 
  /** Performance schema monitoring
  @param locker - PFS identifier
  @param ret - 0 for success and 1 for failure */
  void pfs_end(PSI_mutex_locker *locker, int ret) UNIV_NOTHROW {
    if (locker != nullptr) {
      PSI_MUTEX_CALL(end_mutex_wait)(locker, ret);
    }
  }
 
  /** Performance schema monitoring - register mutex release */
  void pfs_exit() {
    if (m_ptr != nullptr) {
      if (m_ptr->m_enabled) {
        PSI_MUTEX_CALL(unlock_mutex)(m_ptr);
      }
    }
  }
 
  /** Performance schema monitoring - deregister */
  void pfs_del() {
    if (m_ptr != nullptr) {
      PSI_MUTEX_CALL(destroy_mutex)(m_ptr);
      m_ptr = nullptr;
    }
  }
#endif /* UNIV_PFS_MUTEX */
 
 private:
  /** The mutex implementation */
  MutexImpl m_impl;
 
#ifdef UNIV_PFS_MUTEX
  /** The performance schema instrumentation hook. */
  PSI_mutex *m_ptr;
#endif /* UNIV_PFS_MUTEX */
};
 
#endif /* ib0mutex_h */