mariadb/storage/perfschema/pfs_atomic.cc

/* Copyright (c) 2009, 2010, Oracle and/or its affiliates. All rights reserved.

  This program is free software; you can redistribute it and/or modify
  it under the terms of the GNU General Public License as published by
  the Free Software Foundation; version 2 of the License.

  This program is distributed in the hope that it will be useful,
  but WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  GNU General Public License for more details.

  You should have received a copy of the GNU General Public License
  along with this program; if not, write to the Free Software Foundation,
  51 Franklin Street, Suite 500, Boston, MA 02110-1335 USA */

/**
  @file storage/perfschema/pfs_atomic.cc
  Atomic operations (implementation).
*/

#include <my_global.h>
#include <my_pthread.h>
#include "pfs_atomic.h"

/*
  Using SAFE_MUTEX is impossible, because of recursion.
  - code locks mutex X
    - P_S records the event
      - P_S needs an atomic counter A
        - safe mutex called for m_mutex[hash(A)]
          - safe mutex allocates/free memory
            - safe mutex locks THR_LOCK_malloc
              - P_S records the event
                - P_S needs an atomic counter B
                  - safe mutex called for m_mutex[hash(B)]

  When hash(A) == hash(B), safe_mutex complains rightly that
  the mutex is already locked.
  In some cases, A == B, in particular for events_waits_history_long_index.

  In short, the implementation of PFS_atomic should not cause events
  to be recorded in the performance schema.

  Also, because SAFE_MUTEX redefines pthread_mutex_t, etc,
  this code is not inlined in pfs_atomic.h, but located here in pfs_atomic.cc.

  What is needed is a plain, unmodified, pthread_mutex_t.
  This is provided by my_atomic_rwlock_t.
*/

/**
  Internal rwlock array.
  Using a single rwlock for all atomic operations would be a bottleneck.
  Using a rwlock per performance schema structure would be too costly in
  memory, and use too many rwlock.
  The PFS_atomic implementation computes a hash value from the
  atomic variable, to spread the bottleneck across 256 buckets,
  while still providing --transparently for the caller-- an atomic
  operation.
*/
my_atomic_rwlock_t PFS_atomic::m_rwlock_array[256];
static int init_done;

void PFS_atomic::init(void)
{
  uint i;

  for (i=0; i< array_elements(m_rwlock_array); i++)
    my_atomic_rwlock_init(&m_rwlock_array[i]);
  init_done= 1;
}

void PFS_atomic::cleanup(void)
{
  uint i;
  if (!init_done)
    return;
  for (i=0; i< array_elements(m_rwlock_array); i++)
    my_atomic_rwlock_destroy(&m_rwlock_array[i]);
}