From 354bb40e75d94466e91fe6960523612c9d17ccfb Mon Sep 17 00:00:00 2001
From: Karen Arutyunov <karen@codesynthesis.com>
Date: Thu, 2 Nov 2017 23:11:29 +0300
Subject: Add implementation

---
 mysql/mysys/thr_rwlock.c | 139 +++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 139 insertions(+)
 create mode 100644 mysql/mysys/thr_rwlock.c

(limited to 'mysql/mysys/thr_rwlock.c')

diff --git a/mysql/mysys/thr_rwlock.c b/mysql/mysys/thr_rwlock.c
new file mode 100644
index 0000000..cadedda
--- /dev/null
+++ b/mysql/mysys/thr_rwlock.c
@@ -0,0 +1,139 @@
+/* Copyright (c) 2000, 2015, 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, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301  USA */
+
+/* Synchronization - readers / writer thread locks */
+
+#include "thr_rwlock.h"
+
+int rw_pr_init(rw_pr_lock_t *rwlock)
+{
+  native_mutex_init(&rwlock->lock, NULL);
+  native_cond_init(&rwlock->no_active_readers);
+  rwlock->active_readers= 0;
+  rwlock->writers_waiting_readers= 0;
+  rwlock->active_writer= FALSE;
+#ifdef SAFE_MUTEX
+  rwlock->writer_thread= 0;
+#endif
+  return 0;
+}
+
+
+int rw_pr_destroy(rw_pr_lock_t *rwlock)
+{
+  native_cond_destroy(&rwlock->no_active_readers);
+  native_mutex_destroy(&rwlock->lock);
+  return 0;
+}
+
+
+int rw_pr_rdlock(rw_pr_lock_t *rwlock)
+{
+  native_mutex_lock(&rwlock->lock);
+  /*
+    The fact that we were able to acquire 'lock' mutex means
+    that there are no active writers and we can acquire rd-lock.
+    Increment active readers counter to prevent requests for
+    wr-lock from succeeding and unlock mutex.
+  */
+  rwlock->active_readers++;
+  native_mutex_unlock(&rwlock->lock);
+  return 0;
+}
+
+
+int rw_pr_wrlock(rw_pr_lock_t *rwlock)
+{
+  native_mutex_lock(&rwlock->lock);
+
+  if (rwlock->active_readers != 0)
+  {
+    /* There are active readers. We have to wait until they are gone. */
+    rwlock->writers_waiting_readers++;
+
+    while (rwlock->active_readers != 0)
+      native_cond_wait(&rwlock->no_active_readers, &rwlock->lock);
+
+    rwlock->writers_waiting_readers--;
+  }
+
+  /*
+    We own 'lock' mutex so there is no active writers.
+    Also there are no active readers.
+    This means that we can grant wr-lock.
+    Not releasing 'lock' mutex until unlock will block
+    both requests for rd and wr-locks.
+    Set 'active_writer' flag to simplify unlock.
+
+    Thanks to the fact wr-lock/unlock in the absence of
+    contention from readers is essentially mutex lock/unlock
+    with a few simple checks make this rwlock implementation
+    wr-lock optimized.
+  */
+  rwlock->active_writer= TRUE;
+#ifdef SAFE_MUTEX
+  rwlock->writer_thread= my_thread_self();
+#endif
+  return 0;
+}
+
+
+int rw_pr_unlock(rw_pr_lock_t *rwlock)
+{
+  if (rwlock->active_writer)
+  {
+    /* We are unlocking wr-lock. */
+#ifdef SAFE_MUTEX
+    rwlock->writer_thread= 0;
+#endif
+    rwlock->active_writer= FALSE;
+    if (rwlock->writers_waiting_readers)
+    {
+      /*
+        Avoid expensive cond signal in case when there is no contention
+        or it is wr-only.
+
+        Note that from view point of performance it would be better to
+        signal on the condition variable after unlocking mutex (as it
+        reduces number of contex switches).
+
+        Unfortunately this would mean that such rwlock can't be safely
+        used by MDL subsystem, which relies on the fact that it is OK
+        to destroy rwlock once it is in unlocked state.
+      */
+      native_cond_signal(&rwlock->no_active_readers);
+    }
+    native_mutex_unlock(&rwlock->lock);
+  }
+  else
+  {
+    /* We are unlocking rd-lock. */
+    native_mutex_lock(&rwlock->lock);
+    rwlock->active_readers--;
+    if (rwlock->active_readers == 0 &&
+        rwlock->writers_waiting_readers)
+    {
+      /*
+        If we are last reader and there are waiting
+        writers wake them up.
+      */
+      native_cond_signal(&rwlock->no_active_readers);
+    }
+    native_mutex_unlock(&rwlock->lock);
+  }
+  return 0;
+}
+
+
-- 
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