1 files changed, 115 insertions, 22 deletions

diff --git a/mysys/thr_rwlock.c b/mysys/thr_rwlock.c
index ecd12849822..218cfb251c8 100644
--- a/mysys/thr_rwlock.c
+++ b/mysys/thr_rwlock.c
@@ -59,7 +59,7 @@
 *  Mountain View, California  94043
 */
 
-int my_rw_init(my_rw_lock_t *rwp, my_bool *prefer_readers_attr)
+int my_rw_init(my_rw_lock_t *rwp)
 {
   pthread_condattr_t	cond_attr;
 
@@ -74,8 +74,6 @@ int my_rw_init(my_rw_lock_t *rwp, my_bool *prefer_readers_attr)
 #ifdef SAFE_MUTEX
   rwp->write_thread   = 0;
 #endif
-  /* If attribute argument is NULL use default value - prefer writers. */
-  rwp->prefer_readers= prefer_readers_attr ? *prefer_readers_attr : FALSE;
 
   return(0);
 }
@@ -96,8 +94,7 @@ int my_rw_rdlock(my_rw_lock_t *rwp)
   pthread_mutex_lock(&rwp->lock);
 
   /* active or queued writers */
-  while (( rwp->state < 0 ) ||
-         (rwp->waiters && ! rwp->prefer_readers))
+  while (( rwp->state < 0 ) || rwp->waiters)
     pthread_cond_wait( &rwp->readers, &rwp->lock);
 
   rwp->state++;
@@ -109,8 +106,7 @@ int my_rw_tryrdlock(my_rw_lock_t *rwp)
 {
   int res;
   pthread_mutex_lock(&rwp->lock);
-  if ((rwp->state < 0 ) ||
-      (rwp->waiters && ! rwp->prefer_readers))
+  if ((rwp->state < 0 ) || rwp->waiters)
     res= EBUSY;					/* Can't get lock */
   else
   {
@@ -192,30 +188,127 @@ int my_rw_unlock(my_rw_lock_t *rwp)
   return(0);
 }
 
+#endif /* defined(NEED_MY_RW_LOCK) */
+
 
-int rw_pr_init(struct st_my_rw_lock_t *rwlock)
+int rw_pr_init(rw_pr_lock_t *rwlock)
 {
-  my_bool prefer_readers_attr= TRUE;
-  return my_rw_init(rwlock, &prefer_readers_attr);
+  pthread_mutex_init(&rwlock->lock, NULL);
+  pthread_cond_init(&rwlock->no_active_readers, NULL);
+  rwlock->active_readers= 0;
+  rwlock->writers_waiting_readers= 0;
+  rwlock->active_writer= FALSE;
+#ifdef SAFE_MUTEX
+  rwlock->writer_thread= 0;
+#endif
+  return 0;
 }
 
-#else
 
-/*
-  We are on system which has native read/write locks which support
-  preferring of readers.
-*/
+int rw_pr_destroy(rw_pr_lock_t *rwlock)
+{
+  pthread_cond_destroy(&rwlock->no_active_readers);
+  pthread_mutex_destroy(&rwlock->lock);
+  return 0;
+}
 
-int rw_pr_init(rw_pr_lock_t *rwlock)
+
+int rw_pr_rdlock(rw_pr_lock_t *rwlock)
+{
+  pthread_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++;
+  pthread_mutex_unlock(&rwlock->lock);
+  return 0;
+}
+
+
+int rw_pr_wrlock(rw_pr_lock_t *rwlock)
 {
-  pthread_rwlockattr_t rwlock_attr;
+  pthread_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++;
 
-  pthread_rwlockattr_init(&rwlock_attr);
-  pthread_rwlockattr_setkind_np(&rwlock_attr, PTHREAD_RWLOCK_PREFER_READER_NP);
-  pthread_rwlock_init(rwlock, NULL);
-  pthread_rwlockattr_destroy(&rwlock_attr);
+    while (rwlock->active_readers != 0)
+      pthread_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= pthread_self();
+#endif
   return 0;
 }
 
-#endif /* defined(NEED_MY_RW_LOCK) */
+
+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.
+      */
+      pthread_cond_signal(&rwlock->no_active_readers);
+    }
+    pthread_mutex_unlock(&rwlock->lock);
+  }
+  else
+  {
+    /* We are unlocking rd-lock. */
+    pthread_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.
+      */
+      pthread_cond_signal(&rwlock->no_active_readers);
+    }
+    pthread_mutex_unlock(&rwlock->lock);
+  }
+  return 0;
+}
+
+
 #endif /* defined(THREAD) */