A better fix for bug #56405 "Deadlock in the MDL deadlock

detector" that doesn't introduce bug #56715 "Concurrent
transactions + FLUSH result in sporadical unwarranted
deadlock errors".

Deadlock could have occurred when workload containing a mix
of DML, DDL and FLUSH TABLES statements affecting the same
set of tables was executed in a heavily concurrent environment.

This deadlock occurred when several connections tried to
perform deadlock detection in the metadata locking subsystem.
The first connection started traversing wait-for graph,
encountered a sub-graph representing a wait for flush, acquired
LOCK_open and dived into sub-graph inspection. Then it
encountered sub-graph corresponding to wait for metadata lock
and blocked while trying to acquire a rd-lock on
MDL_lock::m_rwlock, since some,other thread had a wr-lock on it.
When this wr-lock was released it could have happened (if there
was another pending wr-lock against this rwlock) that the rd-lock
from the first connection was left unsatisfied but at the same
time the new rd-lock request from the second connection sneaked
in and was satisfied (for this to be possible the second
rd-request should come exactly after the wr-lock is released but
before pending the wr-lock manages to grab rwlock, which is
possible both on Linux and in our own rwlock implementation).
If this second connection continued traversing the wait-for graph
and encountered a sub-graph representing a wait for flush it tried
to acquire LOCK_open and thus the deadlock was created.

The previous patch tried to workaround this problem by not
allowing the deadlock detector to lock LOCK_open mutex if
some other thread doing deadlock detection already owns it
and current search depth is greater than 0. Instead deadlock
was reported. As a result it has introduced bug #56715.

This patch solves this problem in a different way.
It introduces a new rw_pr_lock_t implementation to be used
by MDL subsystem instead of one based on Linux rwlocks or
our own rwlock implementation. This new implementation
never allows situation in which an rwlock is rd-locked and
there is a blocked pending rd-lock. Thus the situation which
has caused this bug becomes impossible with this implementation.

Due to fact that this implementation is optimized for
wr-lock/unlock scenario which is most common in the MDL
subsystem it doesn't introduce noticeable performance
regressions in sysbench tests. Moreover it significantly
improves situation for POINT_SELECT test when many
connections are used.

No test case is provided as this bug is very hard to repeat
in MTR environment but is repeatable with the help of RQG
tests.
This patch also doesn't include a test for bug #56715
"Concurrent transactions + FLUSH result in sporadical
unwarranted deadlock errors" as it takes too much time to
be run as part of normal test-suite runs.

config.h.cmake:
  We no longer need to check for presence of
  pthread_rwlockattr_setkind_np as we no longer
  use Linux-specific implementation of rw_pr_lock_t
  which uses this function.
configure.cmake:
  We no longer need to check for presence of
  pthread_rwlockattr_setkind_np as we no longer
  use Linux-specific implementation of rw_pr_lock_t
  which uses this function.
configure.in:
  We no longer need to check for presence of
  pthread_rwlockattr_setkind_np as we no longer
  use Linux-specific implementation of rw_pr_lock_t
  which uses this function.
include/my_pthread.h:
  Introduced new implementation of rw_pr_lock_t.
  Since it never allows situation in which rwlock is rd-locked
  and there is a blocked pending rd-lock it is not affected by
  bug #56405 "Deadlock in the MDL deadlock detector".
  This implementation is also optimized for wr-lock/unlock
  scenario which is most common in MDL subsystem. So it doesn't
  introduce noticiable performance regressions in sysbench tests
  (compared to old Linux-specific implementation). Moreover it
  significantly improves situation for POINT_SELECT test when
  many connections are used.
  As part of this change removed try-lock part of API for
  this type of lock. It is not used in our code and it would
  be hard to implement correctly within constraints of new
  implementation.
  Finally, removed support of preferring readers from
  my_rw_lock_t implementation as the only user of this
  feature was old rw_pr_lock_t implementation.
include/mysql/psi/mysql_thread.h:
  Removed try-lock part of prlock API.
  It is not used in our code and it would be hard
  to implement correctly within constraints of new
  prlock implementation.
mysys/thr_rwlock.c:
  Introduced new implementation of rw_pr_lock_t.
  Since it never allows situation in which rwlock is rd-locked
  and there is a blocked pending rd-lock it is not affected by
  bug #56405 "Deadlock in the MDL deadlock detector".
  This implementation is also optimized for wr-lock/unlock
  scenario which is most common in MDL subsystem. So it doesn't
  introduce noticiable performance regressions in sysbench tests
  (compared to old Linux-specific implementation). Moreover it
  significantly improves situation for POINT_SELECT test when
  many connections are used.
  Also removed support of preferring readers from
  my_rw_lock_t implementation as the only user of this
  feature was old rw_pr_lock_t implementation.

6 files changed, 185 insertions, 150 deletions

diff --git a/config.h.cmake b/config.h.cmake
index c484edb65a5..7047cc6bbf5 100644
--- a/config.h.cmake
+++ b/config.h.cmake
@@ -220,7 +220,6 @@
 #cmakedefine HAVE_PTHREAD_KEY_DELETE 1
 #cmakedefine HAVE_PTHREAD_KILL 1
 #cmakedefine HAVE_PTHREAD_RWLOCK_RDLOCK 1
-#cmakedefine HAVE_PTHREAD_RWLOCKATTR_SETKIND_NP 1
 #cmakedefine HAVE_PTHREAD_SETPRIO_NP 1
 #cmakedefine HAVE_PTHREAD_SETSCHEDPARAM 1
 #cmakedefine HAVE_PTHREAD_SIGMASK 1
diff --git a/configure.cmake b/configure.cmake
index c2e1dc4647b..91c39fc5b09 100644
--- a/configure.cmake
+++ b/configure.cmake
@@ -346,7 +346,6 @@ CHECK_FUNCTION_EXISTS (pthread_condattr_setclock HAVE_PTHREAD_CONDATTR_SETCLOCK)
 CHECK_FUNCTION_EXISTS (pthread_init HAVE_PTHREAD_INIT)
 CHECK_FUNCTION_EXISTS (pthread_key_delete HAVE_PTHREAD_KEY_DELETE)
 CHECK_FUNCTION_EXISTS (pthread_rwlock_rdlock HAVE_PTHREAD_RWLOCK_RDLOCK)
-CHECK_FUNCTION_EXISTS (pthread_rwlockattr_setkind_np HAVE_PTHREAD_RWLOCKATTR_SETKIND_NP)
 CHECK_FUNCTION_EXISTS (pthread_sigmask HAVE_PTHREAD_SIGMASK)
 CHECK_FUNCTION_EXISTS (pthread_threadmask HAVE_PTHREAD_THREADMASK)
 CHECK_FUNCTION_EXISTS (pthread_yield_np HAVE_PTHREAD_YIELD_NP)
diff --git a/configure.in b/configure.in
index 1f56d958a7f..48385ba2491 100644
--- a/configure.in
+++ b/configure.in
@@ -2169,7 +2169,7 @@ AC_CHECK_FUNCS(alarm bfill bmove bsearch bzero \
   mkstemp mlockall perror poll pread pthread_attr_create mmap mmap64 getpagesize \
   pthread_attr_getstacksize pthread_attr_setstacksize pthread_condattr_create \
   pthread_getsequence_np pthread_key_delete pthread_rwlock_rdlock \
-  pthread_rwlockattr_setkind_np pthread_sigmask \
+  pthread_sigmask \
   readlink realpath rename rint rwlock_init setupterm \
   shmget shmat shmdt shmctl sigaction sigemptyset sigaddset \
   sighold sigset sigthreadmask port_create sleep thr_yield \
diff --git a/include/my_pthread.h b/include/my_pthread.h
index 5cf181596ad..27ab5ba23fe 100644
--- a/include/my_pthread.h
+++ b/include/my_pthread.h
@@ -594,7 +594,7 @@ int my_pthread_fastmutex_lock(my_pthread_fastmutex_t *mp);
 /* Use our own version of read/write locks */
 #define NEED_MY_RW_LOCK 1
 #define rw_lock_t my_rw_lock_t
-#define my_rwlock_init(A,B) my_rw_init((A), 0)
+#define my_rwlock_init(A,B) my_rw_init((A))
 #define rw_rdlock(A) my_rw_rdlock((A))
 #define rw_wrlock(A) my_rw_wrlock((A))
 #define rw_tryrdlock(A) my_rw_tryrdlock((A))
@@ -606,49 +606,82 @@ int my_pthread_fastmutex_lock(my_pthread_fastmutex_t *mp);
 #endif /* USE_MUTEX_INSTEAD_OF_RW_LOCKS */
 
 
-/*
-  Portable read-write locks which prefer readers.
-
-  Required by some algorithms in order to provide correctness.
+/**
+  Portable implementation of special type of read-write locks.
+
+  These locks have two properties which are unusual for rwlocks:
+  1) They "prefer readers" in the sense that they do not allow
+     situations in which rwlock is rd-locked and there is a
+     pending rd-lock which is blocked (e.g. due to pending
+     request for wr-lock).
+     This is a stronger guarantee than one which is provided for
+     PTHREAD_RWLOCK_PREFER_READER_NP rwlocks in Linux.
+     MDL subsystem deadlock detector relies on this property for
+     its correctness.
+  2) They are optimized for uncontended wr-lock/unlock case.
+     This is scenario in which they are most oftenly used
+     within MDL subsystem. Optimizing for it gives significant
+     performance improvements in some of tests involving many
+     connections.
+
+  Another important requirement imposed on this type of rwlock
+  by the MDL subsystem is that it should be OK to destroy rwlock
+  object which is in unlocked state even though some threads might
+  have not yet fully left unlock operation for it (of course there
+  is an external guarantee that no thread will try to lock rwlock
+  which is destroyed).
+  Putting it another way the unlock operation should not access
+  rwlock data after changing its state to unlocked.
+
+  TODO/FIXME: We should consider alleviating this requirement as
+  it blocks us from doing certain performance optimizations.
 */
 
-#if defined(HAVE_PTHREAD_RWLOCK_RDLOCK) && defined(HAVE_PTHREAD_RWLOCKATTR_SETKIND_NP)
-/*
-  On systems which have a way to specify that readers should
-  be preferred through attribute mechanism (e.g. Linux) we use
-  system implementation of read/write locks.
-*/
-#define rw_pr_lock_t pthread_rwlock_t
+typedef struct st_rw_pr_lock_t {
+  /**
+    Lock which protects the structure.
+    Also held for the duration of wr-lock.
+  */
+  pthread_mutex_t lock;
+  /**
+    Condition variable which is used to wake-up
+    writers waiting for readers to go away.
+  */
+  pthread_cond_t no_active_readers;
+  /** Number of active readers. */
+  uint active_readers;
+  /** Number of writers waiting for readers to go away. */
+  uint writers_waiting_readers;
+  /** Indicates whether there is an active writer. */
+  my_bool active_writer;
+#ifdef SAFE_MUTEX
+  /** Thread holding wr-lock (for debug purposes only). */
+  pthread_t writer_thread;
+#endif
+} rw_pr_lock_t;
+
 extern int rw_pr_init(rw_pr_lock_t *);
-#define rw_pr_rdlock(A) pthread_rwlock_rdlock(A)
-#define rw_pr_wrlock(A) pthread_rwlock_wrlock(A)
-#define rw_pr_tryrdlock(A) pthread_rwlock_tryrdlock(A)
-#define rw_pr_trywrlock(A) pthread_rwlock_trywrlock(A)
-#define rw_pr_unlock(A) pthread_rwlock_unlock(A)
-#define rw_pr_destroy(A) pthread_rwlock_destroy(A)
+extern int rw_pr_rdlock(rw_pr_lock_t *);
+extern int rw_pr_wrlock(rw_pr_lock_t *);
+extern int rw_pr_unlock(rw_pr_lock_t *);
+extern int rw_pr_destroy(rw_pr_lock_t *);
+#ifdef SAFE_MUTEX
+#define rw_pr_lock_assert_write_owner(A) \
+  DBUG_ASSERT((A)->active_writer && pthread_equal(pthread_self(), \
+                                                  (A)->writer_thread))
+#define rw_pr_lock_assert_not_write_owner(A) \
+  DBUG_ASSERT(! (A)->active_writer || ! pthread_equal(pthread_self(), \
+                                                      (A)->writer_thread))
+#else
 #define rw_pr_lock_assert_write_owner(A)
 #define rw_pr_lock_assert_not_write_owner(A)
-#else
-/* Otherwise we have to use our own implementation of read/write locks. */
-#define NEED_MY_RW_LOCK 1
-struct st_my_rw_lock_t;
-#define rw_pr_lock_t my_rw_lock_t
-extern int rw_pr_init(struct st_my_rw_lock_t *);
-#define rw_pr_rdlock(A) my_rw_rdlock((A))
-#define rw_pr_wrlock(A) my_rw_wrlock((A))
-#define rw_pr_tryrdlock(A) my_rw_tryrdlock((A))
-#define rw_pr_trywrlock(A) my_rw_trywrlock((A))
-#define rw_pr_unlock(A) my_rw_unlock((A))
-#define rw_pr_destroy(A) my_rw_destroy((A))
-#define rw_pr_lock_assert_write_owner(A) my_rw_lock_assert_write_owner((A))
-#define rw_pr_lock_assert_not_write_owner(A) my_rw_lock_assert_not_write_owner((A))
-#endif /* defined(HAVE_PTHREAD_RWLOCK_RDLOCK) && defined(HAVE_PTHREAD_RWLOCKATTR_SETKIND_NP) */
+#endif /* SAFE_MUTEX */
 
 
 #ifdef NEED_MY_RW_LOCK
 /*
-  On systems which don't support native read/write locks, or don't support
-  read/write locks which prefer readers we have to use own implementation.
+  On systems which don't support native read/write locks we have
+  to use own implementation.
 */
 typedef struct st_my_rw_lock_t {
 	pthread_mutex_t lock;		/* lock for structure		*/
@@ -656,13 +689,12 @@ typedef struct st_my_rw_lock_t {
 	pthread_cond_t	writers;	/* waiting writers		*/
 	int		state;		/* -1:writer,0:free,>0:readers	*/
 	int             waiters;        /* number of waiting writers	*/
-	my_bool         prefer_readers;
 #ifdef SAFE_MUTEX
         pthread_t       write_thread;
 #endif
 } my_rw_lock_t;
 
-extern int my_rw_init(my_rw_lock_t *, my_bool *);
+extern int my_rw_init(my_rw_lock_t *);
 extern int my_rw_destroy(my_rw_lock_t *);
 extern int my_rw_rdlock(my_rw_lock_t *);
 extern int my_rw_wrlock(my_rw_lock_t *);
diff --git a/include/mysql/psi/mysql_thread.h b/include/mysql/psi/mysql_thread.h
index 60b4f5d6ef4..5b8ea3dc5dc 100644
--- a/include/mysql/psi/mysql_thread.h
+++ b/include/mysql/psi/mysql_thread.h
@@ -141,9 +141,7 @@ typedef struct st_mysql_rwlock mysql_rwlock_t;
   @c mysql_prlock_t is a drop-in replacement for @c rw_pr_lock_t.
   @sa mysql_prlock_init
   @sa mysql_prlock_rdlock
-  @sa mysql_prlock_tryrdlock
   @sa mysql_prlock_wrlock
-  @sa mysql_prlock_trywrlock
   @sa mysql_prlock_unlock
   @sa mysql_prlock_destroy
 */
@@ -421,20 +419,6 @@ typedef struct st_mysql_cond mysql_cond_t;
 #endif
 
 /**
-  @def mysql_prlock_tryrdlock(RW)
-  Instrumented rw_pr_tryrdlock.
-  @c mysql_prlock_tryrdlock is a drop-in replacement
-  for @c rw_pr_tryrdlock.
-*/
-#ifdef HAVE_PSI_INTERFACE
-  #define mysql_prlock_tryrdlock(RW) \
-    inline_mysql_prlock_tryrdlock(RW, __FILE__, __LINE__)
-#else
-  #define mysql_prlock_tryrdlock(RW) \
-    inline_mysql_prlock_tryrdlock(RW)
-#endif
-
-/**
   @def mysql_rwlock_trywrlock(RW)
   Instrumented rwlock_trywrlock.
   @c mysql_rwlock_trywrlock is a drop-in replacement
@@ -449,20 +433,6 @@ typedef struct st_mysql_cond mysql_cond_t;
 #endif
 
 /**
-  @def mysql_prlock_trywrlock(RW)
-  Instrumented rw_pr_trywrlock.
-  @c mysql_prlock_trywrlock is a drop-in replacement
-  for @c rw_pr_trywrlock.
-*/
-#ifdef HAVE_PSI_INTERFACE
-  #define mysql_prlock_trywrlock(RW) \
-    inline_mysql_prlock_trywrlock(RW, __FILE__, __LINE__)
-#else
-  #define mysql_prlock_trywrlock(RW) \
-    inline_mysql_prlock_trywrlock(RW)
-#endif
-
-/**
   @def mysql_rwlock_unlock(RW)
   Instrumented rwlock_unlock.
   @c mysql_rwlock_unlock is a drop-in replacement
@@ -905,35 +875,6 @@ static inline int inline_mysql_rwlock_tryrdlock(
   return result;
 }
 
-#ifndef DISABLE_MYSQL_PRLOCK_H
-static inline int inline_mysql_prlock_tryrdlock(
-  mysql_prlock_t *that
-#ifdef HAVE_PSI_INTERFACE
-  , const char *src_file, uint src_line
-#endif
-  )
-{
-  int result;
-#ifdef HAVE_PSI_INTERFACE
-  struct PSI_rwlock_locker *locker= NULL;
-  PSI_rwlock_locker_state state;
-  if (likely(PSI_server && that->m_psi))
-  {
-    locker= PSI_server->get_thread_rwlock_locker(&state, that->m_psi,
-                                                 PSI_RWLOCK_TRYREADLOCK);
-    if (likely(locker != NULL))
-      PSI_server->start_rwlock_rdwait(locker, src_file, src_line);
-  }
-#endif
-  result= rw_pr_tryrdlock(&that->m_prlock);
-#ifdef HAVE_PSI_INTERFACE
-  if (likely(locker != NULL))
-    PSI_server->end_rwlock_rdwait(locker, result);
-#endif
-  return result;
-}
-#endif
-
 static inline int inline_mysql_rwlock_trywrlock(
   mysql_rwlock_t *that
 #ifdef HAVE_PSI_INTERFACE
@@ -961,35 +902,6 @@ static inline int inline_mysql_rwlock_trywrlock(
   return result;
 }
 
-#ifndef DISABLE_MYSQL_PRLOCK_H
-static inline int inline_mysql_prlock_trywrlock(
-  mysql_prlock_t *that
-#ifdef HAVE_PSI_INTERFACE
-  , const char *src_file, uint src_line
-#endif
-  )
-{
-  int result;
-#ifdef HAVE_PSI_INTERFACE
-  struct PSI_rwlock_locker *locker= NULL;
-  PSI_rwlock_locker_state state;
-  if (likely(PSI_server && that->m_psi))
-  {
-    locker= PSI_server->get_thread_rwlock_locker(&state, that->m_psi,
-                                                 PSI_RWLOCK_TRYWRITELOCK);
-    if (likely(locker != NULL))
-      PSI_server->start_rwlock_wrwait(locker, src_file, src_line);
-  }
-#endif
-  result= rw_pr_trywrlock(&that->m_prlock);
-#ifdef HAVE_PSI_INTERFACE
-  if (likely(locker != NULL))
-    PSI_server->end_rwlock_wrwait(locker, result);
-#endif
-  return result;
-}
-#endif
-
 static inline int inline_mysql_rwlock_unlock(
   mysql_rwlock_t *that)
 {
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) */