Avoid taking LOCK_thread_count for thread_count protection

Replaced wait on COND_thread_count with busy waiting with 1 millisecond
sleep.

Aim is to reduce usage of LOCK_thread_count and COND_thread_count.

sql/mysqld.cc

@@ -689,10 +689,7 @@ static std::atomic<char*> shutdown_user;
 
 pthread_key(THD*, THR_THD);
 
-/*
-  LOCK_thread_count protects the following variables:
-  thread_count		Number of threads with THD that servers queries.
-*/
+/** To be removed */
 mysql_mutex_t LOCK_thread_count;
 
 /*
@@ -1794,13 +1791,9 @@ static void close_connections(void)
   /* All threads has now been aborted */
   DBUG_PRINT("quit", ("Waiting for threads to die (count=%u)",
                       uint32_t(thread_count)));
-  mysql_mutex_lock(&LOCK_thread_count);
+
   while (thread_count)
-  {
-    mysql_cond_wait(&COND_thread_count, &LOCK_thread_count);
-    DBUG_PRINT("quit",("One thread died (count=%u)", uint32_t(thread_count)));
-  }
-  mysql_mutex_unlock(&LOCK_thread_count);
+    my_sleep(1000);
 
   DBUG_PRINT("quit",("close_connections thread"));
   DBUG_VOID_RETURN;
@@ -4469,7 +4462,6 @@ static int init_thread_environment()
   mysql_mutex_init(key_LOCK_global_system_variables,
                    &LOCK_global_system_variables, MY_MUTEX_INIT_FAST);
   mysql_mutex_record_order(&LOCK_active_mi, &LOCK_global_system_variables);
-  mysql_mutex_record_order(&LOCK_status, &LOCK_thread_count);
   mysql_prlock_init(key_rwlock_LOCK_system_variables_hash,
                     &LOCK_system_variables_hash);
   mysql_mutex_init(key_LOCK_prepared_stmt_count,

sql/sql_class.cc

@@ -786,11 +786,6 @@ THD::THD(my_thread_id id, bool is_wsrep_applier, bool skip_global_sys_var_lock)
   mysql_mutex_init(key_LOCK_wakeup_ready, &LOCK_wakeup_ready, MY_MUTEX_INIT_FAST);
   mysql_mutex_init(key_LOCK_thd_kill, &LOCK_thd_kill, MY_MUTEX_INIT_FAST);
   mysql_cond_init(key_COND_wakeup_ready, &COND_wakeup_ready, 0);
-  /*
-    LOCK_thread_count goes before LOCK_thd_data - the former is called around
-    'delete thd', the latter - in THD::~THD
-  */
-  mysql_mutex_record_order(&LOCK_thread_count, &LOCK_thd_data);
 
   /* Variables with default values */
   proc_info="login";
@@ -1640,8 +1635,6 @@ THD::~THD()
   DBUG_ENTER("~THD()");
   /* Make sure threads are not available via server_threads.  */
   assert_not_linked();
-  /* This takes a long time so we should not do this under LOCK_thread_count */
-  mysql_mutex_assert_not_owner(&LOCK_thread_count);
 
   /*
     In error cases, thd may not be current thd. We have to fix this so

sql/sql_class.h

@@ -2152,14 +2152,11 @@ struct THD_count
   */
   ~THD_count()
   {
-    uint32_t t= thread_count--;
+#ifndef DBUG_OFF
+    uint32_t t=
+#endif
+    thread_count--;
     DBUG_ASSERT(t > 0);
-    if (t == 1)
-    {
-      mysql_mutex_lock(&LOCK_thread_count);
-      mysql_cond_broadcast(&COND_thread_count);
-      mysql_mutex_unlock(&LOCK_thread_count);
-    }
   }
 };
 

sql/sql_parse.cc

@@ -1077,10 +1077,6 @@ void bootstrap(MYSQL_FILE *file)
     thd->profiling.set_query_source(thd->query(), length);
 #endif
 
-    /*
-      We don't need to obtain LOCK_thread_count here because in bootstrap
-      mode we have only one thread.
-    */
     thd->set_time();
     Parser_state parser_state;
     if (parser_state.init(thd, thd->query(), length))
@@ -8994,9 +8990,6 @@ THD *find_thread_by_id(longlong id, bool query_id)
   @param id                     Thread id or query id
   @param kill_signal            Should it kill the query or the connection
   @param type                   Type of id: thread id or query id
-
-  @note
-    This is written such that we have a short lock on LOCK_thread_count
 */
 
 uint
@@ -9061,8 +9054,6 @@ kill_one_thread(THD *thd, longlong id, killed_state kill_signal, killed_type typ
   @param only_kill_query        Should it kill the query or the connection
 
   @note
-    This is written such that we have a short lock on LOCK_thread_count
-
     If we can't kill all threads because of security issues, no threads
     are killed.
 */