Use rwlock rather than mutex for protecting THD_list

modifications (insert/erase) are protected by write lock
iteration over list is protected by read lock.

This way, threads that iterate over the list (as in SHOW PROCESSLIST,
SHOW GLOBAL STATUS) do not block each other.

sql/mysqld.cc

@@ -890,7 +890,7 @@ PSI_mutex_key key_BINLOG_LOCK_index, key_BINLOG_LOCK_xid_list,
   key_structure_guard_mutex, key_TABLE_SHARE_LOCK_ha_data,
   key_LOCK_error_messages,
   key_LOCK_start_thread,
-  key_LOCK_thread_count, key_Thread_map_mutex, key_LOCK_thread_cache,
+  key_LOCK_thread_count, key_LOCK_thread_cache,
   key_PARTITION_LOCK_auto_inc;
 PSI_mutex_key key_RELAYLOG_LOCK_index;
 PSI_mutex_key key_LOCK_relaylog_end_pos;
@@ -984,7 +984,6 @@ static PSI_mutex_info all_server_mutexes[]=
   { &key_LOCK_commit_ordered, "LOCK_commit_ordered", PSI_FLAG_GLOBAL},
   { &key_LOCK_slave_background, "LOCK_slave_background", PSI_FLAG_GLOBAL},
   { &key_LOCK_thread_count, "LOCK_thread_count", PSI_FLAG_GLOBAL},
-  { &key_Thread_map_mutex, "Thread_map::mutex", PSI_FLAG_GLOBAL },
   { &key_LOCK_thread_cache, "LOCK_thread_cache", PSI_FLAG_GLOBAL},
   { &key_PARTITION_LOCK_auto_inc, "HA_DATA_PARTITION::LOCK_auto_inc", 0},
   { &key_LOCK_slave_state, "LOCK_slave_state", 0},
@@ -1002,7 +1001,8 @@ PSI_rwlock_key key_rwlock_LOCK_grant, key_rwlock_LOCK_logger,
   key_rwlock_LOCK_system_variables_hash, key_rwlock_query_cache_query_lock,
   key_LOCK_SEQUENCE,
   key_rwlock_LOCK_vers_stats, key_rwlock_LOCK_stat_serial,
-  key_rwlock_LOCK_ssl_refresh;
+  key_rwlock_LOCK_ssl_refresh,
+  key_rwlock_THD_list;
 
 static PSI_rwlock_info all_server_rwlocks[]=
 {
@@ -1018,7 +1018,8 @@ static PSI_rwlock_info all_server_rwlocks[]=
   { &key_rwlock_query_cache_query_lock, "Query_cache_query::lock", 0},
   { &key_rwlock_LOCK_vers_stats, "Vers_field_stats::lock", 0},
   { &key_rwlock_LOCK_stat_serial, "TABLE_SHARE::LOCK_stat_serial", 0},
-  { &key_rwlock_LOCK_ssl_refresh, "LOCK_ssl_refresh", PSI_FLAG_GLOBAL }
+  { &key_rwlock_LOCK_ssl_refresh, "LOCK_ssl_refresh", PSI_FLAG_GLOBAL },
+  { &key_rwlock_THD_list, "THD_list::lock", PSI_FLAG_GLOBAL }
 };
 
 #ifdef HAVE_MMAP

sql/mysqld.h

@@ -344,7 +344,7 @@ extern PSI_mutex_key key_BINLOG_LOCK_index, key_BINLOG_LOCK_xid_list,
   key_rpl_group_info_sleep_lock,
   key_structure_guard_mutex, key_TABLE_SHARE_LOCK_ha_data,
   key_LOCK_start_thread,
-  key_LOCK_error_messages, key_LOCK_thread_count, key_Thread_map_mutex,
+  key_LOCK_error_messages,
   key_PARTITION_LOCK_auto_inc;
 extern PSI_mutex_key key_RELAYLOG_LOCK_index;
 extern PSI_mutex_key key_LOCK_relaylog_end_pos;
@@ -361,7 +361,8 @@ extern PSI_rwlock_key key_rwlock_LOCK_grant, key_rwlock_LOCK_logger,
   key_rwlock_LOCK_sys_init_connect, key_rwlock_LOCK_sys_init_slave,
   key_rwlock_LOCK_system_variables_hash, key_rwlock_query_cache_query_lock,
   key_LOCK_SEQUENCE,
-  key_rwlock_LOCK_vers_stats, key_rwlock_LOCK_stat_serial;
+  key_rwlock_LOCK_vers_stats, key_rwlock_LOCK_stat_serial,
+  key_rwlock_THD_list;
 
 #ifdef HAVE_MMAP
 extern PSI_cond_key key_PAGE_cond, key_COND_active, key_COND_pool;

sql/sql_class.h

@@ -6939,7 +6939,7 @@ class ScopedStatementReplication
 class THD_list
 {
   I_List<THD> threads;
-  mutable mysql_mutex_t mutex;
+  mutable mysql_rwlock_t lock;
 
 public:
   /**
@@ -6951,13 +6951,13 @@ class THD_list
   */
   void init()
   {
-    mysql_mutex_init(key_Thread_map_mutex, &mutex, MY_MUTEX_INIT_FAST);
+    mysql_rwlock_init(key_rwlock_THD_list, &lock);
   }
 
   /** Destructor replacement. */
   void destroy()
   {
-    mysql_mutex_destroy(&mutex);
+    mysql_rwlock_destroy(&lock);
   }
 
   /**
@@ -6969,9 +6969,9 @@ class THD_list
   */
   void insert(THD *thd)
   {
-    mysql_mutex_lock(&mutex);
+    mysql_rwlock_wrlock(&lock);
     threads.append(thd);
-    mysql_mutex_unlock(&mutex);
+    mysql_rwlock_unlock(&lock);
   }
 
   /**
@@ -6984,9 +6984,9 @@ class THD_list
   void erase(THD *thd)
   {
     thd->assert_linked();
-    mysql_mutex_lock(&mutex);
+    mysql_rwlock_wrlock(&lock);
     thd->unlink();
-    mysql_mutex_unlock(&mutex);
+    mysql_rwlock_unlock(&lock);
   }
 
   /**
@@ -7002,12 +7002,12 @@ class THD_list
   template <typename T> int iterate(my_bool (*action)(THD *thd, T *arg), T *arg= 0)
   {
     int res= 0;
-    mysql_mutex_lock(&mutex);
+    mysql_rwlock_rdlock(&lock);
     I_List_iterator<THD> it(threads);
     while (auto tmp= it++)
       if ((res= action(tmp, arg)))
         break;
-    mysql_mutex_unlock(&mutex);
+    mysql_rwlock_unlock(&lock);
     return res;
   }
 };