APC: fix LOCK_global_system_variables contention

sql/sys_vars.cc

@@ -5535,18 +5535,20 @@ Sys_slave_net_timeout(
 */
 
 ulonglong Sys_var_multi_source_ulonglong::
-get_master_info_ulonglong_value(THD *thd, ptrdiff_t offset) const
+get_master_info_ulonglong_value(THD *thd, ptrdiff_t offset, bool lock) const
 {
   Master_info *mi;
   ulonglong res= 0;                                  // Default value
-  mysql_mutex_unlock(&LOCK_global_system_variables);
+  if (lock)
+    mysql_mutex_unlock(&LOCK_global_system_variables);
   if ((mi= get_master_info(&thd->variables.default_master_connection,
                            Sql_condition::WARN_LEVEL_WARN)))
   {
     res= *((ulonglong*) (((uchar*) mi) + master_info_offset));
     mi->release();
   }
-  mysql_mutex_lock(&LOCK_global_system_variables);
+  if (lock)
+    mysql_mutex_lock(&LOCK_global_system_variables);
   return res;
 }
   

sql/sys_vars.inl

@@ -2378,6 +2378,14 @@ class Sys_var_multi_source_ulonglong :public Sys_var_ulonglong
 { 
   ptrdiff_t master_info_offset;
   on_multi_source_update_function update_multi_source_variable_func;
+  const uchar *get_value_ptr(THD *thd, bool lock) const
+  {
+    ulonglong *tmp, res;
+    tmp= (ulonglong*) (((uchar*)&(thd->variables)) + offset);
+    res= get_master_info_ulonglong_value(thd, master_info_offset, lock);
+    *tmp= res;
+    return (uchar*) tmp;
+  }
 public:
   Sys_var_multi_source_ulonglong(const char *name_arg,
                              const char *comment, int flag_args,
@@ -2407,17 +2415,13 @@ class Sys_var_multi_source_ulonglong :public Sys_var_ulonglong
   }
   const uchar *session_value_ptr(THD *thd, const LEX_CSTRING *base) const
   {
-    ulonglong *tmp, res;
-    tmp= (ulonglong*) (((uchar*)&(thd->variables)) + offset);
-    res= get_master_info_ulonglong_value(thd, master_info_offset);
-    *tmp= res;
-    return (uchar*) tmp;
+    return get_value_ptr(thd, false);
   }
   const uchar *global_value_ptr(THD *thd, const LEX_CSTRING *base) const
   {
-    return session_value_ptr(thd, base);
+    return get_value_ptr(thd, true);
   }
-  ulonglong get_master_info_ulonglong_value(THD *thd, ptrdiff_t offset) const;
+  ulonglong get_master_info_ulonglong_value(THD *thd, ptrdiff_t offset, bool lock) const;
   bool update_variable(THD *thd, Master_info *mi)
   {
     return update_multi_source_variable_func(this, thd, mi);

storage/perfschema/pfs_variable.cc

@@ -562,13 +562,16 @@ void System_variable::init(THD *target_thd, const SHOW_VAR *show_var,
   /* Get the value of the system variable. */
   String buf(m_value_str, sizeof(m_value_str) - 1, system_charset_info);
 
-  mysql_mutex_lock(&LOCK_global_system_variables);
+  if (query_scope == OPT_GLOBAL || system_var->scope() == sys_var::GLOBAL)
+    mysql_mutex_lock(&LOCK_global_system_variables);
   if (!system_var->val_str_nolock(&buf, target_thd,
                                   system_var->value_ptr(target_thd,
                                                         query_scope,
                                                         &null_clex_str)))
     buf.length(0);
-  mysql_mutex_unlock(&LOCK_global_system_variables);
+
+  if (query_scope == OPT_GLOBAL || system_var->scope() == sys_var::GLOBAL)
+    mysql_mutex_unlock(&LOCK_global_system_variables);
 
   m_value_length= MY_MIN(buf.length(), SHOW_VAR_FUNC_BUFF_SIZE);