7.9.3 The LOCK_ORDER Tool

The MySQL server is a multithreaded application that uses numerous internal locking and lock-related primitives, such as mutexes, rwlocks (including prlocks and sxlocks), conditions, and files. Within the server, the set of lock-related objects changes with implementation of new features and code refactoring for performance improvements. As with any multithreaded application that uses locking primitives, there is always a risk of encountering a deadlock during execution when multiple locks are held at once. For MySQL, the effect of a deadlock is catastrophic, causing a complete loss of service.

To enable detection of lock-acquisition deadlocks and enforcement that runtime execution is free of them, MySQL supports LOCK_ORDER tooling. This enables a lock-order dependency graph to be defined as part of server design, and server runtime checking to ensure that lock acquisition is acyclic and that execution paths comply with the graph.

This section provides information about using the LOCK_ORDER tool, but only at a basic level. For complete details, see the Lock Order section of the MySQL Server Doxygen documentation, available at https://dev.mysql.com/doc/index-other.html.

The LOCK_ORDER tool is intended for debugging the server, not for production use.

To use the LOCK_ORDER tool, follow this procedure:

1. Build MySQL from source, configuring it with the -DWITH_LOCK_ORDER=ON CMake option so that the build includes LOCK_ORDER tooling.

   Note

   With the WITH_LOCK_ORDER option enabled, MySQL builds require the flex program.

2. To run the server with the LOCK_ORDER tool enabled, enable the lock_order system variable at server startup. Several other system variables for LOCK_ORDER configuration are available as well.

3. For MySQL test suite operation, mysql-test-run.pl has a --lock-order option that controls whether to enable the LOCK_ORDER tool during test case execution.

The system variables described following configure operation of the LOCK_ORDER tool, assuming that MySQL has been built to include LOCK_ORDER tooling. The primary variable is lock_order, which indicates whether to enable the LOCK_ORDER tool at runtime:

• If lock_order is disabled (the default), no other LOCK_ORDER system variables have any effect.

• If lock_order is enabled, the other system variables configure which LOCK_ORDER features to enable.

Note

In general, it is intended that the LOCK_ORDER tool be configured by executing mysql-test-run.pl with the --lock-order option, and for mysql-test-run.pl to set LOCK_ORDER system variables to appropriate values.

All LOCK_ORDER system variables must be set at server startup. At runtime, their values are visible but cannot be changed.

Some system variables exist in pairs, such as lock_order_debug_loop and lock_order_trace_loop. For such pairs, the variables are distinguished as follows when the condition occurs with which they are associated:

• If the _debug_ variable is enabled, a debug assertion is raised.

• If the _trace_ variable is enabled, an error is printed to the logs.

Table 7.12 LOCK_ORDER System Variable Summary

Variable Name	Variable Type	Variable Scope
lock_order	Boolean	Global
lock_order_debug_loop	Boolean	Global
lock_order_debug_missing_arc	Boolean	Global
lock_order_debug_missing_key	Boolean	Global
lock_order_debug_missing_unlock	Boolean	Global
lock_order_dependencies	File name	Global
lock_order_extra_dependencies	File name	Global
lock_order_output_directory	Directory name	Global
lock_order_print_txt	Boolean	Global
lock_order_trace_loop	Boolean	Global
lock_order_trace_missing_arc	Boolean	Global
lock_order_trace_missing_key	Boolean	Global
lock_order_trace_missing_unlock	Boolean	Global

• lock_order

  Command-Line Format	--lock-order[={OFF|ON}]
  System Variable	lock_order
  Scope	Global
  Dynamic	No
  SET_VAR Hint Applies	No
  Type	Boolean
  Default Value	OFF

  Whether to enable the LOCK_ORDER tool at runtime. If lock_order is disabled (the default), no other LOCK_ORDER system variables have any effect. If lock_order is enabled, the other system variables configure which LOCK_ORDER features to enable.

  If lock_order is enabled, an error is raised if the server encounters a lock-acquisition sequence that is not declared in the lock-order graph.

• lock_order_debug_loop

  Command-Line Format	--lock-order-debug-loop[={OFF|ON}]
  System Variable	lock_order_debug_loop
  Scope	Global
  Dynamic	No
  SET_VAR Hint Applies	No
  Type	Boolean
  Default Value	OFF

  Whether the LOCK_ORDER tool causes a debug assertion failure when it encounters a dependency that is flagged as a loop in the lock-order graph.

• lock_order_debug_missing_arc

  Command-Line Format	--lock-order-debug-missing-arc[={OFF|ON}]
  System Variable	lock_order_debug_missing_arc
  Scope	Global
  Dynamic	No
  SET_VAR Hint Applies	No
  Type	Boolean
  Default Value	OFF

  Whether the LOCK_ORDER tool causes a debug assertion failure when it encounters a dependency that is not declared in the lock-order graph.

• lock_order_debug_missing_key

  Command-Line Format	--lock-order-debug-missing-key[={OFF|ON}]
  System Variable	lock_order_debug_missing_key
  Scope	Global
  Dynamic	No
  SET_VAR Hint Applies	No
  Type	Boolean
  Default Value	OFF

  Whether the LOCK_ORDER tool causes a debug assertion failure when it encounters an object that is not properly instrumented with the Performance Schema.

• lock_order_debug_missing_unlock

  Command-Line Format	--lock-order-debug-missing-unlock[={OFF|ON}]
  System Variable	lock_order_debug_missing_unlock
  Scope	Global
  Dynamic	No
  SET_VAR Hint Applies	No
  Type	Boolean
  Default Value	OFF

  Whether the LOCK_ORDER tool causes a debug assertion failure when it encounters a lock that is destroyed while still held.

• lock_order_dependencies

  Command-Line Format	--lock-order-dependencies=file_name
  System Variable	lock_order_dependencies
  Scope	Global
  Dynamic	No
  SET_VAR Hint Applies	No
  Type	File name
  Default Value	empty string

  The path to the lock_order_dependencies.txt file that defines the server lock-order dependency graph.

  It is permitted to specify no dependencies. An empty dependency graph is used in this case.

• lock_order_extra_dependencies

  Command-Line Format	--lock-order-extra-dependencies=file_name
  System Variable	lock_order_extra_dependencies
  Scope	Global
  Dynamic	No
  SET_VAR Hint Applies	No
  Type	File name
  Default Value	empty string

  The path to a file containing additional dependencies for the lock-order dependency graph. This is useful to amend the primary server dependency graph, defined in the lock_order_dependencies.txt file, with additional dependencies describing the behavior of third party code. (The alternative is to modify lock_order_dependencies.txt itself, which is not encouraged.)

  If this variable is not set, no secondary file is used.

• lock_order_output_directory

  Command-Line Format	--lock-order-output-directory=dir_name
  System Variable	lock_order_output_directory
  Scope	Global
  Dynamic	No
  SET_VAR Hint Applies	No
  Type	Directory name
  Default Value	empty string

  The directory where the LOCK_ORDER tool writes its logs. If this variable is not set, the default is the current directory.

• lock_order_print_txt

  Command-Line Format	--lock-order-print-txt[={OFF|ON}]
  System Variable	lock_order_print_txt
  Scope	Global
  Dynamic	No
  SET_VAR Hint Applies	No
  Type	Boolean
  Default Value	OFF

  Whether the LOCK_ORDER tool performs a lock-order graph analysis and prints a textual report. The report includes any lock-acquisition cycles detected.

• lock_order_trace_loop

  Command-Line Format	--lock-order-trace-loop[={OFF|ON}]
  System Variable	lock_order_trace_loop
  Scope	Global
  Dynamic	No
  SET_VAR Hint Applies	No
  Type	Boolean
  Default Value	OFF

  Whether the LOCK_ORDER tool prints a trace in the log file when it encounters a dependency that is flagged as a loop in the lock-order graph.

• lock_order_trace_missing_arc

  Command-Line Format	--lock-order-trace-missing-arc[={OFF|ON}]
  System Variable	lock_order_trace_missing_arc
  Scope	Global
  Dynamic	No
  SET_VAR Hint Applies	No
  Type	Boolean
  Default Value	ON

  Whether the LOCK_ORDER tool prints a trace in the log file when it encounters a dependency that is not declared in the lock-order graph.

• lock_order_trace_missing_key

  Command-Line Format	--lock-order-trace-missing-key[={OFF|ON}]
  System Variable	lock_order_trace_missing_key
  Scope	Global
  Dynamic	No
  SET_VAR Hint Applies	No
  Type	Boolean
  Default Value	OFF

  Whether the LOCK_ORDER tool prints a trace in the log file when it encounters an object that is not properly instrumented with the Performance Schema.

• lock_order_trace_missing_unlock

  Command-Line Format	--lock-order-trace-missing-unlock[={OFF|ON}]
  System Variable	lock_order_trace_missing_unlock
  Scope	Global
  Dynamic	No
  SET_VAR Hint Applies	No
  Type	Boolean
  Default Value	ON

  Whether the LOCK_ORDER tool prints a trace in the log file when it encounters a lock that is destroyed while still held.