MySQL 8.4 Reference Manual Including MySQL NDB Cluster 8.4
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:
Build MySQL from source, configuring it with the
-DWITH_LOCK_ORDER=ON
CMake option so that the build includes
LOCK_ORDER tooling.
With the WITH_LOCK_ORDER
option enabled, MySQL builds require the
flex program.
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.
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.
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 7.8 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 |
| 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.
| 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.
| 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.
| 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.
| 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.
| 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.
| 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.
| 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.
| 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.
| 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.
| 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.