| Bug #54673 | It takes too long to get readlock for 'FLUSH TABLES WITH READ LOCK' | ||
|---|---|---|---|
| Submitted: | 21 Jun 2010 16:22 | Modified: | 15 Dec 2010 15:07 |
| Reporter: | Alexey Stroganov | Email Updates: | |
| Status: | Closed | Impact on me: | |
| Category: | MySQL Server: Locking | Severity: | S3 (Non-critical) |
| Version: | 5.1.48, 5.5.5pre | OS: | Any |
| Assigned to: | Dmitry Lenev | CPU Architecture: | Any |
[28 Jun 2010 7:35]
Alexey Stroganov
After code analysis I believe that we have a bug in must_wait condition in lock.cc that doesn't stop new transactions when readlock with FLUSH TABLE WITH READ LOCK was requested.
In case of fast short transactions either BEGIN/COMMIT or LOCK TABLE WRITE/UNLOCK TABLES protect_against_global_read_lock variable is always >0 and execution of FTWRL stucks here:
bool Global_read_lock::lock_global_read_lock(THD *thd)
...
waiting_for_read_lock++;
while (protect_against_global_read_lock && !thd->killed)
{
mysql_cond_wait(&COND_global_read_lock, &LOCK_global_read_lock);
}
...
By design we should stop new transaction when read lock request appears
in bool Global_read_lock::
wait_if_global_read_lock(THD *thd, bool abort_on_refresh,
bool is_not_commit)
...
while (must_wait && ! thd->killed &&
(!abort_on_refresh || !thd->open_tables ||
thd->open_tables->s->version == refresh_version))
{
DBUG_PRINT("signal", ("Waiting for COND_global_read_lock"));
mysql_cond_wait(&COND_global_read_lock, &LOCK_global_read_lock);
DBUG_PRINT("signal", ("Got COND_global_read_lock"));
}
...
but must_wait condition doesn't take into account waiting_for_read_lock counter and such way new transactions don't block and continue execution.
To fix that aspect it should be enough to add waiting_for_read_lock to must_wait condition.
-#define must_wait (global_read_lock && \
+#define must_wait ((global_read_lock || waiting_for_read_lock ) && \
(is_not_commit || \
global_read_lock_blocks_commit))
[20 Oct 2010 20:21]
Bugs System
A patch for this bug has been committed. After review, it may be pushed to the relevant source trees for release in the next version. You can access the patch from: http://lists.mysql.com/commits/121410 3170 Konstantin Osipov 2010-10-21 Bug#54673 It takes too long to get readlock for 'FLUSH TABLES WITH READ LOCK'. Review fixes.
[21 Oct 2010 19:26]
Bugs System
A patch for this bug has been committed. After review, it may be pushed to the relevant source trees for release in the next version. You can access the patch from: http://lists.mysql.com/commits/121564 3173 Konstantin Osipov 2010-10-21 Update comments (Bug#54673, code review)
[21 Oct 2010 19:33]
Bugs System
A patch for this bug has been committed. After review, it may be pushed to the relevant source trees for release in the next version. You can access the patch from: http://lists.mysql.com/commits/121569 3174 Konstantin Osipov 2010-10-21 Bug#54673, code review: fix the coding style in the code that's already changed.
[21 Oct 2010 20:48]
Bugs System
A patch for this bug has been committed. After review, it may be pushed to the relevant source trees for release in the next version. You can access the patch from: http://lists.mysql.com/commits/121579 3175 Konstantin Osipov 2010-10-22 Bug#54673, code review: remove a special case for innobackup.pl, it's unnecessary.
[22 Oct 2010 7:19]
Bugs System
A patch for this bug has been committed. After review, it may be pushed to the relevant source trees for release in the next version. You can access the patch from: http://lists.mysql.com/commits/121627 3175 Konstantin Osipov 2010-10-22 Bug#54673, code review: remove a special case for innobackup.pl, it's unnecessary.
[25 Oct 2010 10:57]
Bugs System
A patch for this bug has been committed. After review, it may be pushed to the relevant source trees for release in the next version. You can access the patch from: http://lists.mysql.com/commits/121792 3178 Konstantin Osipov 2010-10-25 Bug#54673, review comments.
[25 Oct 2010 15:18]
Bugs System
A patch for this bug has been committed. After review, it may be pushed to the relevant source trees for release in the next version. You can access the patch from: http://lists.mysql.com/commits/121837 3178 Konstantin Osipov 2010-10-25 Bug#54673, review comments.
[11 Nov 2010 17:10]
Bugs System
A patch for this bug has been committed. After review, it may be pushed to the relevant source trees for release in the next version. You can access the patch from: http://lists.mysql.com/commits/123652 3179 Dmitry Lenev 2010-11-11 Patch that refactors global read lock implementation and fixes bug #57006 "Deadlock between HANDLER and FLUSH TABLES WITH READ LOCK" and bug #54673 "It takes too long to get readlock for 'FLUSH TABLES WITH READ LOCK'". The first bug manifested itself as a deadlock which occurred when a connection, which had some table open through HANDLER statement, tried to update some data through DML statement while another connection tried to execute FLUSH TABLES WITH READ LOCK concurrently. What happened was that FTWRL in the second connection managed to perform first step of GRL acquisition and thus blocked all upcoming DML. After that it started to wait for table open through HANDLER statement to be flushed. When the first connection tried to execute DML it has started to wait for GRL/the second connection creating deadlock. The second bug manifested itself as starvation of FLUSH TABLES WITH READ LOCK statements in cases when there was a constant stream of concurrent DML statements (in two or more connections). This has happened because requests for protection against GRL which were acquired by DML statements were ignoring presence of pending GRL and thus the latter was starved. This patch solves both these problems by re-implementing GRL using metadata locks. Similar to the old implementation acquisition of GRL in new implementation is two-step. During the first step we block all concurrent DML and DDL statements by acquiring global S metadata lock (each DML and DDL statement acquires global IX lock for its duration). During the second step we block commits by acquiring global S lock in COMMIT namespace (commit code acquires global IX lock in this namespace). Note that unlike in old implementation acquisition of protection against GRL in DML and DDL is semi-automatic. We assume that any statement which should be blocked by GRL will either open and acquires write-lock on tables or acquires metadata locks on objects it is going to modify. For any such statement global IX metadata lock is automatically acquired for its duration. The first problem is solved because waits for GRL become visible to deadlock detector in metadata locking subsystem and thus deadlocks like one in the first bug become impossible. The second problem is solved because global S locks which are used for GRL implementation are given preference over IX locks which are acquired by concurrent DML (and we can switch to fair scheduling in future if needed). Important change: FTWRL/GRL no longer blocks DML and DDL on temporary tables. Before this patch behavior was not consistent in this respect: in some cases DML/DDL statements on temporary tables were blocked while in others they were not. Since the main use cases for FTWRL are various forms of backups and temporary tables are not preserved during backups we have opted for consistently allowing DML/DDL on temporary tables during FTWRL/GRL. Important change: This patch changes thread state names which are used when DML/DDL of FTWRL is waiting for global read lock. It is now either "Waiting for global read lock" or "Waiting for commit lock" depending on the stage on which FTWRL is. Incompatible change: To solve deadlock in events code which was exposed by this patch we have to replace LOCK_event_metadata mutex with metadata locks on events. As result we have to prohibit DDL on events under LOCK TABLES. This patch also adds extensive test coverage for interaction of DML/DDL and FTWRL. Performance of new and old global read lock implementations in sysbench tests were compared. There were no significant difference between new and old implementations. @ mysql-test/include/check_ftwrl_compatible.inc Added helper script which allows to check that a statement is compatible with FLUSH TABLES WITH READ LOCK. @ mysql-test/include/check_ftwrl_incompatible.inc Added helper script which allows to check that a statement is incompatible with FLUSH TABLES WITH READ LOCK. @ mysql-test/include/handler.inc Adjusted test case to the fact that now DROP TABLE closes open HANDLERs for the table to be dropped before checking if there active FTWRL in this connection. @ mysql-test/include/wait_show_condition.inc Fixed small error in the timeout message. The correct name of variable used as parameter for this script is "$condition" and not "$wait_condition". @ mysql-test/r/delayed.result Added test coverage for scenario which triggered assert in metadata locking subsystem. @ mysql-test/r/events_2.result Updated test results after prohibiting event DDL operations under LOCK TABLES. @ mysql-test/r/flush.result Added test coverage for bug #57006 "Deadlock between HANDLER and FLUSH TABLES WITH READ LOCK". @ mysql-test/r/flush_read_lock.result Added test coverage for various aspects of FLUSH TABLES WITH READ LOCK functionality. @ mysql-test/r/flush_read_lock_kill.result Adjusted test case after replacing custom global read lock implementation with one based on metadata locks. Use new debug_sync point. Do not disable concurrent inserts as now InnoDB we always use InnoDB table. @ mysql-test/r/handler_innodb.result Adjusted test case to the fact that now DROP TABLE closes open HANDLERs for the table to be dropped before checking if there active FTWRL in this connection. @ mysql-test/r/handler_myisam.result Adjusted test case to the fact that now DROP TABLE closes open HANDLERs for the table to be dropped before checking if there active FTWRL in this connection. @ mysql-test/r/mdl_sync.result Adjusted test case after replacing custom global read lock implementation with one based on metadata locks. Replaced usage of GRL-specific debug_sync's with appropriate sync points in MDL subsystem. @ mysql-test/suite/perfschema/r/dml_setup_instruments.result Updated test results after removing global COND_global_read_lock condition variable. @ mysql-test/suite/perfschema/r/func_file_io.result Ensure that this test doesn't affect subsequent tests. At the end of its execution enable back P_S instrumentation which this test disables at some point. @ mysql-test/suite/perfschema/r/func_mutex.result Ensure that this test doesn't affect subsequent tests. At the end of its execution enable back P_S instrumentation which this test disables at some point. @ mysql-test/suite/perfschema/r/global_read_lock.result Adjusted test case to take into account that new GRL implementation is based on MDL. @ mysql-test/suite/perfschema/r/server_init.result Adjusted test case after replacing custom global read lock implementation with one based on MDL and replacing LOCK_event_metadata mutex with metadata lock. @ mysql-test/suite/perfschema/t/func_file_io.test Ensure that this test doesn't affect subsequent tests. At the end of its execution enable back P_S instrumentation which this test disables at some point. @ mysql-test/suite/perfschema/t/func_mutex.test Ensure that this test doesn't affect subsequent tests. At the end of its execution enable back P_S instrumentation which this test disables at some point. @ mysql-test/suite/perfschema/t/global_read_lock.test Adjusted test case to take into account that new GRL implementation is based on MDL. @ mysql-test/suite/perfschema/t/server_init.test Adjusted test case after replacing custom global read lock implementation with one based on MDL and replacing LOCK_event_metadata mutex with metadata lock. @ mysql-test/suite/rpl/r/rpl_tmp_table_and_DDL.result Updated test results after prohibiting event DDL under LOCK TABLES. @ mysql-test/t/delayed.test Added test coverage for scenario which triggered assert in metadata locking subsystem. @ mysql-test/t/events_2.test Updated test case after prohibiting event DDL operations under LOCK TABLES. @ mysql-test/t/flush.test Added test coverage for bug #57006 "Deadlock between HANDLER and FLUSH TABLES WITH READ LOCK". @ mysql-test/t/flush_block_commit.test Adjusted test case after changing thread state name which is used when COMMIT waits for FLUSH TABLES WITH READ LOCK from "Waiting for release of readlock" to "Waiting for commit lock". @ mysql-test/t/flush_block_commit_notembedded.test Adjusted test case after changing thread state name which is used when DML waits for FLUSH TABLES WITH READ LOCK. Now we use "Waiting for global read lock" in this case. @ mysql-test/t/flush_read_lock.test Added test coverage for various aspects of FLUSH TABLES WITH READ LOCK functionality. @ mysql-test/t/flush_read_lock_kill-master.opt We no longer need to use make_global_read_lock_block_commit_loop debug tag in this test. Instead we rely on an appropriate debug_sync point in MDL code. @ mysql-test/t/flush_read_lock_kill.test Adjusted test case after replacing custom global read lock implementation with one based on metadata locks. Use new debug_sync point. Do not disable concurrent inserts as now InnoDB we always use InnoDB table. @ mysql-test/t/lock_multi.test Adjusted test case after changing thread state names which are used when DML or DDL waits for FLUSH TABLES WITH READ LOCK to "Waiting for global read lock". @ mysql-test/t/mdl_sync.test Adjusted test case after replacing custom global read lock implementation with one based on metadata locks. Replaced usage of GRL-specific debug_sync's with appropriate sync points in MDL subsystem. Updated thread state names which are used when DDL waits for FTWRL. @ mysql-test/t/trigger_notembedded.test Adjusted test case after changing thread state names which are used when DML or DDL waits for FLUSH TABLES WITH READ LOCK to "Waiting for global read lock". @ sql/event_data_objects.cc Removed Event_queue_element::status/last_executed_changed members and Event_queue_element::update_timing_fields() method. We no longer use this class for updating mysql.events once event is chosen for execution. Accesses to instances of this class in scheduler thread require protection by Event_queue::LOCK_event_queue mutex and we try to avoid updating table while holding this lock. @ sql/event_data_objects.h Removed Event_queue_element::status/last_executed_changed members and Event_queue_element::update_timing_fields() method. We no longer use this class for updating mysql.events once event is chosen for execution. Accesses to instances of this class in scheduler thread require protection by Event_queue::LOCK_event_queue mutex and we try to avoid updating table while holding this lock. @ sql/event_db_repository.cc - Changed Event_db_repository methods to not release all metadata locks once they are done updating mysql.events table. This allows to keep metadata lock protecting against GRL and lock protecting particular event around until corresponding DDL statement is written to the binary log. - Removed logic for conditional update of "status" and "last_executed" fields from update_timing_fields_for_event() method. In the only case when this method is called now "last_executed" is always modified and tracking change of "status" is too much hassle. @ sql/event_db_repository.h Removed logic for conditional update of "status" and "last_executed" fields from Event_db_repository:: update_timing_fields_for_event() method. In the only case when this method is called now "last_executed" is always modified and tracking change of "status" field is too much hassle. @ sql/event_queue.cc Changed event scheduler code not to update mysql.events table while holding Event_queue::LOCK_event_queue mutex. Doing so led to a deadlock with a new GRL implementation. This deadlock didn't occur with old implementation due to fact that code acquiring protection against GRL ignored pending GRL requests (which lead to GRL starvation). One of goals of new implementation is to disallow GRL starvation and so we have to solve problem with this deadlock in a different way. @ sql/events.cc Changed methods of Events class to acquire protection against GRL while perfoming DDL statement and keep it until statement is written to the binary log. Unfortunately this step together with new GRL implementation exposed deadlock involving Events::LOCK_event_metadata and GRL. To solve it Events::LOCK_event_metadata mutex was replaced with a metadata lock on event. As a side-effect events DDL has to be prohibited under LOCK TABLES even in cases when mysql.events table was explicitly locked for write. @ sql/events.h Replaced Events::LOCK_event_metadata mutex with a metadata lock on event. @ sql/ha_ndbcluster.cc Updated code after replacing custom global read lock implementation with one based on MDL. Since MDL subsystem should now be able to detect deadlocks involving metadata locks and GRL there is no need for special handling of active GRL. @ sql/handler.cc Replaced custom implementation of global read lock with one based on metadata locks. Consequently when doing commit instead of calling method of Global_read_lock class to acquire protection against GRL we simply acquire IX in COMMIT namespace. @ sql/lock.cc Replaced custom implementation of global read lock with one based on metadata locks. This step allows to expose wait for GRL to deadlock detector of MDL subsystem and thus succesfully resolve deadlocks similar to one behind bug #57006 "Deadlock between HANDLER and FLUSH TABLES WITH READ LOCK". It also solves problem with GRL starvation described in bug #54673 "It takes too long to get readlock for 'FLUSH TABLES WITH READ LOCK'" since metadata locks used by GRL give preference to FTWRL statement instead of DML statements (if needed in future this can be changed to fair scheduling). Similar to old implementation of acquisition of GRL is two-step. During the first step we block all concurrent DML and DDL statements by acquiring global S metadata lock (each DML and DDL statement acquires global IX lock for its duration). During the second step we block commits by acquiring global S lock in COMMIT namespace (commit code acquires global IX lock in this namespace). Note that unlike in old implementation acquisition of protection against GRL in DML and DDL is semi-automatic. We assume that any statement which should be blocked by GRL will either open and acquires write-lock on tables or acquires metadata locks on objects it is going to modify. For any such statement global IX metadata lock is automatically acquired for its duration. To support this change: - Global_read_lock::lock/unlock_global_read_lock and make_global_read_lock_block_commit methods were changed accordingly. - Global_read_lock::wait_if_global_read_lock() and start_waiting_global_read_lock() methods were dropped. It is now responsibility of code acquiring metadata locks opening tables to acquire protection against GRL by explicitly taking global IX lock with statement duration. - Global variables, mutex and condition variable used by old implementation was removed. - lock_routine_name() was changed to use statement duration for its global IX lock. It was also renamed to lock_object_name() as it now also used to take metadata locks on events. - Global_read_lock::set_explicit_lock_duration() was added which allows not to release locks used for GRL when leaving prelocked mode. @ sql/lock.h - Renamed lock_routine_name() to lock_object_name() and changed its signature to allow its usage for events. - Removed broadcast_refresh() function. It is no longer needed with new GRL implementation. @ sql/log_event.cc Release metadata locks with statement duration at the end of processing legacy event for LOAD DATA. This ensures that replication thread processing such event properly releases its protection against global read lock. @ sql/mdl.cc Changed MDL subsystem to support new MDL-based implementation of global read lock. Added COMMIT and EVENTS namespaces for metadata locks. Changed thread state name for GLOBAL namespace to "Waiting for global read lock". Optimized MDL_map::find_or_insert() method to avoid taking m_mutex mutex when looking up MDL_lock objects for GLOBAL or COMMIT namespaces. We keep pre-created MDL_lock objects for these namespaces around and simply return pointers to these global objects when needed. Changed MDL_lock/MDL_scoped_lock to properly handle notification of insert delayed handler threads when FTWRL takes global S lock. Introduced concept of lock duration. In addition to locks with transaction duration which work in the way which is similar to how locks worked before (i.e. they are released at the end of transaction), locks with statement and explicit duration were introduced. Locks with statement duration are automatically released at the end of statement. Locks with explicit duration require explicit release and obsolete concept of transactional sentinel. * Changed MDL_request and MDL_ticket classes to support notion of duration. * Changed MDL_context to keep locks with different duration in different lists. Changed code handling ticket list to take this into account. * Changed methods responsible for releasing locks to take into account duration of tickets. Particularly public MDL_context::release_lock() method now only can release tickets with explicit duration (there is still internal method which allows to specify duration). To release locks with statement or transaction duration one have to use release_statement/transactional_locks() methods. * Concept of savepoint for MDL subsystem now has to take into account locks with statement duration. Consequently MDL_savepoint class was introduced and methods working with savepoints were updated accordingly. * Added methods which allow to set duration for one or all locks in the context. @ sql/mdl.h Changed MDL subsystem to support new MDL-based implementation of global read lock. Added COMMIT and EVENTS namespaces for metadata locks. Introduced concept of lock duration. In addition to locks with transaction duration which work in the way which is similar to how locks worked before (i.e. they are released at the end of transaction), locks with statement and explicit duration were introduced. Locks with statement duration are automatically released at the end of statement. Locks with explicit duration require explicit release and obsolete concept of transactional sentinel. * Changed MDL_request and MDL_ticket classes to support notion of duration. * Changed MDL_context to keep locks with different duration in different lists. Changed code handling ticket list to take this into account. * Changed methods responsible for releasing locks to take into account duration of tickets. Particularly public MDL_context::release_lock() method now only can release tickets with explicit duration (there is still internal method which allows to specify duration). To release locks with statement or transaction duration one have to use release_statement/transactional_locks() methods. * Concept of savepoint for MDL subsystem now has to take into account locks with statement duration. Consequently MDL_savepoint class was introduced and methods working with savepoints were updated accordingly. * Added methods which allow to set duration for one or all locks in the context. @ sql/mysqld.cc Removed global mutex and condition variables which were used by old implementation of GRL. Also we no longer need to initialize Events::LOCK_event_metadata mutex as it was replaced with metadata locks on events. @ sql/mysqld.h Removed global variable, mutex and condition variables which were used by old implementation of GRL. @ sql/rpl_rli.cc When slave thread closes tables which were open for handling of RBR events ensure that it releases global IX lock which was acquired as protection against GRL. @ sql/sp.cc Adjusted code to the new signature of lock_object/routine_name(), to the fact that one now needs specify duration of lock when initializing MDL_request and to the fact that savepoints for MDL subsystem are now represented by MDL_savepoint class. @ sql/sp_head.cc Ensure that statements in stored procedures release statement metadata locks and thus release their protectiong against GRL in proper moment in time. Adjusted code to the fact that one now needs specify duration of lock when initializing MDL_request. @ sql/sql_admin.cc Adjusted code to the fact that one now needs specify duration of lock when initializing MDL_request. @ sql/sql_base.cc - Implemented support for new approach to acquiring protection against global read lock. We no longer acquire such protection explicitly on the basis of statement flags. Instead we always rely on code which is responsible for acquiring metadata locks on object to be changed acquiring this protection. This is achieved by acquiring global IX metadata lock with statement duration. Code doing this also responsible for checking that current connection has no active GRL by calling an Global_read_lock::can_acquire_protection() method. Changed code in open_table() and lock_table_names() accordingly. Note that as result of this change DDL and DML on temporary tables is always compatible with GRL (before it was incompatible in some cases and compatible in other cases). - To speed-up code acquiring protection against GRL introduced m_has_protection_against_grl member in Open_table_context class. It indicates that protection was already acquired sometime during open_tables() execution and new attempts can be skipped. - Thanks to new GRL implementation calls to broadcast_refresh() became unnecessary and were removed. - Adjusted code to the fact that one now needs specify duration of lock when initializing MDL_request and to the fact that savepoints for MDL subsystem are now represented by MDL_savepoint class. @ sql/sql_base.h Adjusted code to the fact that savepoints for MDL subsystem are now represented by MDL_savepoint class. Also introduced Open_table_context::m_has_protection_against_grl member which allows to avoid acquiring protection against GRL while opening tables if such protection was already acquired. @ sql/sql_class.cc Changed THD::leave_locked_tables_mode() after transactional sentinel for metadata locks was obsoleted by introduction of locks with explicit duration. @ sql/sql_class.h - Adjusted code to the fact that savepoints for MDL subsystem are now represented by MDL_savepoint class. - Changed Global_read_lock class according to changes in global read lock implementation: * wait_if_global_read_lock and start_waiting_global_read_lock are now gone. Instead code needing protection against GRL has to acquire global IX metadata lock with statement duration itself. To help it new can_acquire_protection() was introduced. Also as result of the above change m_protection_count member is gone too. * Added m_mdl_blocks_commits_lock member to store metadata lock blocking commits. * Adjusted code to the fact that concept of transactional sentinel was obsoleted by concept of lock duration. - Removed CF_PROTECT_AGAINST_GRL flag as it is no longer necessary. New GRL implementation acquires protection against global read lock automagically when statement acquires metadata locks on tables or other objects it is going to change. @ sql/sql_db.cc Adjusted code to the fact that one now needs specify duration of lock when initializing MDL_request. @ sql/sql_handler.cc Removed call to broadcast_refresh() function. It is no longer needed with new GRL implementation. Adjusted code after introducing duration concept for metadata locks. Particularly to the fact transactional sentinel was replaced with explicit duration. @ sql/sql_handler.h Renamed mysql_ha_move_tickets_after_trans_sentinel() to mysql_ha_set_explicit_lock_duration() after transactional sentinel was obsoleted by locks with explicit duration. @ sql/sql_insert.cc Adjusted code handling delaying inserts after switching to new GRL implementation. Now connection thread initiating delayed insert has to acquire global IX lock in addition to metadata lock on table being inserted into. This IX lock protects against GRL and similarly to SW lock on table being inserted into has to be passed to handler thread in order to avoid deadlocks. @ sql/sql_lex.cc LEX::protect_against_global_read_lock member is no longer necessary since protection against GRL is automatically taken by code acquiring metadata locks/opening tables. @ sql/sql_lex.h LEX::protect_against_global_read_lock member is no longer necessary since protection against GRL is automatically taken by code acquiring metadata locks/opening tables. @ sql/sql_parse.cc - Implemented support for new approach to acquiring protection against global read lock. We no longer acquire such protection explicitly on the basis of statement flags. Instead we always rely on code which is responsible for acquiring metadata locks on object to be changed acquiring this protection. This is achieved by acquiring global IX metadata lock with statement duration. This lock is automatically released at the end of statement execution. - Changed implementation of CREATE/DROP PROCEDURE/FUNCTION not to release metadata locks and thus protection against of GRL in the middle of statement execution. - Adjusted code to the fact that one now needs specify duration of lock when initializing MDL_request and to the fact that savepoints for MDL subsystem are now represented by MDL_savepoint class. @ sql/sql_prepare.cc Adjusted code to the to the fact that savepoints for MDL subsystem are now represented by MDL_savepoint class. @ sql/sql_rename.cc With new GRL implementation there is no need to explicitly acquire protection against GRL before renaming tables. This happens automatically in code which acquires metadata locks on tables being renamed. @ sql/sql_show.cc Adjusted code to the fact that one now needs specify duration of lock when initializing MDL_request and to the fact that savepoints for MDL subsystem are now represented by MDL_savepoint class. @ sql/sql_table.cc - With new GRL implementation there is no need to explicitly acquire protection against GRL before dropping tables. This happens automatically in code which acquires metadata locks on tables being dropped. - Changed mysql_alter_table() not to release lock on new table name explicitly and to rely on automatic release of locks at the end of statement instead. This was necessary since now MDL_context::release_lock() is supported only for locks for explicit duration. @ sql/sql_trigger.cc With new GRL implementation there is no need to explicitly acquire protection against GRL before changing table triggers. This happens automatically in code which acquires metadata locks on tables which triggers are to be changed. @ sql/sql_update.cc Fix bug exposed by GRL testing. During prepare phase acquire only S metadata locks instead of SW locks to keep prepare of multi-UPDATE compatible with concurrent LOCK TABLES WRITE and global read lock. @ sql/sql_view.cc With new GRL implementation there is no need to explicitly acquire protection against GRL before creating view. This happens automatically in code which acquires metadata lock on view to be created. @ sql/sql_yacc.yy LEX::protect_against_global_read_lock member is no longer necessary since protection against GRL is automatically taken by code acquiring metadata locks/opening tables. @ sql/table.cc Adjusted code to the fact that one now needs specify duration of lock when initializing MDL_request. @ sql/table.h Adjusted code to the fact that one now needs specify duration of lock when initializing MDL_request. @ sql/transaction.cc Replaced custom implementation of global read lock with one based on metadata locks. Consequently when doing commit instead of calling method of Global_read_lock class to acquire protection against GRL we simply acquire IX in COMMIT namespace. Also adjusted code to the fact that MDL savepoint is now represented by MDL_savepoint class.
[11 Nov 2010 17:23]
Bugs System
A patch for this bug has been committed. After review, it may be pushed to the relevant source trees for release in the next version. You can access the patch from: http://lists.mysql.com/commits/123654 3179 Dmitry Lenev 2010-11-11 Patch that refactors global read lock implementation and fixes bug #57006 "Deadlock between HANDLER and FLUSH TABLES WITH READ LOCK" and bug #54673 "It takes too long to get readlock for 'FLUSH TABLES WITH READ LOCK'". The first bug manifested itself as a deadlock which occurred when a connection, which had some table open through HANDLER statement, tried to update some data through DML statement while another connection tried to execute FLUSH TABLES WITH READ LOCK concurrently. What happened was that FTWRL in the second connection managed to perform first step of GRL acquisition and thus blocked all upcoming DML. After that it started to wait for table open through HANDLER statement to be flushed. When the first connection tried to execute DML it has started to wait for GRL/the second connection creating deadlock. The second bug manifested itself as starvation of FLUSH TABLES WITH READ LOCK statements in cases when there was a constant stream of concurrent DML statements (in two or more connections). This has happened because requests for protection against GRL which were acquired by DML statements were ignoring presence of pending GRL and thus the latter was starved. This patch solves both these problems by re-implementing GRL using metadata locks. Similar to the old implementation acquisition of GRL in new implementation is two-step. During the first step we block all concurrent DML and DDL statements by acquiring global S metadata lock (each DML and DDL statement acquires global IX lock for its duration). During the second step we block commits by acquiring global S lock in COMMIT namespace (commit code acquires global IX lock in this namespace). Note that unlike in old implementation acquisition of protection against GRL in DML and DDL is semi-automatic. We assume that any statement which should be blocked by GRL will either open and acquires write-lock on tables or acquires metadata locks on objects it is going to modify. For any such statement global IX metadata lock is automatically acquired for its duration. The first problem is solved because waits for GRL become visible to deadlock detector in metadata locking subsystem and thus deadlocks like one in the first bug become impossible. The second problem is solved because global S locks which are used for GRL implementation are given preference over IX locks which are acquired by concurrent DML (and we can switch to fair scheduling in future if needed). Important change: FTWRL/GRL no longer blocks DML and DDL on temporary tables. Before this patch behavior was not consistent in this respect: in some cases DML/DDL statements on temporary tables were blocked while in others they were not. Since the main use cases for FTWRL are various forms of backups and temporary tables are not preserved during backups we have opted for consistently allowing DML/DDL on temporary tables during FTWRL/GRL. Important change: This patch changes thread state names which are used when DML/DDL of FTWRL is waiting for global read lock. It is now either "Waiting for global read lock" or "Waiting for commit lock" depending on the stage on which FTWRL is. Incompatible change: To solve deadlock in events code which was exposed by this patch we have to replace LOCK_event_metadata mutex with metadata locks on events. As result we have to prohibit DDL on events under LOCK TABLES. This patch also adds extensive test coverage for interaction of DML/DDL and FTWRL. Performance of new and old global read lock implementations in sysbench tests were compared. There were no significant difference between new and old implementations. @ mysql-test/include/check_ftwrl_compatible.inc Added helper script which allows to check that a statement is compatible with FLUSH TABLES WITH READ LOCK. @ mysql-test/include/check_ftwrl_incompatible.inc Added helper script which allows to check that a statement is incompatible with FLUSH TABLES WITH READ LOCK. @ mysql-test/include/handler.inc Adjusted test case to the fact that now DROP TABLE closes open HANDLERs for the table to be dropped before checking if there active FTWRL in this connection. @ mysql-test/include/wait_show_condition.inc Fixed small error in the timeout message. The correct name of variable used as parameter for this script is "$condition" and not "$wait_condition". @ mysql-test/r/delayed.result Added test coverage for scenario which triggered assert in metadata locking subsystem. @ mysql-test/r/events_2.result Updated test results after prohibiting event DDL operations under LOCK TABLES. @ mysql-test/r/flush.result Added test coverage for bug #57006 "Deadlock between HANDLER and FLUSH TABLES WITH READ LOCK". @ mysql-test/r/flush_read_lock.result Added test coverage for various aspects of FLUSH TABLES WITH READ LOCK functionality. @ mysql-test/r/flush_read_lock_kill.result Adjusted test case after replacing custom global read lock implementation with one based on metadata locks. Use new debug_sync point. Do not disable concurrent inserts as now InnoDB we always use InnoDB table. @ mysql-test/r/handler_innodb.result Adjusted test case to the fact that now DROP TABLE closes open HANDLERs for the table to be dropped before checking if there active FTWRL in this connection. @ mysql-test/r/handler_myisam.result Adjusted test case to the fact that now DROP TABLE closes open HANDLERs for the table to be dropped before checking if there active FTWRL in this connection. @ mysql-test/r/mdl_sync.result Adjusted test case after replacing custom global read lock implementation with one based on metadata locks. Replaced usage of GRL-specific debug_sync's with appropriate sync points in MDL subsystem. @ mysql-test/suite/perfschema/r/dml_setup_instruments.result Updated test results after removing global COND_global_read_lock condition variable. @ mysql-test/suite/perfschema/r/func_file_io.result Ensure that this test doesn't affect subsequent tests. At the end of its execution enable back P_S instrumentation which this test disables at some point. @ mysql-test/suite/perfschema/r/func_mutex.result Ensure that this test doesn't affect subsequent tests. At the end of its execution enable back P_S instrumentation which this test disables at some point. @ mysql-test/suite/perfschema/r/global_read_lock.result Adjusted test case to take into account that new GRL implementation is based on MDL. @ mysql-test/suite/perfschema/r/server_init.result Adjusted test case after replacing custom global read lock implementation with one based on MDL and replacing LOCK_event_metadata mutex with metadata lock. @ mysql-test/suite/perfschema/t/func_file_io.test Ensure that this test doesn't affect subsequent tests. At the end of its execution enable back P_S instrumentation which this test disables at some point. @ mysql-test/suite/perfschema/t/func_mutex.test Ensure that this test doesn't affect subsequent tests. At the end of its execution enable back P_S instrumentation which this test disables at some point. @ mysql-test/suite/perfschema/t/global_read_lock.test Adjusted test case to take into account that new GRL implementation is based on MDL. @ mysql-test/suite/perfschema/t/server_init.test Adjusted test case after replacing custom global read lock implementation with one based on MDL and replacing LOCK_event_metadata mutex with metadata lock. @ mysql-test/suite/rpl/r/rpl_tmp_table_and_DDL.result Updated test results after prohibiting event DDL under LOCK TABLES. @ mysql-test/t/delayed.test Added test coverage for scenario which triggered assert in metadata locking subsystem. @ mysql-test/t/events_2.test Updated test case after prohibiting event DDL operations under LOCK TABLES. @ mysql-test/t/flush.test Added test coverage for bug #57006 "Deadlock between HANDLER and FLUSH TABLES WITH READ LOCK". @ mysql-test/t/flush_block_commit.test Adjusted test case after changing thread state name which is used when COMMIT waits for FLUSH TABLES WITH READ LOCK from "Waiting for release of readlock" to "Waiting for commit lock". @ mysql-test/t/flush_block_commit_notembedded.test Adjusted test case after changing thread state name which is used when DML waits for FLUSH TABLES WITH READ LOCK. Now we use "Waiting for global read lock" in this case. @ mysql-test/t/flush_read_lock.test Added test coverage for various aspects of FLUSH TABLES WITH READ LOCK functionality. @ mysql-test/t/flush_read_lock_kill-master.opt We no longer need to use make_global_read_lock_block_commit_loop debug tag in this test. Instead we rely on an appropriate debug_sync point in MDL code. @ mysql-test/t/flush_read_lock_kill.test Adjusted test case after replacing custom global read lock implementation with one based on metadata locks. Use new debug_sync point. Do not disable concurrent inserts as now InnoDB we always use InnoDB table. @ mysql-test/t/lock_multi.test Adjusted test case after changing thread state names which are used when DML or DDL waits for FLUSH TABLES WITH READ LOCK to "Waiting for global read lock". @ mysql-test/t/mdl_sync.test Adjusted test case after replacing custom global read lock implementation with one based on metadata locks. Replaced usage of GRL-specific debug_sync's with appropriate sync points in MDL subsystem. Updated thread state names which are used when DDL waits for FTWRL. @ mysql-test/t/trigger_notembedded.test Adjusted test case after changing thread state names which are used when DML or DDL waits for FLUSH TABLES WITH READ LOCK to "Waiting for global read lock". @ sql/event_data_objects.cc Removed Event_queue_element::status/last_executed_changed members and Event_queue_element::update_timing_fields() method. We no longer use this class for updating mysql.events once event is chosen for execution. Accesses to instances of this class in scheduler thread require protection by Event_queue::LOCK_event_queue mutex and we try to avoid updating table while holding this lock. @ sql/event_data_objects.h Removed Event_queue_element::status/last_executed_changed members and Event_queue_element::update_timing_fields() method. We no longer use this class for updating mysql.events once event is chosen for execution. Accesses to instances of this class in scheduler thread require protection by Event_queue::LOCK_event_queue mutex and we try to avoid updating table while holding this lock. @ sql/event_db_repository.cc - Changed Event_db_repository methods to not release all metadata locks once they are done updating mysql.events table. This allows to keep metadata lock protecting against GRL and lock protecting particular event around until corresponding DDL statement is written to the binary log. - Removed logic for conditional update of "status" and "last_executed" fields from update_timing_fields_for_event() method. In the only case when this method is called now "last_executed" is always modified and tracking change of "status" is too much hassle. @ sql/event_db_repository.h Removed logic for conditional update of "status" and "last_executed" fields from Event_db_repository:: update_timing_fields_for_event() method. In the only case when this method is called now "last_executed" is always modified and tracking change of "status" field is too much hassle. @ sql/event_queue.cc Changed event scheduler code not to update mysql.events table while holding Event_queue::LOCK_event_queue mutex. Doing so led to a deadlock with a new GRL implementation. This deadlock didn't occur with old implementation due to fact that code acquiring protection against GRL ignored pending GRL requests (which lead to GRL starvation). One of goals of new implementation is to disallow GRL starvation and so we have to solve problem with this deadlock in a different way. @ sql/events.cc Changed methods of Events class to acquire protection against GRL while perfoming DDL statement and keep it until statement is written to the binary log. Unfortunately this step together with new GRL implementation exposed deadlock involving Events::LOCK_event_metadata and GRL. To solve it Events::LOCK_event_metadata mutex was replaced with a metadata lock on event. As a side-effect events DDL has to be prohibited under LOCK TABLES even in cases when mysql.events table was explicitly locked for write. @ sql/events.h Replaced Events::LOCK_event_metadata mutex with a metadata lock on event. @ sql/ha_ndbcluster.cc Updated code after replacing custom global read lock implementation with one based on MDL. Since MDL subsystem should now be able to detect deadlocks involving metadata locks and GRL there is no need for special handling of active GRL. @ sql/handler.cc Replaced custom implementation of global read lock with one based on metadata locks. Consequently when doing commit instead of calling method of Global_read_lock class to acquire protection against GRL we simply acquire IX in COMMIT namespace. @ sql/lock.cc Replaced custom implementation of global read lock with one based on metadata locks. This step allows to expose wait for GRL to deadlock detector of MDL subsystem and thus succesfully resolve deadlocks similar to one behind bug #57006 "Deadlock between HANDLER and FLUSH TABLES WITH READ LOCK". It also solves problem with GRL starvation described in bug #54673 "It takes too long to get readlock for 'FLUSH TABLES WITH READ LOCK'" since metadata locks used by GRL give preference to FTWRL statement instead of DML statements (if needed in future this can be changed to fair scheduling). Similar to old implementation of acquisition of GRL is two-step. During the first step we block all concurrent DML and DDL statements by acquiring global S metadata lock (each DML and DDL statement acquires global IX lock for its duration). During the second step we block commits by acquiring global S lock in COMMIT namespace (commit code acquires global IX lock in this namespace). Note that unlike in old implementation acquisition of protection against GRL in DML and DDL is semi-automatic. We assume that any statement which should be blocked by GRL will either open and acquires write-lock on tables or acquires metadata locks on objects it is going to modify. For any such statement global IX metadata lock is automatically acquired for its duration. To support this change: - Global_read_lock::lock/unlock_global_read_lock and make_global_read_lock_block_commit methods were changed accordingly. - Global_read_lock::wait_if_global_read_lock() and start_waiting_global_read_lock() methods were dropped. It is now responsibility of code acquiring metadata locks opening tables to acquire protection against GRL by explicitly taking global IX lock with statement duration. - Global variables, mutex and condition variable used by old implementation was removed. - lock_routine_name() was changed to use statement duration for its global IX lock. It was also renamed to lock_object_name() as it now also used to take metadata locks on events. - Global_read_lock::set_explicit_lock_duration() was added which allows not to release locks used for GRL when leaving prelocked mode. @ sql/lock.h - Renamed lock_routine_name() to lock_object_name() and changed its signature to allow its usage for events. - Removed broadcast_refresh() function. It is no longer needed with new GRL implementation. @ sql/log_event.cc Release metadata locks with statement duration at the end of processing legacy event for LOAD DATA. This ensures that replication thread processing such event properly releases its protection against global read lock. @ sql/mdl.cc Changed MDL subsystem to support new MDL-based implementation of global read lock. Added COMMIT and EVENTS namespaces for metadata locks. Changed thread state name for GLOBAL namespace to "Waiting for global read lock". Optimized MDL_map::find_or_insert() method to avoid taking m_mutex mutex when looking up MDL_lock objects for GLOBAL or COMMIT namespaces. We keep pre-created MDL_lock objects for these namespaces around and simply return pointers to these global objects when needed. Changed MDL_lock/MDL_scoped_lock to properly handle notification of insert delayed handler threads when FTWRL takes global S lock. Introduced concept of lock duration. In addition to locks with transaction duration which work in the way which is similar to how locks worked before (i.e. they are released at the end of transaction), locks with statement and explicit duration were introduced. Locks with statement duration are automatically released at the end of statement. Locks with explicit duration require explicit release and obsolete concept of transactional sentinel. * Changed MDL_request and MDL_ticket classes to support notion of duration. * Changed MDL_context to keep locks with different duration in different lists. Changed code handling ticket list to take this into account. * Changed methods responsible for releasing locks to take into account duration of tickets. Particularly public MDL_context::release_lock() method now only can release tickets with explicit duration (there is still internal method which allows to specify duration). To release locks with statement or transaction duration one have to use release_statement/transactional_locks() methods. * Concept of savepoint for MDL subsystem now has to take into account locks with statement duration. Consequently MDL_savepoint class was introduced and methods working with savepoints were updated accordingly. * Added methods which allow to set duration for one or all locks in the context. @ sql/mdl.h Changed MDL subsystem to support new MDL-based implementation of global read lock. Added COMMIT and EVENTS namespaces for metadata locks. Introduced concept of lock duration. In addition to locks with transaction duration which work in the way which is similar to how locks worked before (i.e. they are released at the end of transaction), locks with statement and explicit duration were introduced. Locks with statement duration are automatically released at the end of statement. Locks with explicit duration require explicit release and obsolete concept of transactional sentinel. * Changed MDL_request and MDL_ticket classes to support notion of duration. * Changed MDL_context to keep locks with different duration in different lists. Changed code handling ticket list to take this into account. * Changed methods responsible for releasing locks to take into account duration of tickets. Particularly public MDL_context::release_lock() method now only can release tickets with explicit duration (there is still internal method which allows to specify duration). To release locks with statement or transaction duration one have to use release_statement/transactional_locks() methods. * Concept of savepoint for MDL subsystem now has to take into account locks with statement duration. Consequently MDL_savepoint class was introduced and methods working with savepoints were updated accordingly. * Added methods which allow to set duration for one or all locks in the context. @ sql/mysqld.cc Removed global mutex and condition variables which were used by old implementation of GRL. Also we no longer need to initialize Events::LOCK_event_metadata mutex as it was replaced with metadata locks on events. @ sql/mysqld.h Removed global variable, mutex and condition variables which were used by old implementation of GRL. @ sql/rpl_rli.cc When slave thread closes tables which were open for handling of RBR events ensure that it releases global IX lock which was acquired as protection against GRL. @ sql/sp.cc Adjusted code to the new signature of lock_object/routine_name(), to the fact that one now needs specify duration of lock when initializing MDL_request and to the fact that savepoints for MDL subsystem are now represented by MDL_savepoint class. @ sql/sp_head.cc Ensure that statements in stored procedures release statement metadata locks and thus release their protectiong against GRL in proper moment in time. Adjusted code to the fact that one now needs specify duration of lock when initializing MDL_request. @ sql/sql_admin.cc Adjusted code to the fact that one now needs specify duration of lock when initializing MDL_request. @ sql/sql_base.cc - Implemented support for new approach to acquiring protection against global read lock. We no longer acquire such protection explicitly on the basis of statement flags. Instead we always rely on code which is responsible for acquiring metadata locks on object to be changed acquiring this protection. This is achieved by acquiring global IX metadata lock with statement duration. Code doing this also responsible for checking that current connection has no active GRL by calling an Global_read_lock::can_acquire_protection() method. Changed code in open_table() and lock_table_names() accordingly. Note that as result of this change DDL and DML on temporary tables is always compatible with GRL (before it was incompatible in some cases and compatible in other cases). - To speed-up code acquiring protection against GRL introduced m_has_protection_against_grl member in Open_table_context class. It indicates that protection was already acquired sometime during open_tables() execution and new attempts can be skipped. - Thanks to new GRL implementation calls to broadcast_refresh() became unnecessary and were removed. - Adjusted code to the fact that one now needs specify duration of lock when initializing MDL_request and to the fact that savepoints for MDL subsystem are now represented by MDL_savepoint class. @ sql/sql_base.h Adjusted code to the fact that savepoints for MDL subsystem are now represented by MDL_savepoint class. Also introduced Open_table_context::m_has_protection_against_grl member which allows to avoid acquiring protection against GRL while opening tables if such protection was already acquired. @ sql/sql_class.cc Changed THD::leave_locked_tables_mode() after transactional sentinel for metadata locks was obsoleted by introduction of locks with explicit duration. @ sql/sql_class.h - Adjusted code to the fact that savepoints for MDL subsystem are now represented by MDL_savepoint class. - Changed Global_read_lock class according to changes in global read lock implementation: * wait_if_global_read_lock and start_waiting_global_read_lock are now gone. Instead code needing protection against GRL has to acquire global IX metadata lock with statement duration itself. To help it new can_acquire_protection() was introduced. Also as result of the above change m_protection_count member is gone too. * Added m_mdl_blocks_commits_lock member to store metadata lock blocking commits. * Adjusted code to the fact that concept of transactional sentinel was obsoleted by concept of lock duration. - Removed CF_PROTECT_AGAINST_GRL flag as it is no longer necessary. New GRL implementation acquires protection against global read lock automagically when statement acquires metadata locks on tables or other objects it is going to change. @ sql/sql_db.cc Adjusted code to the fact that one now needs specify duration of lock when initializing MDL_request. @ sql/sql_handler.cc Removed call to broadcast_refresh() function. It is no longer needed with new GRL implementation. Adjusted code after introducing duration concept for metadata locks. Particularly to the fact transactional sentinel was replaced with explicit duration. @ sql/sql_handler.h Renamed mysql_ha_move_tickets_after_trans_sentinel() to mysql_ha_set_explicit_lock_duration() after transactional sentinel was obsoleted by locks with explicit duration. @ sql/sql_insert.cc Adjusted code handling delaying inserts after switching to new GRL implementation. Now connection thread initiating delayed insert has to acquire global IX lock in addition to metadata lock on table being inserted into. This IX lock protects against GRL and similarly to SW lock on table being inserted into has to be passed to handler thread in order to avoid deadlocks. @ sql/sql_lex.cc LEX::protect_against_global_read_lock member is no longer necessary since protection against GRL is automatically taken by code acquiring metadata locks/opening tables. @ sql/sql_lex.h LEX::protect_against_global_read_lock member is no longer necessary since protection against GRL is automatically taken by code acquiring metadata locks/opening tables. @ sql/sql_parse.cc - Implemented support for new approach to acquiring protection against global read lock. We no longer acquire such protection explicitly on the basis of statement flags. Instead we always rely on code which is responsible for acquiring metadata locks on object to be changed acquiring this protection. This is achieved by acquiring global IX metadata lock with statement duration. This lock is automatically released at the end of statement execution. - Changed implementation of CREATE/DROP PROCEDURE/FUNCTION not to release metadata locks and thus protection against of GRL in the middle of statement execution. - Adjusted code to the fact that one now needs specify duration of lock when initializing MDL_request and to the fact that savepoints for MDL subsystem are now represented by MDL_savepoint class. @ sql/sql_prepare.cc Adjusted code to the to the fact that savepoints for MDL subsystem are now represented by MDL_savepoint class. @ sql/sql_rename.cc With new GRL implementation there is no need to explicitly acquire protection against GRL before renaming tables. This happens automatically in code which acquires metadata locks on tables being renamed. @ sql/sql_show.cc Adjusted code to the fact that one now needs specify duration of lock when initializing MDL_request and to the fact that savepoints for MDL subsystem are now represented by MDL_savepoint class. @ sql/sql_table.cc - With new GRL implementation there is no need to explicitly acquire protection against GRL before dropping tables. This happens automatically in code which acquires metadata locks on tables being dropped. - Changed mysql_alter_table() not to release lock on new table name explicitly and to rely on automatic release of locks at the end of statement instead. This was necessary since now MDL_context::release_lock() is supported only for locks for explicit duration. @ sql/sql_trigger.cc With new GRL implementation there is no need to explicitly acquire protection against GRL before changing table triggers. This happens automatically in code which acquires metadata locks on tables which triggers are to be changed. @ sql/sql_update.cc Fix bug exposed by GRL testing. During prepare phase acquire only S metadata locks instead of SW locks to keep prepare of multi-UPDATE compatible with concurrent LOCK TABLES WRITE and global read lock. @ sql/sql_view.cc With new GRL implementation there is no need to explicitly acquire protection against GRL before creating view. This happens automatically in code which acquires metadata lock on view to be created. @ sql/sql_yacc.yy LEX::protect_against_global_read_lock member is no longer necessary since protection against GRL is automatically taken by code acquiring metadata locks/opening tables. @ sql/table.cc Adjusted code to the fact that one now needs specify duration of lock when initializing MDL_request. @ sql/table.h Adjusted code to the fact that one now needs specify duration of lock when initializing MDL_request. @ sql/transaction.cc Replaced custom implementation of global read lock with one based on metadata locks. Consequently when doing commit instead of calling method of Global_read_lock class to acquire protection against GRL we simply acquire IX in COMMIT namespace. Also adjusted code to the fact that MDL savepoint is now represented by MDL_savepoint class.
[12 Nov 2010 13:58]
Bugs System
A patch for this bug has been committed. After review, it may be pushed to the relevant source trees for release in the next version. You can access the patch from: http://lists.mysql.com/commits/123730 3182 Dmitry Lenev 2010-11-12 Follow-up for patch fixing bug #57006 "Deadlock between HANDLER and FLUSH TABLES WITH READ LOCK" and bug #54673 "It takes too long to get readlock for 'FLUSH TABLES WITH READ LOCK'". Disable execution of flush_read_lock.test on embedded server. This test uses too many statements which are not supported by embedded server.
[5 Dec 2010 12:41]
Bugs System
Pushed into mysql-trunk 5.6.1 (revid:alexander.nozdrin@oracle.com-20101205122447-6x94l4fmslpbttxj) (version source revid:alexander.nozdrin@oracle.com-20101205122447-6x94l4fmslpbttxj) (merge vers: 5.6.1) (pib:23)
[15 Dec 2010 15:07]
Paul DuBois
Noted in 5.5.8, 5.6.1 changelogs. Deadlock occurred when a connection that had some table open through a HANDLER statement tried to update data through a DML statement while another connection tried to execute FLUSH TABLES WITH READ LOCK concurrently. Starvation of FLUSH TABLES WITH READ LOCK statements occurred when there was a constant load of concurrent DML statements in two or more connections. These problems resulted from the global read lock implementation, which was reimplemented with the following consequences: * To solve deadlock in event-handling code that was exposed by this patch, the LOCK_event_metadata mutex as replaced with metadata locks on events. As result, DDL on events is now prohibited under LOCK TABLES. This is an incompatible change. * The global read lock (FLUSH TABLES WITH READ LOCK) no longer blocks DML and DDL on temporary tables. Before this patch, server behavior was not consistent in this respect: in some cases, DML/DDL statements on temporary tables were blocked; in others, they were not. Since the main use cases for FLUSH TABLES WITH READ LOCK are various forms of backups and temporary tables are not preserved during backups, the server now consistently allows DML/DDL on temporary tables under the global read lock. * Thread state names are now either "Waiting for global read lock" or "Waiting for commit lock", depending on which stage FLUSH TABLES WITH READ LOCK is in.
[16 Dec 2010 22:31]
Bugs System
Pushed into mysql-5.5 5.5.9 (revid:jonathan.perkin@oracle.com-20101216101358-fyzr1epq95a3yett) (version source revid:jonathan.perkin@oracle.com-20101216101358-fyzr1epq95a3yett) (merge vers: 5.5.9) (pib:24)
Description: - I've started sysbench/OLTP_RW workload for MyISAM with 16 threads for 120 sec - OLTP_RW - one short transaction that consists of several SQL stmts wrapped with LOCK TABLES WRITE/UNLOCK TABLES - I've executed 'FLUSH TABLES WITH READ LOCK' in parallel It was unable to acquire global read lock during execution of the test mysql> flush tables with read lock; Query OK, 0 rows affected (1 min 56.55 sec) +-----+------+-----------------+--------+---------+------+-------------------------+------------------------------------+ | Id | User | Host | db | Command | Time | State | Info | +-----+------+-----------------+--------+---------+------+-------------------------+------------------------------------+ | 6 | root | localhost:33680 | sbtest | Query | 0 | Waiting for table | LOCK TABLES sbtest WRITE | | 7 | root | localhost:33681 | sbtest | Query | 0 | Waiting for table | LOCK TABLES sbtest WRITE | | 8 | root | localhost:33682 | sbtest | Query | 0 | Waiting for table | LOCK TABLES sbtest WRITE | | 9 | root | localhost:33683 | sbtest | Query | 0 | Waiting for table | LOCK TABLES sbtest WRITE | | 10 | root | localhost:33684 | sbtest | Query | 0 | Waiting for table | LOCK TABLES sbtest WRITE | | 11 | root | localhost:33685 | sbtest | Query | 0 | Waiting for table | LOCK TABLES sbtest WRITE | | 12 | root | localhost:33686 | sbtest | Query | 0 | Waiting for table | LOCK TABLES sbtest WRITE | | 13 | root | localhost:33687 | sbtest | Query | 0 | Waiting for table | LOCK TABLES sbtest WRITE | | 14 | root | localhost:33688 | sbtest | Query | 0 | Waiting for table | LOCK TABLES sbtest WRITE | | 15 | root | localhost:33689 | sbtest | Execute | 0 | Writing to net | UPDATE sbtest set k=k+1 where id=? | | 16 | root | localhost:33690 | sbtest | Query | 0 | Waiting for table | LOCK TABLES sbtest WRITE | | 17 | root | localhost:33691 | sbtest | Query | 0 | Waiting for table | LOCK TABLES sbtest WRITE | | 18 | root | localhost:33692 | sbtest | Query | 0 | Waiting for table | LOCK TABLES sbtest WRITE | | 19 | root | localhost:33693 | sbtest | Query | 0 | Waiting for table | LOCK TABLES sbtest WRITE | | 20 | root | localhost:33694 | sbtest | Query | 0 | Waiting for table | LOCK TABLES sbtest WRITE | | 21 | root | localhost:33695 | sbtest | Query | 0 | Waiting for table | LOCK TABLES sbtest WRITE | | 22 | root | localhost | | Query | 117 | Waiting to get readlock | flush tables with read lock | | 196 | root | localhost | | Query | 0 | | show processlist | +-----+------+-----------------+--------+---------+------+-------------------------+------------------------------------+ 'flush table with read lock' stucks here: #0 0x00000032e0e0b309 in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0 #1 0x00000000005dde27 in inline_mysql_cond_wait () at ../include/mysql/psi/mysql_thread.h:1064 #2 Global_read_lock::lock_global_read_lock (this=0x21bb110, thd=0x21b9c10) at lock.cc:1137 ---Type <return> to continue, or q <return> to quit--- #3 0x00000000005f9873 in reload_acl_and_cache (thd=0x21b9c10, options=16388, tables=0x0, write_to_binlog=0x7f5d3ed66ecf) at sql_parse.cc:6843 #4 0x00000000005fae8b in mysql_execute_command (thd=0x21b9c10) at sql_parse.cc:3997 #5 0x00000000005ff792 in mysql_parse (thd=0x21b9c10, inBuf=0x21b6c40 "flush tables with read lock", length=27, parser_state=0x7f5d3ed67ca0) at sql_parse.cc:5889 #6 0x00000000006002df in dispatch_command (command=COM_QUERY, thd=0x21b9c10, packet=0x21bc881 "flush tables with read lock", packet_length=<value optimized out>) at sql_parse.cc:1114 The same symptoms are observed and for InnoDB starting from ~128 threads. After discussing issue with Dmitry Lenev my understanding is that server should react on 'flush tables with read lock' stmt following way - should allow to finish all currently running transactions and then process 'flush tables with read lock'. However in this case we observe situation when intensive short running transaction prevents execution of 'flush tables with read lock' until load is finished. According to Dmitry behavior looks not correct and should be investigated. How to repeat: - Start server - Start sysbench/OLTP_RW workload with 16(MyISAM)/128(InnoDB) threads - Execute 'FLUSH TABLES WITH READ LOCK' in parallel