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:
None 
Category:MySQL Server: Locking Severity:S3 (Non-critical)
Version:5.1.48, 5.5.5pre OS:Any
Assigned to: Dmitry Lenev CPU Architecture:Any

[21 Jun 2010 16:22] Alexey Stroganov
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
[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)