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Patch that changes approach to how we acquire metadata

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locks for DML statements and changes the way MDL locks
are acquired/granted in contended case.

Instead of backing-off when a lock conflict is encountered
and waiting for it to go away before restarting open_tables()
process we now wait for lock to be released without releasing
any previously acquired locks. If conflicting lock goes away
we resume opening tables. If waiting leads to a deadlock we
try to resolve it by backing-off and restarting open_tables()
immediately.

As result both waiting for possibility to acquire and
acquiring of a metadata lock now always happen within the
same MDL API call. This has allowed to make release of a lock
and granting it to the most appropriate pending request an
atomic operation.
Thanks to this it became possible to wake up during release
of lock only those waiters which requests can be satisfied
at the moment as well as wake up only one waiter in case
when granting its request would prevent all other requests
from being satisfied. This solves thundering herd problem
which occured in cases when we were releasing some lock and
woke up many waiters for SNRW or X locks (this was the issue
in bug#52289 "performance regression for MyISAM in sysbench
OLTP_RW test".
This also allowed to implement more fair (FIFO) scheduling
among waiters with the same priority.
It also opens the door for introducing new types of requests
for metadata locks such as low-prio SNRW lock which is
necessary in order to support LOCK TABLES LOW_PRIORITY WRITE.

Notice that after this sometimes can report ER_LOCK_DEADLOCK
error in cases in which it has not happened before.
Particularly we will always report this error if waiting for
conflicting lock has happened in the middle of transaction
and resulted in a deadlock. Before this patch the error was
not reported if deadlock could have been resolved by backing
off all metadata locks acquired by the current statement.

mysql-test/r/mdl_sync.result:
  Added test coverage for some aspects of deadlock handling in
  metadata locking subsystem.
  Adjusted test case after removing back-off in general case
  when conflicting metadata lock is encountered during
  open_tables() (now this happens only if waiting for
  conflicting lock to go away leads to a deadlock).
mysql-test/r/sp_sync.result:
  Adjusted test case after removing back-off in general case
  when conflicting metadata lock is encountered during
  open_tables() (now this happens only if waiting for
  conflicting lock to go away leads to a deadlock).
mysql-test/suite/perfschema/r/dml_setup_instruments.result:
  Adjusted test results after renaming MDL_context::
  m_waiting_for_lock rwlock to m_LOCK_waiting_for.
mysql-test/suite/rpl/r/rpl_sp.result:
  Adjusted test case after implementing new approach to
  acquiring metadata locks in open_tables(). We no longer
  release all MDL locks acquired by statement before waiting
  for conflicting lock to go away. As result DROP FUNCTION
  statement has to wait for DML statement which managed to
  acquire metadata lock on function being dropped and now
  waits for other conflicting metadata lock to go away.
mysql-test/suite/rpl/t/rpl_sp.test:
  Adjusted test case after implementing new approach to
  acquiring metadata locks in open_tables(). We no longer
  release all MDL locks acquired by statement before waiting
  for conflicting lock to go away. As result DROP FUNCTION
  statement has to wait for DML statement which managed to
  acquire metadata lock on function being dropped and now
  waits for other conflicting metadata lock to go away.
mysql-test/t/mdl_sync.test:
  Added test coverage for some aspects of deadlock handling in
  metadata locking subsystem.
  Adjusted test case after removing back-off in general case
  when conflicting metadata lock is encountered during
  open_tables() (now this happens only if waiting for
  conflicting lock to go away leads to a deadlock).
mysql-test/t/sp_sync.test:
  Adjusted test case after removing back-off in general case
  when conflicting metadata lock is encountered during
  open_tables() (now this happens only if waiting for
  conflicting lock to go away leads to a deadlock).
sql/mdl.cc:
  Changed MDL subsystem to support new approach to acquring
  metadata locks in open tables and more fair and efficient
  scheduling of metadata locks. To implement this:
  - Made releasing of the lock and granting it to the most
    appropriate pending request atomic operation. As result it
    became possible to wake up only those waiters requests from
    which can be satisfied at the moment as well as wake-up
    only one waiter in case when granting its request would
    prevent all other requests from being satisfied.
    This solved thundering herd problem which occured in cases
    when we were releasing some lock and woke up many waiters
    for SNRW or X locks (this was the issue in Bug #52289
    "performance regression for MyISAM in sysbench OLTP_RW
    test".
    To emphasize above changes wake_up_waiters() was renamed
    to MDL_context::reschedule_waiters().
  - Changed code to add tickets for new requests to the back of
    waiters queue and to select tickets to be satisfied from
    the head of the queue if possible (this makes scheduling of
    requests with the same priority fair). To be able to do
    this efficiently we now use for waiting and granted queues
    version of I_P_List class which provides fast push_back()
    method.
  - Members and methods of MDL_context related to sending
    and waiting for signal were moved to separate MDL_wait
    class.
  - Since in order to avoid race conditions we must grant the
    lock only to the context which was not chosen as a victim
    of deadlock, killed or aborted due to timeout
    MDL_wait::set_status() (former awake()) was changed not to
    send signal if signal slot is already occupied and to
    indicate this fact through its return value. As another
    consequence MDL_wait::timed_wait() method was changed to
    handle timeout (optionally) and abort due to kill as
    signals which make signal slot occupied.
  - Renamed MDL_context::acquire_lock_impl() to acquire_lock().
    Changed it to be able correctly process requests for shared
    locks when there are open HANDLERs, made this method more
    optimized for acquisition of shared locks. As part of this
    change moved code common between try_acquire_lock() and
    acquire_lock() to new try_acquire_lock_impl() method.
    Also adjusted acquire_lock()'s code to take into account
    the fact that in cases when lock is granted as result of
    MDL_context::reschedule_waiters() call (i.e. when it is
    granted after waiting for lock conflict to go away)
    updating MDL_lock state is responsibility of the thread
    calling reschedule_waiters().
  - Changed MDL_context::find_deadlock() to send VICTIM
    signal even if victim is the context which has initiated
    deadlock detection. This is required in order to avoid
    races in cases when the same context simultaneously is
    chosen as a victim and its request for lock is satisfied.
    As result return value of this method became unnecessary
    and it was changed to return void.
    Adjusted MDL_lock::find_deadlock() method to take into
    account that now there can be a discrepancy between
    MDL_context::m_waiting_for value being set and real state
    of the ticket this member points to.
  - Renamed MDL_context::m_waiting_for_lock to m_LOCK_waiting_for
    and MDL_context::stop_waiting() to done_waiting_for().
  - Finally, removed MDL_context::wait_for_lock() method.
sql/mdl.h:
  Changed MDL subsystem to support new approach to acquring
  metadata locks in open tables and more fair and efficient
  scheduling of metadata locks. To implement this:
  - Members and methods of MDL_context related to sending
    and waiting for signal were moved to separate MDL_wait
    class.
  - Since now in order to avoid race conditions we must grant
    the lock only to the context which was not chosen as a
    victim of deadlock, killed or aborted due to timeout
    MDL_wait::set_status (former awake()) was changed not to
    send signal if signal slot is already occupied and to
    indicate this fact through its return value.
    Also NORMAL_WAKE_UP signal became GRANTED, and timeouts
    and aborts due to kill became full blown signals rather
    than simple return values.
  - MDL_wait::timed_wait() now takes extra parameter that
    indicates whether signal should be set if timeout is
    reached.
  - Enabled fast push_back() operation in MDL_context::m_tickets
    list to make move_ticket_after_trans_sentinel() method more
    efficient.
  - Removed MDL_context::wait_for_lock() method.
  - Renamed MDL_context::m_waiting_for_lock to m_LOCK_waiting_for
    and MDL_context::stop_waiting() to done_waiting_for().
  - MDL_context::acquire_lock_impl() became acquire_lock().
  - Introduced MDL_context::try_acquire_lock_impl() as a
    place for code shared by try_acquire_lock and
    acquire_lock().
  - Due to fact that now VICTIM signal is sent even if victim
    is the context which has initiated deadlock detection
    find_deadlock() no longer needs a return value.
sql/sql_base.cc:
  Implemented new approach to acquiring metadata locks in
  open_tables(). We no longer perform back-off when conflicting
  metadata lock is encountered. Instead we wait for this lock
  to go away while holding all locks which were acquired so
  far. Back-off is only used in situation when further waiting
  will cause a deadlock which could be avoided by performing
  back-off and restarting open_tables() process. Absence of
  waiting between back-off and restart of acquiring metadata
  locks can't lead to livelocks as MDL subsystem was changed
  to make release of lock and granting it to waiting lock
  an atomic action, so back-off will automatically give way
  to other participants of deadlock loop.
  Accordingly:
  - open_table_get_mdl_lock() and open_and_process_routine()
    were changed to wait for conflicting metadata lock to
    go away without back-off. Only if such wait leads to a
    deadlock back-off is requested. As part of this change
    new error handler class was introduced which converts,
    if possible, ER_LOCK_DEADLOCK error to a request for
    back-off and re-start of open_tables() process.
  - Open_table_context::recover_from_failed_open() was changed
    not to wait in case of metadata lock conflict. Instead we
    immediately proceed to re-acquiring locks.
  - Open_table_context::request_backoff_action() now always
    emits error if back-off is requested in the middle of
    transaction as we can't be sure that releasing lock
    which were acquired only by current statement will
    resolve a deadlock. Before this patch such situations were
    successfully detected thanks to the fact that we called
    MDL_context::wait_for_lock() method in
    recover_from_failed_open().
  - In order to avoid deadlocks open_tables() code was adjusted
    to flush open HANDLERs for which there are pending requests
    for X locks before restarting the process of acquiring
    metadata locks.
  - Changed close_tables_for_reopen() not to reset MDL_request
    for tables belonging to the tail of prelocking list. It is
    no longer necessary as these MDL_request objects won't be
    used for any waiting.
  - Adjusted comment in tdc_wait_for_old_version() to avoid
    mentioning removed MDL_context::wait_for_lock() method.
sql/sql_base.h:
  As we no longer wait for conflicting metadata lock away in
  Open_table_context::recover_from_failed_open() method,
  Open_table_context::OT_WAIT_MDL_LOCK action was renamed to
  OT_MDL_CONFLICT.
  Also Open_table_context::m_failed_mdl_request became
  unnecessary and was removed.
sql/sql_plist.h:
  Extended I_P_List template to support efficient push_back()
  operation if it is parameterized with an appropriate policy
  class.
sql/sql_show.cc:
  Adjusted code after removal of MDL_context::wait_for_lock()
  method. Now if one needs to acquire metadata lock with waiting
  one has to use a variant of MDL_context::acquire_lock() method.
This commit is contained in:
Dmitry Lenev
2010-06-07 11:06:55 +04:00
parent 142a162c66
commit 9dbd9ce185
13 changed files with 1076 additions and 521 deletions
Download Patch File Download Diff File Expand all files Collapse all files

250
sql/sql_base.cc
View File

@@ -2364,6 +2364,66 @@ void table_share_release_hook(void *share)
}
/**
An error handler which converts, if possible, ER_LOCK_DEADLOCK error
that can occur when we are trying to acquire a metadata lock to
a request for back-off and re-start of open_tables() process.
*/
class MDL_deadlock_handler : public Internal_error_handler
{
public:
MDL_deadlock_handler(Open_table_context *ot_ctx_arg)
: m_ot_ctx(ot_ctx_arg), m_is_active(FALSE)
{}
virtual ~MDL_deadlock_handler() {}
virtual bool handle_condition(THD *thd,
uint sql_errno,
const char* sqlstate,
MYSQL_ERROR::enum_warning_level level,
const char* msg,
MYSQL_ERROR ** cond_hdl);
private:
/** Open table context to be used for back-off request. */
Open_table_context *m_ot_ctx;
/**
Indicates that we are already in the process of handling
ER_LOCK_DEADLOCK error. Allows to re-emit the error from
the error handler without falling into infinite recursion.
*/
bool m_is_active;
};
bool MDL_deadlock_handler::handle_condition(THD *,
uint sql_errno,
const char*,
MYSQL_ERROR::enum_warning_level,
const char*,
MYSQL_ERROR ** cond_hdl)
{
*cond_hdl= NULL;
if (! m_is_active && sql_errno == ER_LOCK_DEADLOCK)
{
/* Disable the handler to avoid infinite recursion. */
m_is_active= TRUE;
(void) m_ot_ctx->request_backoff_action(Open_table_context::OT_MDL_CONFLICT,
NULL);
m_is_active= FALSE;
/*
If the above back-off request failed, a new instance of
ER_LOCK_DEADLOCK error was emitted. Thus the current
instance of error condition can be treated as handled.
*/
return TRUE;
}
return FALSE;
}
/**
Try to acquire an MDL lock for a table being opened.
@@ -2432,19 +2492,61 @@ open_table_get_mdl_lock(THD *thd, Open_table_context *ot_ctx,
ot_ctx->add_request(mdl_request);
if (thd->mdl_context.try_acquire_lock(mdl_request))
return TRUE;
if (mdl_request->ticket == NULL)
if (flags & MYSQL_OPEN_FAIL_ON_MDL_CONFLICT)
{
if (flags & MYSQL_OPEN_FAIL_ON_MDL_CONFLICT)
/*
When table is being open in order to get data for I_S table,
we might have some tables not only open but also locked (e.g. when
this happens under LOCK TABLES or in a stored function).
As a result by waiting on a conflicting metadata lock to go away
we may create a deadlock which won't entirely belong to the
MDL subsystem and thus won't be detectable by this subsystem's
deadlock detector.
To avoid such situation we skip the trouble-making table if
there is a conflicting lock.
*/
if (thd->mdl_context.try_acquire_lock(mdl_request))
return TRUE;
if (mdl_request->ticket == NULL)
{
my_error(ER_WARN_I_S_SKIPPED_TABLE, MYF(0),
mdl_request->key.db_name(), mdl_request->key.name());
return TRUE;
}
if (ot_ctx->request_backoff_action(Open_table_context::OT_WAIT_MDL_LOCK,
mdl_request, NULL))
}
else
{
/*
We are doing a normal table open. Let us try to acquire a metadata
lock on the table. If there is a conflicting lock, acquire_lock()
will wait for it to go away. Sometimes this waiting may lead to a
deadlock, with the following results:
1) If a deadlock is entirely within MDL subsystem, it is
detected by the deadlock detector of this subsystem.
ER_LOCK_DEADLOCK error is produced. Then, the error handler
that is installed prior to the call to acquire_lock() attempts
to request a back-off and retry. Upon success, ER_LOCK_DEADLOCK
error is suppressed, otherwise propagated up the calling stack.
2) Otherwise, a deadlock may occur when the wait-for graph
includes edges not visible to the MDL deadlock detector.
One such example is a wait on an InnoDB row lock, e.g. when:
conn C1 gets SR MDL lock on t1 with SELECT * FROM t1
conn C2 gets a row lock on t2 with SELECT * FROM t2 FOR UPDATE
conn C3 gets in and waits on C1 with DROP TABLE t0, t1
conn C2 continues and blocks on C3 with SELECT * FROM t0
conn C1 deadlocks by waiting on C2 by issuing SELECT * FROM
t2 LOCK IN SHARE MODE.
Such circular waits are currently only resolved by timeouts,
e.g. @@innodb_lock_wait_timeout or @@lock_wait_timeout.
*/
MDL_deadlock_handler mdl_deadlock_handler(ot_ctx);
thd->push_internal_handler(&mdl_deadlock_handler);
bool result= thd->mdl_context.acquire_lock(mdl_request,
ot_ctx->get_timeout());
thd->pop_internal_handler();
if (result && !ot_ctx->can_recover_from_failed_open())
return TRUE;
}
*mdl_ticket= mdl_request->ticket;
@@ -2542,7 +2644,7 @@ bool open_table(THD *thd, TABLE_LIST *table_list, MEM_ROOT *mem_root,
if (thd->version != refresh_version)
{
(void) ot_ctx->request_backoff_action(Open_table_context::OT_WAIT_TDC,
NULL, NULL);
NULL);
DBUG_RETURN(TRUE);
}
}
@@ -2754,7 +2856,7 @@ bool open_table(THD *thd, TABLE_LIST *table_list, MEM_ROOT *mem_root,
/* Someone did a refresh while thread was opening tables */
mysql_mutex_unlock(&LOCK_open);
(void) ot_ctx->request_backoff_action(Open_table_context::OT_WAIT_TDC,
NULL, NULL);
NULL);
DBUG_RETURN(TRUE);
}
@@ -2885,7 +2987,7 @@ bool open_table(THD *thd, TABLE_LIST *table_list, MEM_ROOT *mem_root,
release_table_share(share);
mysql_mutex_unlock(&LOCK_open);
(void) ot_ctx->request_backoff_action(Open_table_context::OT_WAIT_TDC,
NULL, NULL);
NULL);
DBUG_RETURN(TRUE);
}
/* Force close at once after usage */
@@ -2926,13 +3028,13 @@ bool open_table(THD *thd, TABLE_LIST *table_list, MEM_ROOT *mem_root,
{
share->version= 0;
(void) ot_ctx->request_backoff_action(Open_table_context::OT_DISCOVER,
NULL, table_list);
table_list);
}
else if (share->crashed)
{
share->version= 0;
(void) ot_ctx->request_backoff_action(Open_table_context::OT_REPAIR,
NULL, table_list);
table_list);
}
goto err_unlock;
@@ -3789,8 +3891,7 @@ end_with_lock_open:
/** Open_table_context */
Open_table_context::Open_table_context(THD *thd, uint flags)
:m_failed_mdl_request(NULL),
m_failed_table(NULL),
:m_failed_table(NULL),
m_start_of_statement_svp(thd->mdl_context.mdl_savepoint()),
m_global_mdl_request(NULL),
m_timeout(flags & MYSQL_LOCK_IGNORE_TIMEOUT ?
@@ -3834,37 +3935,44 @@ MDL_request *Open_table_context::get_global_mdl_request(THD *thd)
bool
Open_table_context::
request_backoff_action(enum_open_table_action action_arg,
MDL_request *mdl_request, TABLE_LIST *table)
TABLE_LIST *table)
{
/*
We are inside a transaction that already holds locks and have
met a broken table or a table which needs re-discovery.
Performing any recovery action requires acquiring an exclusive
metadata lock on this table. Doing that with locks breaks the
metadata locking protocol and might lead to deadlocks,
so we report an error.
A back off action may be one of the three kinds:
However, if we have only met a conflicting lock or an old
TABLE version, and just need to wait for the conflict to
disappear/old version to go away, allow waiting.
While waiting, we use a simple empiric to detect
deadlocks: we never wait on someone who's waiting too.
Waiting will be done after releasing metadata locks acquired
by this statement.
* We met a broken table that needs repair, or a table that
is not present on this MySQL server and needs re-discovery.
To perform the action, we need an exclusive metadata lock on
the table. Acquiring an X lock while holding other shared
locks is very deadlock-prone. If this is a multi- statement
transaction that holds metadata locks for completed
statements, we don't do it, and report an error instead.
* Our attempt to acquire an MDL lock lead to a deadlock,
detected by the MDL deadlock detector. The current
session was chosen a victim. If this is a multi-statement
transaction that holds metadata locks for completed statements,
restarting locking for the current statement may lead
to a livelock. Thus, again, if m_has_locks is set,
we report an error. Otherwise, when there are no metadata
locks other than which belong to this statement, we can
try to recover from error by releasing all locks and
restarting the pre-locking.
* Finally, we could have met a TABLE_SHARE with old version.
Again, if this is a first statement in a transaction we can
close all tables, release all metadata locks and wait for
the old version to go away. Otherwise, waiting with MDL locks
may lead to criss-cross wait between this connection and a
connection that has an open table and waits on a metadata lock,
i.e. to a deadlock.
Since there is no way to detect such a deadlock, we prevent
it by reporting an error.
*/
if (m_has_locks && action_arg != OT_WAIT_MDL_LOCK)
if (m_has_locks)
{
my_error(ER_LOCK_DEADLOCK, MYF(0));
return TRUE;
}
m_action= action_arg;
/*
If waiting for metadata lock is requested, a pointer to
MDL_request object for which we were unable to acquire the
lock is required.
*/
DBUG_ASSERT(m_action != OT_WAIT_MDL_LOCK || mdl_request);
m_failed_mdl_request= mdl_request;
/*
If auto-repair or discovery are requested, a pointer to table
list element must be provided.
@@ -3900,9 +4008,7 @@ recover_from_failed_open(THD *thd)
/* Execute the action. */
switch (m_action)
{
case OT_WAIT_MDL_LOCK:
result= thd->mdl_context.wait_for_lock(m_failed_mdl_request,
get_timeout());
case OT_MDL_CONFLICT:
break;
case OT_WAIT_TDC:
result= tdc_wait_for_old_versions(thd, &m_mdl_requests, get_timeout());
@@ -3973,7 +4079,6 @@ recover_from_failed_open(THD *thd)
TABLE_LIST element, set when we need auto-discovery or repair,
for safety.
*/
m_failed_mdl_request= NULL;
m_failed_table= NULL;
/* Prepare for possible another back-off. */
m_action= OT_NO_ACTION;
@@ -4097,16 +4202,25 @@ open_and_process_routine(THD *thd, Query_tables_list *prelocking_ctx,
*/
DBUG_ASSERT(rt->mdl_request.type == MDL_SHARED);
if (thd->mdl_context.try_acquire_lock(&rt->mdl_request))
/*
Waiting for a conflicting metadata lock to go away may
lead to a deadlock, detected by MDL subsystem.
If possible, we try to resolve such deadlocks by releasing all
metadata locks and restarting the pre-locking process.
To prevent the error from polluting the diagnostics area
in case of successful resolution, install a special error
handler for ER_LOCK_DEADLOCK error.
*/
MDL_deadlock_handler mdl_deadlock_handler(ot_ctx);
thd->push_internal_handler(&mdl_deadlock_handler);
bool result= thd->mdl_context.acquire_lock(&rt->mdl_request,
ot_ctx->get_timeout());
thd->pop_internal_handler();
if (result)
DBUG_RETURN(TRUE);
if (rt->mdl_request.ticket == NULL)
{
/* A lock conflict. Someone's trying to modify SP metadata. */
ot_ctx->request_backoff_action(Open_table_context::OT_WAIT_MDL_LOCK,
&rt->mdl_request, NULL);
DBUG_RETURN(TRUE);
}
DEBUG_SYNC(thd, "after_shared_lock_pname");
/* Ensures the routine is up-to-date and cached, if exists. */
@@ -4613,18 +4727,6 @@ bool open_tables(THD *thd, TABLE_LIST **start, uint *counter, uint flags,
bool has_prelocking_list;
DBUG_ENTER("open_tables");
/*
Close HANDLER tables which are marked for flush or against which there
are pending exclusive metadata locks. Note that we do this not to avoid
deadlocks (calls to mysql_ha_flush() in mdl_wait_for_locks() and
tdc_wait_for_old_version() are enough for this) but in order to have
a point during statement execution at which such HANDLERs are closed
even if they don't create problems for current thread (i.e. to avoid
having DDL blocked by HANDLERs opened for long time).
*/
if (thd->handler_tables_hash.records)
mysql_ha_flush(thd);
/*
temporary mem_root for new .frm parsing.
TODO: variables for size
@@ -4633,6 +4735,17 @@ bool open_tables(THD *thd, TABLE_LIST **start, uint *counter, uint flags,
thd->current_tablenr= 0;
restart:
/*
Close HANDLER tables which are marked for flush or against which there
are pending exclusive metadata locks. This is needed both in order to
avoid deadlocks and to have a point during statement execution at
which such HANDLERs are closed even if they don't create problems for
the current session (i.e. to avoid having a DDL blocked by HANDLERs
opened for a long time).
*/
if (thd->handler_tables_hash.records)
mysql_ha_flush(thd);
has_prelocking_list= thd->lex->requires_prelocking();
table_to_open= start;
sroutine_to_open= (Sroutine_hash_entry**) &thd->lex->sroutines_list.first;
@@ -5578,15 +5691,6 @@ void close_tables_for_reopen(THD *thd, TABLE_LIST **tables,
/* We have to cleanup translation tables of views. */
tmp->cleanup_items();
}
/*
Metadata lock requests for tables from extended part of prelocking set
are part of list of requests to be waited for in Open_table_context.
So to satisfy assumptions in MDL_context::wait_for_locks(), which will
performs the waiting, we have to reset MDL_request::ticket values for
them as well.
*/
for (tmp= first_not_own_table; tmp; tmp= tmp->next_global)
tmp->mdl_request.ticket= NULL;
close_thread_tables(thd);
thd->mdl_context.rollback_to_savepoint(start_of_statement_svp);
}
@@ -8692,8 +8796,12 @@ tdc_wait_for_old_versions(THD *thd, MDL_request_list *mdl_requests,
while (!thd->killed)
{
/*
Here we have situation as in mdl_wait_for_locks() we need to
get rid of offending HANDLERs to avoid deadlock.
We have to get rid of HANDLERs which are open by this thread
and have old TABLE versions. Otherwise we might get a deadlock
in situation when we are waiting for an old TABLE object which
corresponds to a HANDLER open by another session. And this
other session waits for our HANDLER object to get closed.
TODO: We should also investigate in which situations we have
to broadcast on COND_refresh because of this.
*/