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Bug #48541 Deadlock between LOCK_open and LOCK_mdl

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The reason for the deadlock was an improper exit from
MDL_context::wait_for_locks() which caused mysys_var->current_mutex to remain
LOCK_mdl even though LOCK_mdl was no longer held by that connection. 

This could for example lead to a deadlock in the following way:
1) INSERT DELAYED tries to open a table but fails, and trying to recover it
calls wait_for_locks().
2) Due to a pending exclusive request, wait_for_locks() fails and exits without
resetting mysys_var->current_mutex for the delayed insert handler thread. So it
continues to point to LOCK_mdl.
3) The handler thread manages to open a table.
4) A different connection takes LOCK_open and tries to take LOCK_mdl.
5) FLUSH TABLES from a third connection notices that the handler thread has a
table open, and tries to kill it. This involves locking mysys_var->current_mutex
while having LOCK_open locked. Since current_mutex mistakenly points to LOCK_mdl,
we have a deadlock.

This patch makes sure MDL_EXIT_COND() is called before exiting wait_for_locks().
This clears mysys->current_mutex which resolves the issue. 

An assert is added to recover_from_failed_open_table_attempt() after
wait_for_locks() is called, to check that current_mutex is indeed reset.
With this assert in place, existing tests in (e.g.) mdl_sync.test will fail
without this patch.
This commit is contained in:
Jon Olav Hauglid
2009-12-16 12:32:11 +01:00
parent 13348d3769
commit ddea504a7d
2 changed files with 3 additions and 1 deletions
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3
sql/mdl.cc
View File

@ -1254,7 +1254,8 @@ MDL_context::wait_for_locks(MDL_request_list *mdl_requests)
} }
if (!mdl_request) if (!mdl_request)
{ {
pthread_mutex_unlock(&LOCK_mdl); /* As a side-effect MDL_EXIT_COND() unlocks LOCK_mdl. */
MDL_EXIT_COND(m_thd, mysys_var, old_msg);
break; break;
} }
pthread_cond_wait(&COND_mdl, &LOCK_mdl); pthread_cond_wait(&COND_mdl, &LOCK_mdl);

1
sql/sql_base.cc
View File

@ -3693,6 +3693,7 @@ recover_from_failed_open_table_attempt(THD *thd, TABLE_LIST *table)
case OT_WAIT: case OT_WAIT:
result= (thd->mdl_context.wait_for_locks(&m_mdl_requests) || result= (thd->mdl_context.wait_for_locks(&m_mdl_requests) ||
tdc_wait_for_old_versions(thd, &m_mdl_requests)); tdc_wait_for_old_versions(thd, &m_mdl_requests));
DBUG_ASSERT(thd->mysys_var->current_mutex == NULL);
break; break;
case OT_DISCOVER: case OT_DISCOVER:
{ {