diff options
Diffstat (limited to 'storage/myisammrg')
| -rw-r--r-- | storage/myisammrg/ha_myisammrg.cc | 27 |
1 files changed, 25 insertions, 2 deletions
diff --git a/storage/myisammrg/ha_myisammrg.cc b/storage/myisammrg/ha_myisammrg.cc index 466c66ec769..26e54d61101 100644 --- a/storage/myisammrg/ha_myisammrg.cc +++ b/storage/myisammrg/ha_myisammrg.cc @@ -478,8 +478,31 @@ int ha_myisammrg::add_children_list(void) /* Set the expected table version, to not cause spurious re-prepare. */ child_l->set_table_ref_id(mrg_child_def->get_child_table_ref_type(), mrg_child_def->get_child_def_version()); - /* Use the same metadata lock type for children. */ - child_l->mdl_request.set_type(parent_l->mdl_request.type); + /* + For statements which acquire a SNW metadata lock on a parent table and + then later try to upgrade it to an X lock (e.g. ALTER TABLE), SNW + locks should be also taken on the children tables. + + Otherwise we end up in a situation where the thread trying to upgrade SNW + to X lock on the parent also holds a SR metadata lock and a read + thr_lock.c lock on the child. As a result, another thread might be + blocked on the thr_lock.c lock for the child after successfully acquiring + a SR or SW metadata lock on it. If at the same time this second thread + has a shared metadata lock on the parent table or there is some other + thread which has a shared metadata lock on the parent and is waiting for + this second thread, we get a deadlock. This deadlock cannot be properly + detected by the MDL subsystem as part of the waiting happens within + thr_lock.c. By taking SNW locks on the child tables we ensure that any + thread which waits for a thread doing SNW -> X upgrade, does this within + the MDL subsystem and thus potential deadlocks are exposed to the deadlock + detector. + + We don't do the same thing for SNRW locks as this would allow + DDL on implicitly locked underlying tables of a MERGE table. + */ + if (! thd->locked_tables_mode && + parent_l->mdl_request.type == MDL_SHARED_NO_WRITE) + child_l->mdl_request.set_type(MDL_SHARED_NO_WRITE); /* Link TABLE_LIST object into the children list. */ if (this->children_last_l) child_l->prev_global= this->children_last_l; |