Fix for sporadical crashes of lock_multi_bug38499.test

Dmitry Lenev · Dmitry Lenev · commit cf23a807a184 · 2010-02-01T17:38:50.000+03:00
caused by patch which implemented new type-of-operation-aware
metadata locks and added a wait-for graph based deadlock
detector to the MDL subsystem (this patch fixed bug #46272
"MySQL 5.4.4, new MDL: unnecessary deadlock" and bug #37346
"innodb does not detect deadlock between update and alter
table").

Crashes were caused by a race in MDL_context::try_acquire_lock().
This method added MDL_ticket to the list of granted tickets and
released lock protecting list before setting MDL_ticket::m_lock.
Thus some other thread was able to see ticket without properly
set m_lock member for some short period of time. If this thread
called method involving this member during this period crash
happened.

This fix ensures that MDL_ticket::m_lock is set in all cases
when ticket is added to granted/pending lists in MDL_lock.
--BZR--
revision-id: dlenev@mysql.com-20100201143850-wgkic8b873rra0zo
property-branch-nick: mysql-next-4284-nl-push
property-file-info: ld7:file_id40:mdl.cc-20080523121737-j62pi0m62eaw1hq6-17:message324:We must set MDL_ticket::m_lock member before adding ticket
property-file-info: to the list of granted tickets, since such tickets can be
property-file-info: accessed by other threads which might call methods using
property-file-info: this member.
property-file-info: Added assert which ensures that all MDL_tickets which are
property-file-info: added to the granted/pending lists have properly set
property-file-info: MDL_ticket::m_lock member.4:path10:sql/mdl.cced7:file_id39:mdl.h-20080523121748-o4y2wcq3maotb9do-17:message120:Adjusted comment describing MDL_ticket::m_lock member to
property-file-info: reflect current reality.
property-file-info: Added accessor method for this member.4:path9:sql/mdl.hee
testament3-sha1: c51a82262c714dad4615811908b8ef1f87b5c276
diff --git a/sql/mdl.cc b/sql/mdl.cc
@@ -838,6 +838,12 @@ void MDL_lock::Ticket_list::clear_bit_if_not_in_list(enum_mdl_type type)
 
 void MDL_lock::Ticket_list::add_ticket(MDL_ticket *ticket)
 {
+  /*
+    Ticket being added to the list must have MDL_ticket::m_lock set,
+    since for such tickets methods accessing this member might be
+    called by other threads.
+  */
+  DBUG_ASSERT(ticket->get_lock());
   m_list.push_front(ticket);
   m_bitmap|= MDL_BIT(ticket->get_type());
 }
@@ -1317,11 +1323,10 @@ MDL_context::try_acquire_lock(MDL_request *mdl_request)
 
   if (lock->can_grant_lock(mdl_request->type, this))
   {
+    ticket->m_lock= lock;
     lock->m_granted.add_ticket(ticket);
     rw_unlock(&lock->m_rwlock);
 
-    ticket->m_lock= lock;
-
     m_tickets.push_front(ticket);
 
     mdl_request->ticket= ticket;
diff --git a/sql/mdl.h b/sql/mdl.h
@@ -397,6 +397,7 @@ class MDL_ticket
            m_type == MDL_EXCLUSIVE;
   }
   enum_mdl_type get_type() const { return m_type; }
+  MDL_lock *get_lock() const { return m_lock; }
   void downgrade_exclusive_lock(enum_mdl_type type);
 
   bool has_stronger_or_equal_type(enum_mdl_type type) const;
@@ -423,7 +424,9 @@ class MDL_ticket
   */
   MDL_context *m_ctx;
 
-  /** Pointer to the lock object for this lock ticket. Context private. */
+  /**
+    Pointer to the lock object for this lock ticket. Externally accessible.
+  */
   MDL_lock *m_lock;
 
 private:
