Fix for:

Bug #4810 "deadlock with KILL when the victim was in a wait state"
(I included mutex unlock into exit_cond() for future safety)
and BUG#4827 "KILL while START SLAVE may lead to replication slave crash"


sql/lock.cc:
  we did exit_cond() before unlock(LOCK_open), which led to deadlocks with THD::awake(). Fixing this.
sql/log.cc:
  mutex unlock is now included in exit_cond()
sql/repl_failsafe.cc:
  we did exit_cond() before unlock(LOCK_rpl_status), which led to deadlocks with THD::awake(). Fixing this.
sql/slave.cc:
  we did exit_cond() before unlock(cond_lock), which led to deadlocks with THD::awake(). Fixing this.
  Fixing also that if killed while waiting for slave thread to start, we don't release the mutex
  (that caused a double release of the mutex => crash).
sql/sql_class.h:
  comments about exit_cond()/enter_cond().
  Mutex unlock is now included in exit_cond() so that it's always done in the good order.
sql/sql_table.cc:
  unlock is now included in exit_cond().

1 files changed, 9 insertions, 7 deletions

diff --git a/sql/lock.cc b/sql/lock.cc
index 5010d115a6c..9ea1ce96175 100644
--- a/sql/lock.cc
+++ b/sql/lock.cc
@@ -692,15 +692,14 @@ bool lock_global_read_lock(THD *thd)
     while (protect_against_global_read_lock && !thd->killed)
       pthread_cond_wait(&COND_refresh, &LOCK_open);
     waiting_for_read_lock--;
-    thd->exit_cond(old_message);
     if (thd->killed)
     {
-      (void) pthread_mutex_unlock(&LOCK_open);
+      thd->exit_cond(old_message);
       DBUG_RETURN(1);
     }
     thd->global_read_lock=1;
     global_read_lock++;
-    (void) pthread_mutex_unlock(&LOCK_open);
+    thd->exit_cond(old_message);
   }
   DBUG_RETURN(0);
 }
@@ -721,11 +720,12 @@ void unlock_global_read_lock(THD *thd)
 bool wait_if_global_read_lock(THD *thd, bool abort_on_refresh)
 {
   const char *old_message;
-  bool result=0;
+  bool result= 0, need_exit_cond;
   DBUG_ENTER("wait_if_global_read_lock");
 
+  LINT_INIT(old_message);
   (void) pthread_mutex_lock(&LOCK_open);
-  if (global_read_lock)
+  if (need_exit_cond= (bool)global_read_lock)
   {
     if (thd->global_read_lock)		// This thread had the read locks
     {
@@ -740,11 +740,13 @@ bool wait_if_global_read_lock(THD *thd, bool abort_on_refresh)
       (void) pthread_cond_wait(&COND_refresh,&LOCK_open);
     if (thd->killed)
       result=1;
-    thd->exit_cond(old_message);
   }
   if (!abort_on_refresh && !result)
     protect_against_global_read_lock++;
-  pthread_mutex_unlock(&LOCK_open);
+  if (unlikely(need_exit_cond)) // global read locks are rare
+    thd->exit_cond(old_message);
+  else
+    pthread_mutex_unlock(&LOCK_open);
   DBUG_RETURN(result);
 }