1 files changed, 13 insertions, 0 deletions
diff --git a/sql/sql_class.h b/sql/sql_class.h
index 64fed055c80..59ac8ff0483 100644
--- a/sql/sql_class.h
+++ b/sql/sql_class.h
@@ -885,6 +885,12 @@ public:
   void close_active_vio();
 #endif  
   void awake(bool prepare_to_die);
+  /*
+    For enter_cond() / exit_cond() to work the mutex must be got before
+    enter_cond() (in 4.1 an assertion will soon ensure this); this mutex is
+    then released by exit_cond(). Use must be:
+    lock mutex; enter_cond(); your code; exit_cond().
+  */
   inline const char* enter_cond(pthread_cond_t *cond, pthread_mutex_t* mutex,
 			  const char* msg)
   {
@@ -896,6 +902,13 @@ public:
   }
   inline void exit_cond(const char* old_msg)
   {
+    /*
+      Putting the mutex unlock in exit_cond() ensures that
+      mysys_var->current_mutex is always unlocked _before_ mysys_var->mutex is
+      locked (if that would not be the case, you'll get a deadlock if someone
+      does a THD::awake() on you).
+    */
+    pthread_mutex_unlock(mysys_var->current_mutex);
     pthread_mutex_lock(&mysys_var->mutex);
     mysys_var->current_mutex = 0;
     mysys_var->current_cond = 0;