bzr commit into mysql-5.6-next-mr branch (dlenev:3052) Bug#46272

[Dmitry Lenev]

#At file:///home/dlenev/src/bzr/mysql-next-4284-st-review/ based on revid:dlenev@stripped

 3052 Dmitry Lenev	2010-01-18
      A prerequisite patch for the fix for Bug#46272
      "Bug #46272 MySQL 5.4.4, new MDL: unnecessary deadlock".
      Instead of a single global mutex that protects the MDL
      subsystem, use a mutex per one MDL lock.
      
      Work on after review comments in progress.

    modified:
      sql/mdl.h
=== modified file 'sql/mdl.h'
--- a/sql/mdl.h	2010-01-16 09:27:03 +0000
+++ b/sql/mdl.h	2010-01-18 20:03:25 +0000
@@ -121,6 +121,9 @@ public:
     return (m_length == rhs->m_length &&
             memcmp(m_ptr, rhs->m_ptr, m_length) == 0);
   }
+  /**
+    Compare two MDL keys lexicographically.
+  */
   int cmp(const MDL_key *rhs) const
   {
     int res;
@@ -413,6 +416,9 @@ public:
 
   inline THD *get_thd() const { return m_thd; }
 
+  /**
+    Wake up context which is waiting for a change of MDL_lock state.
+  */
   void wake_up()
   {
     pthread_cond_signal(&m_cond);
@@ -424,6 +430,9 @@ public:
   bool acquire_global_shared_lock();
   void release_global_shared_lock();
 
+  /**
+    Check if this context owns global lock of particular type.
+  */
   bool is_global_lock_owner(enum_mdl_type type_arg)
   {
     MDL_request mdl_request;


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