Bug#17083851 BACKPORT BUG#11765744 TO 5.1, 5.5 AND 5.6

      
Description:
Original fix Bug#11765744 changed mutex to read write lock
to avoid multiple recursive lock acquire operation on 
LOCK_status mutex.  
On Windows, locking read-write lock recursively is not safe. 
Slim read-write locks, which MySQL uses if they are supported by
Windows version, do not support recursion according to their 
documentation. For our own implementation of read-write lock, 
which is used in cases when Windows version doesn't support SRW,
recursive locking of read-write lock can easily lead to deadlock
if there are concurrent lock requests.
      
Fix:  
This patch reverts the previous fix for bug#11765744 that used
read-write locks. Instead problem of recursive locking for
LOCK_status mutex is solved by tracking recursion level using 
counter in THD object and acquiring lock only once when we enter 
fill_status() function first time. 

1 files changed, 8 insertions, 2 deletions

diff --git a/sql/sql_show.cc b/sql/sql_show.cc
index 2811483a0b1..fbd163f4667 100644
--- a/sql/sql_show.cc
+++ b/sql/sql_show.cc
@@ -6215,14 +6215,20 @@ int fill_status(THD *thd, TABLE_LIST *tables, COND *cond)
     tmp1= &thd->status_var;
   }
 
-  mysql_mutex_lock(&LOCK_status);
+  /*
+    Avoid recursive acquisition of LOCK_status in cases when WHERE clause
+    represented by "cond" contains subquery on I_S.SESSION/GLOBAL_STATUS.
+  */
+  if (thd->fill_status_recursion_level++ == 0) 
+    mysql_mutex_lock(&LOCK_status);
   if (option_type == OPT_GLOBAL)
     calc_sum_of_all_status(&tmp);
   res= show_status_array(thd, wild,
                          (SHOW_VAR *)all_status_vars.buffer,
                          option_type, tmp1, "", tables->table,
                          upper_case_names, cond);
-  mysql_mutex_unlock(&LOCK_status);
+  if (thd->fill_status_recursion_level-- == 1) 
+    mysql_mutex_unlock(&LOCK_status);
   DBUG_RETURN(res);
 }