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From:Dmitry Lenev Date:February 15 2010 12:37pm
Subject:bzr commit into mysql-5.5-next-mr branch (dlenev:3099) Bug#51093
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#At file:///home/dlenev/src/bzr/mysql-next-4284-bg51093/ based on revid:jon.hauglid@stripped

 3099 Dmitry Lenev	2010-02-15
      Fix for bug #51093 "Crash (possibly stack overflow) in 
      On some platforms deadlock detector in metadata locking 
      subsystem under certain conditions might have exhausted
      stack space causing server crashes.
      Particularly this caused failures of rqg_mdl_stability
      test on Solaris in PushBuild.
      During search for deadlock MDL deadlock detector could 
      sometimes encounter loop in the waiters graph in which 
      MDL_context which has started search for a deadlock 
      does not participate. In such case our algorithm will 
      continue looping assuming that either this deadlock will 
      be resolved by MDL_context which has created it (i.e.
      by one of loop participants) or maximum search depth
      will be reached. 
      Since max search depth was set to 1000 in the latter case 
      on platforms where each iteration of deadlock search 
      algorithm needs more than DEFAULT_STACK_SIZE/1000 bytes 
      of stack (around 192 bytes for 32-bit and around 256 bytes 
      for 64-bit platforms) we might have exhausted stack space.
      This patch solves this problem by reducing maximum search
      depth for MDL deadlock detector to 32. This should be safe
      at the moment as it is unlikely that each iteration of the 
      current deadlock detector algorithm will consume more than 
      1K of stack (thus total amount of stack required can't be
      more than 32K) and we require at least 80K of stack in order
      to open any table. Also this value should be (hopefully) big
      enough to not cause too much false deadlock errors (there
      is an anecdotal evidence that real-life deadlocks are
      typically shorter than that).
      Additional reasearch should be conducted in future in order
      to determine the more optimal value of maximum search depth.
      This patch does not include test case as existing
      rqg_mdl_stability test can serve as one.

=== modified file 'sql/'
--- a/sql/	2010-02-15 10:23:34 +0000
+++ b/sql/	2010-02-15 12:37:48 +0000
@@ -74,7 +74,20 @@ public:
   MDL_context *start;
   MDL_context *victim;
   uint current_search_depth;
-  static const uint MAX_SEARCH_DEPTH= 1000;
+  /**
+    Maximum depth for deadlock searches. After this depth is
+    achieved we will unconditionally declare that there is a
+    deadlock.
+    @note This depth should be small enough to avoid stack
+          being exhausted by recursive search algorithm.
+    TODO: Find out what is the optimal value for this parameter.
+          Current value is safe, but probably sub-optimal,
+          as there is an anecdotal evidence that real-life
+          deadlocks are even shorter typically.
+  */
+  static const uint MAX_SEARCH_DEPTH= 32;

Attachment: [text/bzr-bundle] bzr/
bzr commit into mysql-5.5-next-mr branch (dlenev:3099) Bug#51093Dmitry Lenev15 Feb