Invaldating a subset of a sufficiently large query cache can take a long time.
During this time the server is efficiently frozen and no other operation can
be executed. This patch addresses this problem by setting a time limit on
how long time a dictionary access request can take before giving up on the 
attempt. This patch does not work for query cache invalidations issued by
DROP, ALTER or RENAME TABLE operations.


sql/sql_cache.cc:
  Changed mutex locking to a timed spinn lock. If access to query cache dictionary
  takes "a long time" (currently more than 0.1 seconds) the system fall backs on
  ordinary statement execution.