lp:~vlad-lesin/percona-server/5.6-bug-1376934

Created by Vlad Lesin on 2015-01-11 and last modified on 2015-01-13
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726. By Vlad Lesin on 2015-01-13

Bug #1376934 fix.

Per query variable setting for max_statement_time failed because statement
timer was set before statement variable was set.

This commit fixes this issue.

718. By Alexey Kopytov on 2014-12-23

Bug #1405076: Deadlock in RELEASE_LOCK()

The RELEASE_LOCK() implementation introduced by the multiple user level
locks patch can result in a deadlock under the following conditions:

- connection #1 calls RELEASE_LOCK() for a previously acquired lock. In
  which case MDL_lock::remove_ticket() is called, which write-locks
  MDL_lock::m_rwlock corresponding to the user-level MDL object. With
  that lock held, MDL_map_partition::remove() is called which locks
  MDL_map_partition::m_mutex protecting the hash of MDL locks belonging
  to an MDL partition.

- connection #2 calls RELEASE_LOCK() simultaneously for the same lock
  being released by connection #1. Since connection #2 did not own the
  lock, it calls MDL_map_partition::get_lock_owner() to check if 0 or
  NULL should be returned (i.e. if the lock exists). That function also
  locks both MDL_map_partition::m_mutex and MDL_lock::m_rwlock(), but in
  the reverse order as compared to connection #1.

With the right timing for the above events we get each thread waiting
for a lock acquired by the other thread, i.e. a deadlock.

Fixed by avoiding to lock MDL_lock::m_rwlock with
MDL_map_partition::m_mutex locked. There is already infrastructure to
release the latter and acquire the former and guarantee that a reference
to an MDL_lock object is valid at the same time. It is implemented in
MDL_map_partition::move_from_hash_to_lock_mutex(), so the fix utilizes
it to remove the deadlock condition.

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