Fixed as follows:

diff:
=== modified file 'Percona-Server/storage/innobase/lock/lock0lock.cc'
--- Percona-Server/storage/innobase/lock/lock0lock.cc   2014-03-09 13:26:24 +0000
+++ Percona-Server/storage/innobase/lock/lock0lock.cc   2014-03-17 19:19:58 +0000
@@ -1639,7 +1639,6 @@
                        is in the queue*/
                } else if (lock->trx != trx) {
                        if (wsrep_log_conflicts) {
-                               mutex_enter(&trx_sys->mutex);
                                if (bf_this)
                                        fputs("\n*** Priority TRANSACTION:\n",
                                              stderr);
@@ -1656,7 +1655,6 @@
                                              stderr);
                                trx_print_latched(stderr, lock->trx, 3000);

-                               mutex_exit(&trx_sys->mutex);
                                fputs("*** WAITING FOR THIS LOCK TO BE GRANTED:\n",
                                      stderr);
 

------------------------------------------------------------
revno: 752
fixes bug: https://launchpad.net/bugs/1293624
committer: Raghavendra D Prabhu <email address hidden>
branch nick: pxc56
timestamp: Tue 2014-03-18 00:49:58 +0530
message:
  Bug#1293624: Enabling wsrep_log_conflicts dynamically causes node to hang

  Fix merge regression of Bug#1234382 from codership-5.6 tree.

  This merge caused double acquisition of trx_sys->mutex.


  There are issues with codership  fix of trx_sys->mutex acquisition in wsrep_kill_victim.

  =======================
  query: (null)
  2014-03-17 22:15:43 13605 [Note] WSREP: WSREP rollbacker aborted thd: (5 140736823138048)
  2014-03-17 22:15:45 13605 [Note] WSREP: Deadlock error for: (null)
  InnoDB: sync levels should be > 298 but a level is 297
  Mutex '&trx->mutex'
  InnoDB: Locked mutex: addr 0x7fffb8037a80 thread 140736630683392 file /media/Oort/ncode/percona-xtradb-cluster/pxc56/Percona-Server/storage/innobase/lock/lock0lock.cc line 2455
  InnoDB: sync_thread_levels_g(array, 298) does not hold!
  2014-03-17 22:16:05 7fffcce0f700 InnoDB: Assertion failure in thread 140736630683392 in file sync0sync.cc line 1268
  InnoDB: We intentionally generate a memory trap.
  InnoDB: Submit a detailed bug report to http://bugs.mysql.com.
  InnoDB: If you get repeated assertion failures or crashes, even
  InnoDB: immediately after the mysqld startup, there may be
  InnoDB: corruption in the InnoDB tablespace. Please refer to
  InnoDB: http://dev.mysql.com/doc/refman/5.6/en/forcing-innodb-recovery.html
  InnoDB: about forcing recovery.

  ====================================

  This is because, it violates the latching order :


  #define SYNC_TRX_SYS          298
  #define SYNC_TRX              297

  from sync0sync.h

  ie. always trx_sys->mutex before trx->mutex.


  The fixes introduced in earlier revision 490 does this by acquiring trx_sys->mutex much higher  in lock_rec_lock_slow.


  However, wsrep_log_conflicts in wsrep_kill_victim is not completely thread-safe.

  It is called from several locations:

  wsrep_kill_victim(GLOBAL_SYMBOL_REF,0)
          -lock_rec_other_has_conflicting(Percona-Server/storage/innobase/lock/lock0lock.cc,1704)
                  --lock_rec_lock_slow(Percona-Server/storage/innobase/lock/lock0lock.cc,2484)
                          ---lock_rec_lock(Percona-Server/storage/innobase/lock/lock0lock.cc,2577)
                  --lock_rec_lock_slow(Percona-Server/storage/innobase/lock/lock0lock.cc,2488)
                          ---lock_rec_lock(Percona-Server/storage/innobase/lock/lock0lock.cc,2577)
                  --lock_rec_insert_check_and_lock(Percona-Server/storage/innobase/lock/lock0lock.cc,6367)
                          ---nonnull(Percona-Server/storage/innobase/btr/btr0cur.cc,1270)
          -lock_table_other_has_incompatible(Percona-Server/storage/innobase/lock/lock0lock.cc,4732)
                  --lock_table(Percona-Server/storage/innobase/lock/lock0lock.cc,4791)
                          ---main(Percona-Server/client/mysqlimport.c,697)
                          ---truncate_table(Percona-Server/sql/sql_truncate.cc,453)
                          ---ib_table_lock(Percona-Server/storage/innobase/api/api0api.cc,3461)
                          ---ib_trx_lock_table_with_retry(Percona-Server/storage/innobase/api/api0misc.cc,81)
                          ---fill_innodb_locks_from_cache(Percona-Server/storage/innobase/handler/i_s.cc,972)
                          ---fill_innodb_locks_from_cache(Percona-Server/storage/innobase/handler/i_s.cc,973)
                          ---row_ins_foreign_check_on_constraint(Percona-Server/storage/innobase/row/row0ins.cc,1138)
                          ---row_ins_check_foreign_constraint(Percona-Server/storage/innobase/row/row0ins.cc,1543)
                          ---row_ins_step(Percona-Server/storage/innobase/row/row0ins.cc,3335)
                          ---row_merge_lock_table(Percona-Server/storage/innobase/row/row0merge.cc,2515)
                          ---row_lock_table_autoinc_for_mysql(Percona-Server/storage/innobase/row/row0mysql.cc,1119)
                          ---row_lock_table_for_mysql(Percona-Server/storage/innobase/row/row0mysql.cc,1191)
                          ---row_lock_table_for_mysql(Percona-Server/storage/innobase/row/row0mysql.cc,1195)
                          ---row_mysql_lock_table(Percona-Server/storage/innobase/row/row0mysql.cc,3126)
                          ---row_sel_step(Percona-Server/storage/innobase/row/row0sel.cc,2088)
                          ---row_search_for_mysql(Percona-Server/storage/innobase/row/row0sel.cc,4085)
                          ---row_upd_step(Percona-Server/storage/innobase/row/row0upd.cc,2964)
                          ---i_s_locks_row_validate(Percona-Server/storage/innobase/trx/trx0i_s.cc,431)
                          ---fill_locks_row(Percona-Server/storage/innobase/trx/trx0i_s.cc,810)
                          ---fill_locks_row(Percona-Server/storage/innobase/trx/trx0i_s.cc,815)
                  --lock_table_ix_resurrect(Percona-Server/storage/innobase/lock/lock0lock.cc,4845)
                          ---lock_rec_expl_exist_on_page(Percona-Server/storage/innobase/include/lock0lock.h,475)
                          ---trx_resurrect_table_locks(Percona-Server/storage/innobase/trx/trx0trx.cc,662)
                  --lock_table_queue_validate(Percona-Server/storage/innobase/lock/lock0lock.cc,5884)
                          ---lock_validate_table_locks(Percona-Server/storage/innobase/lock/lock0lock.cc,6142)



  However, note, that trx_print_latched is a read-only procedure, and
  while there is a risk of crash due to non-existent transaction, that can
  be ruled out due to semantics of wsrep_kill_victim acting only on active
  transactions.



  However, for future, this logic needs to be moved to much higher level like srv_printf_innodb_monitor.