Looks like thread 38 and 33 have contending locks and both are doing a full table scan:

--
Thread 38 (Thread 0x7ff571abe700 (LWP 6433)):
+bt
#0  0x00007ffff7bcda5c in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
#1  0x00000000009baee3 in os_cond_wait (event=0x525fa580, reset_sig_count=1) at /usr/src/debug/Percona-XtraDB-Cluster-5.6.15/storage/innobase/os/os0sync.cc:214
#2  os_event_wait_low (event=0x525fa580, reset_sig_count=1) at /usr/src/debug/Percona-XtraDB-Cluster-5.6.15/storage/innobase/os/os0sync.cc:610
#3  0x0000000000a2b00a in sync_array_wait_event (arr=0x1504d80, index=0) at /usr/src/debug/Percona-XtraDB-Cluster-5.6.15/storage/innobase/sync/sync0arr.cc:433
#4  0x0000000000a2d461 in mutex_spin_wait (_mutex=0x7fe8e95f3088, high_priority=false, file_name=0xc2b610 "/mnt/workspace/percona-xtradb-cluster-5.6-rpms/label_exp/centos6-64/target/BUILD/Percona-XtraDB-Cluster-5.6.15/storage/innobase/lock/lock0lock.cc", line=6478) at /us
r/src/debug/Percona-XtraDB-Cluster-5.6.15/storage/innobase/sync/sync0sync.cc:671
#5  0x0000000000992abc in mutex_enter_func (mutex=0x7fe8e95f3088, file_name=0xc2b610 "/mnt/workspace/percona-xtradb-cluster-5.6-rpms/label_exp/centos6-64/target/BUILD/Percona-XtraDB-Cluster-5.6.15/storage/innobase/lock/lock0lock.cc", line=6478) at /usr/src/debug/Percona-X
traDB-Cluster-5.6.15/storage/innobase/include/sync0sync.ic:273
#6  pfs_mutex_enter_func (mutex=0x7fe8e95f3088, file_name=0xc2b610 "/mnt/workspace/percona-xtradb-cluster-5.6-rpms/label_exp/centos6-64/target/BUILD/Percona-XtraDB-Cluster-5.6.15/storage/innobase/lock/lock0lock.cc", line=6478) at /usr/src/debug/Percona-XtraDB-Cluster-5.6.
15/storage/innobase/include/sync0sync.ic:350
#7  0x0000000000996890 in lock_rec_convert_impl_to_expl (block=0x7fec37f04c00, rec=0x7fedb7178082 "", index=0x7fe8c801f238, offsets=0x7ff571ab9810) at /usr/src/debug/Percona-XtraDB-Cluster-5.6.15/storage/innobase/lock/lock0lock.cc:6478
#8  0x0000000000999ab1 in lock_clust_rec_read_check_and_lock (flags=<value optimized out>, block=0x7fec37f04c00, rec=<value optimized out>, index=0x7fe8c801f238, offsets=<value optimized out>, mode=<value optimized out>, gap_mode=1024, thr=0x7fe89c014170) at /usr/src/debug/Percona-XtraDB-Cluster-5.6.15/storage/innobase/lock/lock0lock.cc:6820
#9  0x0000000000a0b4ff in sel_set_rec_lock (block=0x7fec37f04c00, rec=0x7fedb7178082 "", index=0x7fe8c801f238, offsets=0x7ff571ab9810, mode=<value optimized out>, type=<value optimized out>, thr=0x7fe89c014170) at /usr/src/debug/Percona-XtraDB-Cluster-5.6.15/storage/innobase/row/row0sel.cc:1012
#10 0x0000000000a11679 in row_search_for_mysql (buf=0x7fe89c0127b0 "\376\rafrica\\dariogusv", mode=1, prebuilt=0x7fe89c013a78, match_mode=<value optimized out>, direction=<value optimized out>) at /usr/src/debug/Percona-XtraDB-Cluster-5.6.15/storage/innobase/row/row0sel.cc:4506
#11 0x0000000000968857 in ha_innobase::general_fetch (this=0x7fe89c010a80, buf=0x7fe89c0127b0 "\376\rafrica\\dariogusv", direction=1, match_mode=0) at /usr/src/debug/Percona-XtraDB-Cluster-5.6.15/storage/innobase/handler/ha_innodb.cc:8969
#12 0x00000000005c087c in handler::ha_rnd_next (this=0x7fe89c010a80, buf=0x7fe89c0127b0 "\376\rafrica\\dariogusv") at /usr/src/debug/Percona-XtraDB-Cluster-5.6.15/sql/handler.cc:2798
#13 0x000000000089fec8 in Rows_log_event::do_table_scan_and_update (this=0x7fe89c00fd80, rli=0x7fe89c00bed0) at /usr/src/debug/Percona-XtraDB-Cluster-5.6.15/sql/log_event.cc:10862
#14 0x00000000008a59df in Rows_log_event::do_apply_event (this=0x7fe89c00fd80, rli=0x7fe89c00bed0) at /usr/src/debug/Percona-XtraDB-Cluster-5.6.15/sql/log_event.cc:11247
#15 0x00000000008a6c38 in Log_event::apply_event (this=0x7fe89c00fd80, rli=0x7fe89c00bed0) at /usr/src/debug/Percona-XtraDB-Cluster-5.6.15/sql/log_event.cc:2997
#16 0x00000000005bd38e in wsrep_apply_events (ctx=0x7fe89c000990, buf=0x7ff56800de0f, buf_len=444, flags=<value optimized out>, meta=<value optimized out>) at /usr/src/debug/Percona-XtraDB-Cluster-5.6.15/sql/wsrep_applier.cc:150
#17 wsrep_apply_cb (ctx=0x7fe89c000990, buf=0x7ff56800de0f, buf_len=444, flags=<value optimized out>, meta=<value optimized out>) at /usr/src/debug/Percona-XtraDB-Cluster-5.6.15/sql/wsrep_applier.cc:226
#18 0x00007fff8bcfa291 in galera::TrxHandle::apply (this=0x7ff56800d9d0, recv_ctx=0x7fe89c000990, apply_cb=0x5bcdc0 <wsrep_apply_cb(void*, void const*, size_t, uint32_t, wsrep_trx_meta_t const*)>, meta=...) at galera/src/trx_handle.cpp:304
#19 0x00007fff8bd333f5 in apply_trx_ws (recv_ctx=0x7fe89c000990, apply_cb=0x5bcdc0 <wsrep_apply_cb(void*, void const*, size_t, uint32_t, wsrep_trx_meta_t const*)>, commit_cb=0x5bc9a0 <wsrep_commit_cb(void*, uint32_t, wsrep_trx_meta_t const*, wsrep_bool_t*, bool)>, trx=..., meta=...) at galera/src/replicator_smm.cpp:39
#20 0x00007fff8bd34263 in galera::ReplicatorSMM::apply_trx (this=0x1471b50, recv_ctx=0x7fe89c000990, trx=0x7ff56800d9d0) at galera/src/replicator_smm.cpp:419
#21 0x00007fff8bd3efa2 in galera::ReplicatorSMM::recv_IST (this=0x1471b50, recv_ctx=0x7fe89c000990) at galera/src/replicator_str.cpp:760
#22 0x00007fff8bd2f10f in galera::ReplicatorSMM::async_recv (this=0x1471b50, recv_ctx=0x7fe89c000990) at galera/src/replicator_smm.cpp:357
#23 0x00007fff8bd43ba3 in galera_recv (gh=<value optimized out>, recv_ctx=<value optimized out>) at galera/src/wsrep_provider.cpp:231
#24 0x00000000005be11f in wsrep_replication_process (thd=0x7fe89c000990) at /usr/src/debug/Percona-XtraDB-Cluster-5.6.15/sql/wsrep_thd.cc:309
#25 0x00000000005ae42e in start_wsrep_THD (arg=0x5be0d0) at /usr/src/debug/Percona-XtraDB-Cluster-5.6.15/sql/mysqld.cc:5502
#26 0x00007ffff7bc9ddb in start_thread () from /lib64/libpthread.so.0
#27 0x00007ffff60f1a1d in clone () from /lib64/libc.so.6


Thread 33 (Thread 0x7ff571c03700 (LWP 6428)):
+bt
#0  0x00007ffff6043ba5 in raise () from /lib64/libc.so.6
#1  0x00007ffff60454bb in abort () from /lib64/libc.so.6
#2  0x00000000009950db in lock_rec_has_to_wait (mode=1027, block=<value optimized out>, heap_no=3, trx=0x7fe8c8017be8) at /usr/src/debug/Percona-XtraDB-Cluster-5.6.15/storage/innobase/lock/lock0lock.cc:1051
#3  lock_rec_other_has_conflicting (mode=1027, block=<value optimized out>, heap_no=3, trx=0x7fe8c8017be8) at /usr/src/debug/Percona-XtraDB-Cluster-5.6.15/storage/innobase/lock/lock0lock.cc:1702
#4  0x000000000099973f in lock_rec_lock_slow (impl=0, mode=1027, block=0x7fec37f04d80, heap_no=3, index=0x7fe8c801f238, thr=0x7fe8c8017390) at /usr/src/debug/Percona-XtraDB-Cluster-5.6.15/storage/innobase/lock/lock0lock.cc:2484
#5  lock_rec_lock (impl=0, mode=1027, block=0x7fec37f04d80, heap_no=3, index=0x7fe8c801f238, thr=0x7fe8c8017390) at /usr/src/debug/Percona-XtraDB-Cluster-5.6.15/storage/innobase/lock/lock0lock.cc:2578
#6  0x0000000000999ae3 in lock_clust_rec_read_check_and_lock (flags=<value optimized out>, block=0x7fec37f04d80, rec=<value optimized out>, index=0x7fe8c801f238, offsets=<value optimized out>, mode=<value optimized out>, gap_mode=1024, thr=0x7fe8c8017390) at /usr/src/debug/Percona-XtraDB-Cluster-5.6.15/storage/innobase/lock/lock0lock.cc:6831
#7  0x0000000000a0b4ff in sel_set_rec_lock (block=0x7fec37f04d80, rec=0x7fedb717c127 "", index=0x7fe8c801f238, offsets=0x7ff571bfe810, mode=<value optimized out>, type=<value optimized out>, thr=0x7fe8c8017390) at /usr/src/debug/Percona-XtraDB-Cluster-5.6.15/storage/innobase/row/row0sel.cc:1012
#8  0x0000000000a11679 in row_search_for_mysql (buf=0x7fe8c8015dc0 "\376\020australia\\kevindutler", mode=1, prebuilt=0x7fe8c8016c98, match_mode=<value optimized out>, direction=<value optimized out>) at /usr/src/debug/Percona-XtraDB-Cluster-5.6.15/storage/innobase/row/row0sel.cc:4506
#9  0x0000000000968857 in ha_innobase::general_fetch (this=0x7fe8c8014090, buf=0x7fe8c8015dc0 "\376\020australia\\kevindutler", direction=1, match_mode=0) at /usr/src/debug/Percona-XtraDB-Cluster-5.6.15/storage/innobase/handler/ha_innodb.cc:8969
#10 0x00000000005c087c in handler::ha_rnd_next (this=0x7fe8c8014090, buf=0x7fe8c8015dc0 "\376\020australia\\kevindutler") at /usr/src/debug/Percona-XtraDB-Cluster-5.6.15/sql/handler.cc:2798
#11 0x000000000089fec8 in Rows_log_event::do_table_scan_and_update (this=0x7fe8c800fd80, rli=0x7fe8c800bed0) at /usr/src/debug/Percona-XtraDB-Cluster-5.6.15/sql/log_event.cc:10862
#12 0x00000000008a59df in Rows_log_event::do_apply_event (this=0x7fe8c800fd80, rli=0x7fe8c800bed0) at /usr/src/debug/Percona-XtraDB-Cluster-5.6.15/sql/log_event.cc:11247
#13 0x00000000008a6c38 in Log_event::apply_event (this=0x7fe8c800fd80, rli=0x7fe8c800bed0) at /usr/src/debug/Percona-XtraDB-Cluster-5.6.15/sql/log_event.cc:2997
#14 0x00000000005bd38e in wsrep_apply_events (ctx=0x7fe8c8000990, buf=0x7ff56800b61f, buf_len=454, flags=<value optimized out>, meta=<value optimized out>) at /usr/src/debug/Percona-XtraDB-Cluster-5.6.15/sql/wsrep_applier.cc:150
#15 wsrep_apply_cb (ctx=0x7fe8c8000990, buf=0x7ff56800b61f, buf_len=454, flags=<value optimized out>, meta=<value optimized out>) at /usr/src/debug/Percona-XtraDB-Cluster-5.6.15/sql/wsrep_applier.cc:226
#16 0x00007fff8bcfa291 in galera::TrxHandle::apply (this=0x7ff56800b1e0, recv_ctx=0x7fe8c8000990, apply_cb=0x5bcdc0 <wsrep_apply_cb(void*, void const*, size_t, uint32_t, wsrep_trx_meta_t const*)>, meta=...) at galera/src/trx_handle.cpp:304
#17 0x00007fff8bd333f5 in apply_trx_ws (recv_ctx=0x7fe8c8000990, apply_cb=0x5bcdc0 <wsrep_apply_cb(void*, void const*, size_t, uint32_t, wsrep_trx_meta_t const*)>, commit_cb=0x5bc9a0 <wsrep_commit_cb(void*, uint32_t, wsrep_trx_meta_t const*, wsrep_bool_t*, bool)>, trx=..., meta=...) at galera/src/replicator_smm.cpp:39
#18 0x00007fff8bd34263 in galera::ReplicatorSMM::apply_trx (this=0x1471b50, recv_ctx=0x7fe8c8000990, trx=0x7ff56800b1e0) at galera/src/replicator_smm.cpp:419
#19 0x00007fff8bd3efa2 in galera::ReplicatorSMM::recv_IST (this=0x1471b50, recv_ctx=0x7fe8c8000990) at galera/src/replicator_str.cpp:760
#20 0x00007fff8bd2f10f in galera::ReplicatorSMM::async_recv (this=0x1471b50, recv_ctx=0x7fe8c8000990) at galera/src/replicator_smm.cpp:357
#21 0x00007fff8bd43ba3 in galera_recv (gh=<value optimized out>, recv_ctx=<value optimized out>) at galera/src/wsrep_provider.cpp:231
#22 0x00000000005be11f in wsrep_replication_process (thd=0x7fe8c8000990) at /usr/src/debug/Percona-XtraDB-Cluster-5.6.15/sql/wsrep_thd.cc:309
#23 0x00000000005ae42e in start_wsrep_THD (arg=0x5be0d0) at /usr/src/debug/Percona-XtraDB-Cluster-5.6.15/sql/mysqld.cc:5502
#24 0x00007ffff7bc9ddb in start_thread () from /lib64/libpthread.so.0
#25 0x00007ffff60f1a1d in clone () from /lib64/libc.so.6


The index is a clustered index created automatically by InnoDB for tables that do not have implicit primary key