Attached via gdb and obtained backtrace:

Thread 11 (Thread 0x7fc1775fe700 (LWP 1692)):
#0  0x0000003e5140b43c in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
#1  0x00000000005bd6d3 in os_cond_wait (event=0x26d3c70, reset_sig_count=1) at /mnt/xb-2.1-log-archive/mysql-5.6/storage/innobase/os/os0sync.cc:214
#2  os_event_wait_low (event=0x26d3c70, reset_sig_count=1) at /mnt/xb-2.1-log-archive/mysql-5.6/storage/innobase/os/os0sync.cc:610
#3  0x000000000055cbfa in os_aio_simulated_handle (global_segment=0, message1=0x7fc1775fdd98, message2=0x7fc1775fdd90, type=0x7fc1775fdd88) at /mnt/xb-2.1-log-archive/mysql-5.6/storage/innobase/os/os0file.cc:5437
#4  0x00000000005704e3 in fil_aio_wait (segment=0) at /mnt/xb-2.1-log-archive/mysql-5.6/storage/innobase/fil/fil0fil.cc:5547
#5  0x000000000059e178 in io_handler_thread (arg=<value optimized out>) at /mnt/xb-2.1-log-archive/mysql-5.6/storage/innobase/srv/srv0start.cc:478
#6  0x0000003e51407851 in start_thread () from /lib64/libpthread.so.0
#7  0x0000003e50ce890d in clone () from /lib64/libc.so.6

Thread 10 (Thread 0x7fc177fff700 (LWP 1693)):
#0  0x0000003e5140b43c in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
#1  0x00000000005bd6d3 in os_cond_wait (event=0x26d3d00, reset_sig_count=1) at /mnt/xb-2.1-log-archive/mysql-5.6/storage/innobase/os/os0sync.cc:214
#2  os_event_wait_low (event=0x26d3d00, reset_sig_count=1) at /mnt/xb-2.1-log-archive/mysql-5.6/storage/innobase/os/os0sync.cc:610
#3  0x000000000055cbfa in os_aio_simulated_handle (global_segment=1, message1=0x7fc177ffed98, message2=0x7fc177ffed90, type=0x7fc177ffed88) at /mnt/xb-2.1-log-archive/mysql-5.6/storage/innobase/os/os0file.cc:5437
#4  0x00000000005704e3 in fil_aio_wait (segment=1) at /mnt/xb-2.1-log-archive/mysql-5.6/storage/innobase/fil/fil0fil.cc:5547
#5  0x000000000059e178 in io_handler_thread (arg=<value optimized out>) at /mnt/xb-2.1-log-archive/mysql-5.6/storage/innobase/srv/srv0start.cc:478
#6  0x0000003e51407851 in start_thread () from /lib64/libpthread.so.0
#7  0x0000003e50ce890d in clone () from /lib64/libc.so.6

Thread 9 (Thread 0x7fc17d4b1700 (LWP 1694)):
#0  0x0000003e5140b43c in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
#1  0x00000000005bd6d3 in os_cond_wait (event=0x26d3d90, reset_sig_count=1442) at /mnt/xb-2.1-log-archive/mysql-5.6/storage/innobase/os/os0sync.cc:214
#2  os_event_wait_low (event=0x26d3d90, reset_sig_count=1442) at /mnt/xb-2.1-log-archive/mysql-5.6/storage/innobase/os/os0sync.cc:610
#3  0x000000000055cbfa in os_aio_simulated_handle (global_segment=2, message1=0x7fc17d4b0d98, message2=0x7fc17d4b0d90, type=0x7fc17d4b0d88) at /mnt/xb-2.1-log-archive/mysql-5.6/storage/innobase/os/os0file.cc:5437
#4  0x00000000005704e3 in fil_aio_wait (segment=2) at /mnt/xb-2.1-log-archive/mysql-5.6/storage/innobase/fil/fil0fil.cc:5547
#5  0x000000000059e178 in io_handler_thread (arg=<value optimized out>) at /mnt/xb-2.1-log-archive/mysql-5.6/storage/innobase/srv/srv0start.cc:478
#6  0x0000003e51407851 in start_thread () from /lib64/libpthread.so.0
#7  0x0000003e50ce890d in clone () from /lib64/libc.so.6

Thread 8 (Thread 0x7fc17cab0700 (LWP 1695)):
#0  0x0000003e5140b43c in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
#1  0x00000000005bd6d3 in os_cond_wait (event=0x26d3e20, reset_sig_count=297) at /mnt/xb-2.1-log-archive/mysql-5.6/storage/innobase/os/os0sync.cc:214
#2  os_event_wait_low (event=0x26d3e20, reset_sig_count=297) at /mnt/xb-2.1-log-archive/mysql-5.6/storage/innobase/os/os0sync.cc:610
#3  0x000000000055cbfa in os_aio_simulated_handle (global_segment=3, message1=0x7fc17caafd98, message2=0x7fc17caafd90, type=0x7fc17caafd88) at /mnt/xb-2.1-log-archive/mysql-5.6/storage/innobase/os/os0file.cc:5437
#4  0x00000000005704e3 in fil_aio_wait (segment=3) at /mnt/xb-2.1-log-archive/mysql-5.6/storage/innobase/fil/fil0fil.cc:5547
#5  0x000000000059e178 in io_handler_thread (arg=<value optimized out>) at /mnt/xb-2.1-log-archive/mysql-5.6/storage/innobase/srv/srv0start.cc:478
#6  0x0000003e51407851 in start_thread () from /lib64/libpthread.so.0
#7  0x0000003e50ce890d in clone () from /lib64/libc.so.6

Thread 7 (Thread 0x7fc170595700 (LWP 1696)):
#0  0x0000003e5140b43c in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
#1  0x00000000005bd6d3 in os_cond_wait (event=0x26d3eb0, reset_sig_count=71) at /mnt/xb-2.1-log-archive/mysql-5.6/storage/innobase/os/os0sync.cc:214
#2  os_event_wait_low (event=0x26d3eb0, reset_sig_count=71) at /mnt/xb-2.1-log-archive/mysql-5.6/storage/innobase/os/os0sync.cc:610
#3  0x000000000055cbfa in os_aio_simulated_handle (global_segment=4, message1=0x7fc170594d98, message2=0x7fc170594d90, type=0x7fc170594d88) at /mnt/xb-2.1-log-archive/mysql-5.6/storage/innobase/os/os0file.cc:5437
#4  0x00000000005704e3 in fil_aio_wait (segment=4) at /mnt/xb-2.1-log-archive/mysql-5.6/storage/innobase/fil/fil0fil.cc:5547
#5  0x000000000059e178 in io_handler_thread (arg=<value optimized out>) at /mnt/xb-2.1-log-archive/mysql-5.6/storage/innobase/srv/srv0start.cc:478
#6  0x0000003e51407851 in start_thread () from /lib64/libpthread.so.0
#7  0x0000003e50ce890d in clone () from /lib64/libc.so.6

Thread 6 (Thread 0x7fc16fb94700 (LWP 1697)):
#0  0x0000003e5140b43c in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
#1  0x00000000005bd6d3 in os_cond_wait (event=0x26d3f40, reset_sig_count=34) at /mnt/xb-2.1-log-archive/mysql-5.6/storage/innobase/os/os0sync.cc:214
#2  os_event_wait_low (event=0x26d3f40, reset_sig_count=34) at /mnt/xb-2.1-log-archive/mysql-5.6/storage/innobase/os/os0sync.cc:610
#3  0x000000000055cbfa in os_aio_simulated_handle (global_segment=5, message1=0x7fc16fb93d98, message2=0x7fc16fb93d90, type=0x7fc16fb93d88) at /mnt/xb-2.1-log-archive/mysql-5.6/storage/innobase/os/os0file.cc:5437
#4  0x00000000005704e3 in fil_aio_wait (segment=5) at /mnt/xb-2.1-log-archive/mysql-5.6/storage/innobase/fil/fil0fil.cc:5547
#5  0x000000000059e178 in io_handler_thread (arg=<value optimized out>) at /mnt/xb-2.1-log-archive/mysql-5.6/storage/innobase/srv/srv0start.cc:478
#6  0x0000003e51407851 in start_thread () from /lib64/libpthread.so.0
#7  0x0000003e50ce890d in clone () from /lib64/libc.so.6

Thread 5 (Thread 0x7fc16f193700 (LWP 1698)):
#0  0x0000003e5140b43c in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
#1  0x00000000005bd6d3 in os_cond_wait (event=0x26d3fd0, reset_sig_count=1400) at /mnt/xb-2.1-log-archive/mysql-5.6/storage/innobase/os/os0sync.cc:214
#2  os_event_wait_low (event=0x26d3fd0, reset_sig_count=1400) at /mnt/xb-2.1-log-archive/mysql-5.6/storage/innobase/os/os0sync.cc:610
#3  0x000000000055cbfa in os_aio_simulated_handle (global_segment=6, message1=0x7fc16f192d98, message2=0x7fc16f192d90, type=0x7fc16f192d88) at /mnt/xb-2.1-log-archive/mysql-5.6/storage/innobase/os/os0file.cc:5437
#4  0x00000000005704e3 in fil_aio_wait (segment=6) at /mnt/xb-2.1-log-archive/mysql-5.6/storage/innobase/fil/fil0fil.cc:5547
#5  0x000000000059e178 in io_handler_thread (arg=<value optimized out>) at /mnt/xb-2.1-log-archive/mysql-5.6/storage/innobase/srv/srv0start.cc:478
#6  0x0000003e51407851 in start_thread () from /lib64/libpthread.so.0
#7  0x0000003e50ce890d in clone () from /lib64/libc.so.6

Thread 4 (Thread 0x7fc16e792700 (LWP 1699)):
#0  0x0000003e5140b43c in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
#1  0x00000000005bd6d3 in os_cond_wait (event=0x26d4060, reset_sig_count=350) at /mnt/xb-2.1-log-archive/mysql-5.6/storage/innobase/os/os0sync.cc:214
#2  os_event_wait_low (event=0x26d4060, reset_sig_count=350) at /mnt/xb-2.1-log-archive/mysql-5.6/storage/innobase/os/os0sync.cc:610
#3  0x000000000055cbfa in os_aio_simulated_handle (global_segment=7, message1=0x7fc16e791d98, message2=0x7fc16e791d90, type=0x7fc16e791d88) at /mnt/xb-2.1-log-archive/mysql-5.6/storage/innobase/os/os0file.cc:5437
#4  0x00000000005704e3 in fil_aio_wait (segment=7) at /mnt/xb-2.1-log-archive/mysql-5.6/storage/innobase/fil/fil0fil.cc:5547
#5  0x000000000059e178 in io_handler_thread (arg=<value optimized out>) at /mnt/xb-2.1-log-archive/mysql-5.6/storage/innobase/srv/srv0start.cc:478
#6  0x0000003e51407851 in start_thread () from /lib64/libpthread.so.0
#7  0x0000003e50ce890d in clone () from /lib64/libc.so.6

Thread 3 (Thread 0x7fc16dd91700 (LWP 1700)):
#0  0x0000003e5140b43c in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
#1  0x00000000005bd6d3 in os_cond_wait (event=0x26d40f0, reset_sig_count=76) at /mnt/xb-2.1-log-archive/mysql-5.6/storage/innobase/os/os0sync.cc:214
#2  os_event_wait_low (event=0x26d40f0, reset_sig_count=76) at /mnt/xb-2.1-log-archive/mysql-5.6/storage/innobase/os/os0sync.cc:610
#3  0x000000000055cbfa in os_aio_simulated_handle (global_segment=8, message1=0x7fc16dd90d98, message2=0x7fc16dd90d90, type=0x7fc16dd90d88) at /mnt/xb-2.1-log-archive/mysql-5.6/storage/innobase/os/os0file.cc:5437
#4  0x00000000005704e3 in fil_aio_wait (segment=8) at /mnt/xb-2.1-log-archive/mysql-5.6/storage/innobase/fil/fil0fil.cc:5547
#5  0x000000000059e178 in io_handler_thread (arg=<value optimized out>) at /mnt/xb-2.1-log-archive/mysql-5.6/storage/innobase/srv/srv0start.cc:478
#6  0x0000003e51407851 in start_thread () from /lib64/libpthread.so.0
#7  0x0000003e50ce890d in clone () from /lib64/libc.so.6

Thread 2 (Thread 0x7fc16d390700 (LWP 1701)):
#0  0x0000003e5140b43c in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
#1  0x00000000005bd6d3 in os_cond_wait (event=0x26d4180, reset_sig_count=78) at /mnt/xb-2.1-log-archive/mysql-5.6/storage/innobase/os/os0sync.cc:214
#2  os_event_wait_low (event=0x26d4180, reset_sig_count=78) at /mnt/xb-2.1-log-archive/mysql-5.6/storage/innobase/os/os0sync.cc:610
#3  0x000000000055cbfa in os_aio_simulated_handle (global_segment=9, message1=0x7fc16d38fd98, message2=0x7fc16d38fd90, type=0x7fc16d38fd88) at /mnt/xb-2.1-log-archive/mysql-5.6/storage/innobase/os/os0file.cc:5437
#4  0x00000000005704e3 in fil_aio_wait (segment=9) at /mnt/xb-2.1-log-archive/mysql-5.6/storage/innobase/fil/fil0fil.cc:5547
#5  0x000000000059e178 in io_handler_thread (arg=<value optimized out>) at /mnt/xb-2.1-log-archive/mysql-5.6/storage/innobase/srv/srv0start.cc:478
#6  0x0000003e51407851 in start_thread () from /lib64/libpthread.so.0
#7  0x0000003e50ce890d in clone () from /lib64/libc.so.6

Thread 1 (Thread 0x7fc17d4b37e0 (LWP 1686)):
#0  0x0000003e50ce14f3 in select () from /lib64/libc.so.6
#1  0x000000000049e43b in os_thread_sleep (tm=<value optimized out>) at /mnt/xb-2.1-log-archive/mysql-5.6/storage/innobase/os/os0thread.cc:255
#2  0x00000000004fb2a0 in recv_apply_hashed_log_recs (allow_ibuf=0) at /mnt/xb-2.1-log-archive/mysql-5.6/storage/innobase/log/log0recv.cc:1941
#3  0x00000000004fc78f in recv_scan_log_recs (available_memory=88080384, store_to_hash=1, buf=<value optimized out>, len=<value optimized out>, start_lsn=<value optimized out>, contiguous_lsn=<value optimized out>, group_scanned_lsn=0x7fff4967c178)
    at /mnt/xb-2.1-log-archive/mysql-5.6/storage/innobase/log/log0recv.cc:2913
#4  0x00000000004feb66 in recv_group_scan_log_recs (type=78656949, limit_lsn=18446744073709551615, min_flushed_lsn=1625997, max_flushed_lsn=1625997) at /mnt/xb-2.1-log-archive/mysql-5.6/storage/innobase/log/log0recv.cc:2961
#5  recv_recovery_from_checkpoint_start_func (type=78656949, limit_lsn=18446744073709551615, min_flushed_lsn=1625997, max_flushed_lsn=1625997) at /mnt/xb-2.1-log-archive/mysql-5.6/storage/innobase/log/log0recv.cc:3224
#6  0x00000000005a24a1 in innobase_start_or_create_for_mysql () at /mnt/xb-2.1-log-archive/mysql-5.6/storage/innobase/srv/srv0start.cc:2312
#7  0x00000000004526bf in innodb_init () at xtrabackup.cc:1412
#8  xtrabackup_prepare_func () at xtrabackup.cc:5388
#9  0x0000000000454959 in main (argc=0, argv=0x24a33a0) at xtrabackup.cc:5948