Not really sure what to provide 
… But in gdb, I did:
(gdb) t 7 (from the pj_ioqueue_lock_key you are referring to)
(gdb) bt
#0 __futex_abstimed_wait_common64 (private=<optimized out>, cancel=<optimized out>, abstime=0x0, op=393, expected=0, futex_word=0x5588e09590d0)
at ./nptl/futex-internal.c:57
#1 __futex_abstimed_wait_common (futex_word=0x5588e09590d0, expected=0, clockid=<optimized out>, abstime=0x0, private=<optimized out>, cancel=<optimized out>)
at ./nptl/futex-internal.c:87
#2 0x00007f2b913d0d60 in __new_sem_wait_slow64 (sem=<optimized out>, abstime=0x0, clockid=0) at ./nptl/sem_waitcommon.c:223
#3 0x00005588d867643b in ast_sem_wait ()
#4 0x00005588d8676514 in default_tps_processing_function ()
#5 0x00005588d868ea5b in dummy_start ()
#6 0x00007f2b913c91c4 in rseq_register_current_thread (do_rseq=<optimized out>, self=0x7f2b8ea946c0) at ../sysdeps/unix/sysv/linux/rseq-internal.h:70
#7 start_thread (arg=<optimized out>) at ./nptl/pthread_create.c:373
#8 0x00007f2b9144985c in __closefrom_fallback (from=0, dirfd_fallback=<optimized out>) at ../sysdeps/unix/sysv/linux/closefrom_fallback.c:52
#9 0x0000000000000000 in ()
Let me know if you want to see something else!