init_worker_by_lua workers stuck on gc_traverse_tab

Howdy.
Anyone ever encountered something like this?
It's not something I can reproduce unfortunately but I encountrred it several timer now.

#0  0x00007f12771747ae in gc_traverse_tab (t=0x7edcdddc5a28, g=0x7f1276be73f0) at lj_gc.c:217
#1  propagatemark (g=0x7f1276be73f0) at lj_gc.c:333
#2  0x00007f1277175798 in lj_gc_fullgc (L=0x7f1276be7380) at lj_gc.c:804
#3  0x00007f127718763d in lua_gc (L=L@entry=0x7f1276be7380, what=what@entry=2, data=data@entry=0) at lj_api.c:1300
#4  0x00005625f14a8e57 in ngx_http_lua_report (log=<optimized out>, L=0x7f1276be7380, status=<optimized out>, prefix=<optimized out>) at gcore/modules/ngx_http_lua_module/src/ngx_http_lua_util.c:4514
#5  0x00005625f14c16fe in ngx_http_lua_init_worker (cycle=0x56285cd1e058) at gcore/modules/ngx_http_lua_module/src/ngx_http_lua_initworkerby.c:298
#6  0x00005625f13ac05e in ngx_worker_process_init (cycle=cycle@entry=0x56285cd1e058, worker=worker@entry=1) at src/os/unix/ngx_process_cycle.c:1150
#7  0x00005625f13ac59d in ngx_worker_process_cycle (cycle=cycle@entry=0x56285cd1e058, data=data@entry=0x1) at src/os/unix/ngx_process_cycle.c:922
#8  0x00005625f13aa5d5 in ngx_spawn_process (cycle=cycle@entry=0x56285cd1e058, proc=proc@entry=0x5625f13ac560 <ngx_worker_process_cycle>, data=data@entry=0x1, name=name@entry=0x5625f188ddbe "worker process", respawn=respawn@entry=-4) at src/os/unix/ngx_process.c:203
#9  0x00005625f13abc48 in ngx_start_worker_processes (cycle=cycle@entry=0x56285cd1e058, n=20, type=type@entry=-4) at src/os/unix/ngx_process_cycle.c:533
#10 0x00005625f13ae2cc in ngx_master_process_cycle (cycle=0x56285cd1e058) at src/os/unix/ngx_process_cycle.c:419
#11 0x00005625f13796da in main (argc=<optimized out>, argv=<optimized out>) at src/core/nginx.c:489
[Inferior 1 (process 2740) detached]

Start by reading ngx_http_lua_init_worker in gcore/modules/ngx_http_lua_module/src/ngx_http_lua_initworkerby.c and the ngx_http_lua_report call in ngx_http_lua_util.c, then compare them with the LuaJIT frames in lj_gc.c and lj_api.c. Attempt to reproduce the worker stall around init_worker_by_lua; done means the cause is identified and a documented next step is available.