I was upgrading Rails to 6.0.3.6 from 6.0.3.4 on my Raspberry Pi, after bundle update, ffi gem updated to version 1.15.
But it encounters:

symbol lookup error: ***/gems/ffi-1.15.0/lib/ffi_c.so: undefined symbol: pthread_atfork

After I downgrade ffi gem to 1.14.2 (specify it in Gemfile), the problem disappeared.

I'll leave this issue closed, just in case anyone finds the same problem.