> Which part of the function is not async-signal safe? It doesn't interact
> with any file descriptors nor does it use any syscalls except for
> getpid() and time().

gettimeofday() and RAND_add()

The functions which are guaranteed to be async-signal safe are listed here:
https://www.securecoding.cert.org/confluence/display/seccode/SIG30-C.+Call+only+asynchronous-safe+functions+within+signal+handlers

And here's what David Butenhof says about pthread_atfork():
"""
The real answer is that pthread_atfork() is a completely useless and stupid
mechanism that was a well intentioned but ultimately pointless attempt to
carve a "back door" solution out of an inherently insoluable design
conflict.
"""

More here:
https://groups.google.com/forum/#!msg/comp.programming.threads/ThHE32-vRsg/3j-YICgSQzoJ

IMO it's really an openssl issue, and it should be fixed there (mainly
because they are the only one in position of setuping a safe
atfork-hook, like the glibc does for malloc & co).