After 12.x upgrade: mysqld RET _umtx_op -1 errno 45 Operation not supported

[Marcin Cieslak]

Hello,

for archival purposes I am running an ancient (__FreeBSD_version 602100) 
jail with mysqld inside.

This was working fine when the jail host was running 10.4. After upgrade to 12.x
(r345375) mysqld process starts but it hangs before PID is created and the
socket opened.

Quick ktrace on the process shows this coming up all the time:

 50766 mysqld   CALL  _umtx_op(0x966eaa0,UMTX_OP_RESERVED0,0x18cab,0,0)
 50766 mysqld   RET   _umtx_op -1 errno 45 Operation not supported

What has changed, can I somehow bring it up again? (I understood that
we keep compatibility with very old userland).

Marcin