By default, Squid connects directly to origin servers for SSL requests. But if you must force SSL requests through a parent, first tell Squid it can not go direct for SSL:
acl SSL method CONNECT
never_direct allow SSL
With this in place, Squid should pick one of your parents to
use for SSL requests. If you want it to pick a particular parent,
you must use the cache_host_acl configuration:
cache_peer parent1 parent 3128 3130
cache_peer parent2 parent 3128 3130
cache_host_acl parent2 !SSL
The above lines tell Squid to NOT use parent2 for SSL, so it
should always use parent1.
It is a know limitation when using Async I/O on Linux. The Linux Threads package steals (uses internally) the SIGUSR1 signal that squid uses to rotate logs.
In order to not disturb the threads package SIGUSR1 use is disabled in Squid when threads is enabled on Linux.