I'm proposing an API to restore the functionality to resume or suspend a specified thread from execution.
This interface was implemented in the past in user-space inside pthread(3) with the M:N thread model (with help from removed pthread_dbg). http://netbsd.org/~kamil/patch-00028-pt_suspend-pt_resume.txt This code is close to FreeBSD and shares the same request names (PT_RESUME and PT_SUSPEND), however on NetBSD we pass the full pair of tracee's pid_t and thread's lwpid_t. FreeBSD specifies just thread ID, which is insufficient on NetBSD, as a single tracer can control multiple tracees and face duplicated lwpid_t. I've added an interface to detect if a specific LWP has been suspended (or not) with extending the PT_LWPINFO interface with a new pl_event value PL_EVENT_SUSPENDED (next to PL_EVENT_NONE and PL_EVENT_SIGNAL). There is a new check preventing deadlocks and ptrace(2) can set with this patch new errno EDEADLK. I haven't checked the existing code but it appears that we can deadlock tracee with current PT_CONTINUE and friends.