hyc@symas.com writes:

...

slapd/cancel.c sets o_abandon before o_cancel. Thus it's possible for the canceled operation to obey o_abandon before o_cancel gets set.

Cancel is just best-effort, why not just set o_cancel before o_abandon?

Memory coherence. Without mutex protection, the cancelled thread might still see the o_abandon change before the o_cancel change. And thus not send a response, nor update info for the Cancel operation to react to.