On Thu, Feb 21, 2019 at 03:54:10PM +0100, Sylvain Munaut wrote: This should be perfectly fine, and the exact same approach has fixed other sequence-of-events problems earlier: transition the state first, and then take actions. Re deferring events: We can't globally change events to deferred, exactly because of the reasons mentioned: - passing along a data pointer to a local variable. - returning error code if transition is not allowed (evaluated in rare cases). Also, in general, it is safer to use the "more deterministic" way of immediate event triggering. If events get stored in sequence, maybe some other message that gets rx'd in-between (maybe it already was in the select() queue before we added events to be triggered) gets handled in some intermediate state in-between action and effect; that opens up variations in code paths that would otherwise be guaranteed to always happen in exactly identical sequence. It could still be useful to have some defer() API. I recently faced the need for a deferred action. My particular scenario is: a child object tells its parent that it is gone from its FSM cleanup implementation, and the parent deallocates; which also free()s the child. In pseudo code: struct main_obj { struct child_obj *child; }; struct child_obj { struct osmo_fsm_inst *fi; struct main_obj *parent; }; child_obj_alloc(struct main_obj *parent) { /* our common "FSM instance owns object" scheme: */ fi = osmo_fsm_inst_alloc(talloc_ctx = parent); child = talloc_zero(talloc_ctx = fi, struct child_obj); child->fi = fi; parent->child = child; } child_obj_fsm_cleanup(struct child_obj *child) { main_obj_remove_child(child) } main_obj_remove_child(main) { /* Without children, I have no purpose. Bye. */ talloc_free(main); } --> heap use after free in osmo_fsm_inst_term(child->fi), after calling child->fi->cleanup(). The point is that the child is allocated as a talloc child of the main object, and the main object is wired to deallocate itself when no children are left. When I call osmo_fsm_inst_term() on the child, its cleanup function tells the parent that it is gone, and the parent completely deallocates itself, still within the cleanup code path. But the osmo_fsm_inst code then continues to talloc_free() the child->fi after it called the cleanup function -- though the entire object was already freed in the cleanup function. (The actual case in osmo-msc:neels/ho: the msc subscriber struct having MSC-{A,I,T} roles, and as soon as the MSC-A role is gone, or there is neither an MSC-I nor MSC-T role, the entire subscriber must be released.) The solution I picked in the end was to also add an FSM instance to the main_obj, because if the child FSM is an osmo_fsm_inst_alloc_child(), it will trigger an event to the parent *after* all other handling is done. So the entire main_obj FSM instance only exists to receive the parent_term event after the child FSM is deallocated, instead of being notified of a released child already in the cleanup function. I considered solving like this instead: child_obj_fsm_cleanup(struct child_obj *child) { defer( main_obj_remove_child, child->parent ); } where defer could be implemented as an osmo_timer with zero delay. Triggering events could work in the same way, i.e. the defer implementation does not have to be osmo_fsm specific. But also, in this instance, it is quite possible that something else happens in-between the child deallocation and the deferred event handling. If some msg gets handled while the parent still points at the child, that would pose a use-after-free risk. Of course we could set parent->child = NULL in the cleanup callback and defer the deallocation ... but the point being that deferring actions opens up non-determinisms that can be non-trivial to handle well. Having a separate full FSM instance might be a bit of overkill, but has the benefit of allocation/deallocation and event logging. ~N