On Tue, Feb 13, 2024 at 08:51:26PM +0100, Toke Høiland-Jørgensen wrote: > Kumar Kartikeya Dwivedi <memxor@xxxxxxxxx> writes: > > > On Tue, 13 Feb 2024 at 18:46, Benjamin Tissoires <bentiss@xxxxxxxxxx> wrote: > >> > >> On Feb 12 2024, Alexei Starovoitov wrote: > >> > On Mon, Feb 12, 2024 at 10:21 AM Benjamin Tissoires > >> > <benjamin.tissoires@xxxxxxxxxx> wrote: > >> > > > >> > > On Mon, Feb 12, 2024 at 6:46 PM Toke Høiland-Jørgensen <toke@xxxxxxxxxx> wrote: > >> > > > > >> > > > Benjamin Tissoires <benjamin.tissoires@xxxxxxxxxx> writes: > >> > > > > >> [...] > >> > I agree that workqueue delegation fits into the bpf_timer concept and > >> > a lot of code can and should be shared. > >> > >> Thanks Alexei for the detailed answer. I've given it an attempt but still can not > >> figure it out entirely. > >> > >> > All the lessons(bugs) learned with bpf_timer don't need to be re-discovered :) > >> > Too bad, bpf_timer_set_callback() doesn't have a flag argument, > >> > so we need a new kfunc to set a sleepable callback. > >> > Maybe > >> > bpf_timer_set_sleepable_cb() ? > >> > >> OK. So I guess I should drop Toke's suggestion with the bpf_timer_ini() flag? > >> > >> > The verifier will set is_async_cb = true for it (like it does for regular cb-s). > >> > And since prog->aux->sleepable is kinda "global" we need another > >> > per subprog flag: > >> > bool is_sleepable: 1; > >> > >> done (in push_callback_call()) > >> > >> > > >> > We can factor out a check "if (prog->aux->sleepable)" into a helper > >> > that will check that "global" flag and another env->cur_state->in_sleepable > >> > flag that will work similar to active_rcu_lock. > >> > >> done (I think), cf patch 2 below > >> > >> > Once the verifier starts processing subprog->is_sleepable > >> > it will set cur_state->in_sleepable = true; > >> > to make all subprogs called from that cb to be recognized as sleepable too. > >> > >> That's the point I don't know where to put the new code. > >> > > > > I think that would go in the already existing special case for > > push_async_cb where you get the verifier state of the async callback. > > You can make setting the boolean in that verifier state conditional on > > whether it's your kfunc/helper you're processing taking a sleepable > > callback. > > > >> It seems the best place would be in do_check(), but I am under the impression > >> that the code of the callback is added at the end of the instruction list, meaning > >> that I do not know where it starts, and which subprog index it corresponds to. > >> > >> > > >> > A bit of a challenge is what to do with global subprogs, > >> > since they're verified lazily. They can be called from > >> > sleepable and non-sleepable contex. Should be solvable. > >> > >> I must confess this is way over me (and given that I didn't even managed to make > >> the "easy" case working, that might explain things a little :-P ) > >> > > > > I think it will be solvable but made somewhat difficult by the fact > > that even if we mark subprog_info of some global_func A as > > in_sleepable, so that we explore it as sleepable during its > > verification, we might encounter later another global_func that calls > > a global func, already explored as non-sleepable, in sleepable > > context. In this case I think we need to redo the verification of that > > global func as sleepable once again. It could be that it is called > > from both non-sleepable and sleepable contexts, so both paths > > (in_sleepable = true, and in_sleepable = false) need to be explored, > > or we could reject such cases, but it might be a little restrictive. > > > > Some common helper global func unrelated to caller context doing some > > auxiliary work, called from sleepable timer callback and normal main > > subprog might be an example where rejection will be prohibitive. > > > > An approach might be to explore main and global subprogs once as we do > > now, and then keep a list of global subprogs that need to be revisited > > as in_sleepable (due to being called from a sleepable context) and > > trigger do_check_common for them again, this might have to be repeated > > as the list grows on each iteration, but eventually we will have > > explored all of them as in_sleepable if need be, and the loop will > > end. Surely, this trades off logical simplicity of verifier code with > > redoing verification of global subprogs again. > > > > To add items to such a list, for each global subprog we encounter that > > needs to be analyzed as in_sleepable, we will also collect all its > > callee global subprogs by walking its instructions (a bit like > > check_max_stack_depth does). > > Sorry if I'm being dense, but why is all this needed if it's already > possible to just define the timer callback from a program type that > allows sleeping, and then set the actual timeout from a different > program that is not sleepable? Isn't the set_sleepable_cb() kfunc just a > convenience then? Or did I misunderstand and it's not actually possible > to mix callback/timer arming from different program types? More than just convience. bpf_set_sleepable_cb() might need to be called from non-sleepable and there could be no way to hack it around with fake sleepable entry. bpf_timer_cancel() clears callback_fn. So if prog wants to bpf_timer_start() and later bpf_timer_cancel() it would need to bpf_set_sleepable_cb() every time before bpf_timer_start(). And at that time it might be in non-sleepable ctx.
