Re: [PATCH bpf-next v1 08/10] bpf: Add bpf_task_work_schedule_* kfuncs with KF_IMPLICIT_ARGS

[Andrii Nakryiko]

On Mon, Jan 12, 2026 at 10:54 AM Ihor Solodrai <ihor.solodrai@xxxxxxxxx> wrote:
>
> On 1/9/26 1:56 PM, Ihor Solodrai wrote:
> > On 1/9/26 1:49 PM, Alexei Starovoitov wrote:
> >> On Fri, Jan 9, 2026 at 1:39 PM Ihor Solodrai <ihor.solodrai@xxxxxxxxx> wrote:
> >>>
> >>> [...]
> >>>
> >>>> I feel bpf_task_work_schedule_resume() is ok to break, since it's so new.
> >>>> We can remove bpf_task_work_schedule_[resume|singal]_impl()
> >>>> to avoid carrying forward forever.
> >>>>
> >>>> bpf_stream_vprintk_impl() is not that clear. I would remove it too.
> >>>
> >>> That leaves only bpf_wq_set_callback_impl(). Can we break that too?
> >>
> >> Sounds like Benjamin is ok removing it.
> >> So I think we can indeed remove them all.
> >>
> >>> Then there won't be legacy cases at all. It was introduced in v6.16
> >>> along the with __prog suffix [1][2].
> >>>
> >>> If we go this route, we could clean up __prog support/docs too.
> >>>
> >>> I think it's worth it to make an "all or nothing" decision here:
> >>> either break all 4 existing kfuncs, or backwards-support all of them.
> >>
> >> I don't see why "all or nothing" is a good thing.
> >> It won't be "all" anyway.
> >> We have bpf_rbtree_add_impl(), bpf_list_push_front_impl(), etc.
> >> And those we cannot remove. sched-ext is using them.
> >> Another few categories are bpf_obj_new_impl(), bpf_obj_drop_impl().
> >> There are not __prog type, but conceptually the same thing and
> >> KF_IMPLICIT_ARGS should support them too eventually.
> >
> > I was thinking we could remove/simplify code relevant to backwards
> > compat of existing _impl kfuncs. But you're right, if we start using
> > implicit args for other types/kfuncs, the "legacy" case still has to
> > work.
> >
> > Ok, in the next revision I'll remove all the __prog users, but leave
> > the "legacy" case support in place for future use.
>
> I just had an off-list chat with Andrii, and we agreed that leaving
> the existing _impl kfuncs supported may be a good idea.
>
> It doesn't cost us much: we keep the mechanism for legacy functions
> anyways, so supporting bpf_wq_set_callback_impl() and co only requires
> keeping definitions in the kernel.
>
> The only benefit of *removing* these _impl functions is that we could
> clean up __prog support.
>
> But having backwards compat seems like a better deal.
> What do you think?
>

I think that bit of __prog clean up might be worth doing after all.
The biggest difference for users if we remove _impl stuff from new
kernels would be an extra explicit xxx_impl kfunc declaration (it
won't come from vmlinux.h anymore) and then the following call pattern
(which probably will be hidden in the macro anyways):

if (xxx_impl)
    xxx_impl(..., NULL);
else
    xxx(...);


This will work on old and new kernels alike, so ok, let's just drop
all the _impl stuff. Can't do that for bpf_obj_new_impl() because
that's already used more widely, but for task_work, wq, stream -- they
are all fresh and not yet properly used in production.


>
> >
> >>
> >>
> >>> git tag --contains bc049387b41f | grep -v rc
> >>> v6.16
> >>> v6.17
> >>> v6.18
> >>>
> >>> [1] https://lore.kernel.org/all/20250513142812.1021591-1-memxor@xxxxxxxxx/
> >>> [2] https://lore.kernel.org/all/20240420-bpf_wq-v2-13-6c986a5a741f@xxxxxxxxxx/
> >>>
> >>>
> >
>