On Thu, Jan 13, 2022 at 9:22 PM Kumar Kartikeya Dwivedi <memxor@xxxxxxxxx> wrote: > > On Fri, Jan 14, 2022 at 10:19:50AM IST, Kumar Kartikeya Dwivedi wrote: > > On Fri, Jan 14, 2022 at 04:02:11AM IST, Alexei Starovoitov wrote: > > > On Tue, Jan 11, 2022 at 11:34:20PM +0530, Kumar Kartikeya Dwivedi wrote: > > > > [...] > > > > + /* Make sure all updates are visible before we go to MODULE_STATE_LIVE, > > > > + * pairs with smp_rmb in btf_try_get_module (for success case). > > > > + * > > > > + * btf_populate_kfunc_set(...) > > > > + * smp_wmb() <-----------. > > > > + * mod->state = LIVE | if (mod->state == LIVE) > > > > + * | atomic_inc_nz(mod) > > > > + * `---------> smp_rmb() > > > > + * btf_kfunc_id_set_contains(...) > > > > + */ > > > > + smp_wmb(); > > > > > > This comment somehow implies that mod->state = LIVE > > > and if (mod->state == LIVE && try_mod_get) can race. > > > That's not the case. > > > The patch 1 closed the race. > > > btf_kfunc_id_set_contains() will be called only on LIVE modules. > > > At that point all __init funcs of the module including register_btf_kfunc_id_set() > > > have completed. > > > This smp_wmb/rmb pair serves no purpose. > > > Unless I'm missing something? > > > > > > > Right, I'm no expert on memory ordering, but even if we closed the race, to me > > there seems to be no reason why the CPU cannot reorder the stores to tab (or its > > hook/type slot) with mod->state = LIVE store. > > > > Usually, the visibility is handled by whatever lock is used to register the > > module somewhere in some subsystem, as the next acquirer can see all updates > > from the previous registration. > > > > In this case, we're directly assigning a pointer without holding any locks etc. > > While it won't be concurrently accessed until module state is LIVE, it is > > necessary to make all updates visible in the right order (that is, once state is > > LIVE, everything stored previously in struct btf for module is also visible). > > > > Once mod->state = LIVE is visible, we will start accessing kfunc_set_tab, but if > > previous stores to it were not visible by then, we'll access a badly-formed > > kfunc_set_tab. > > > > For this particular case, you can think of mod->state = LIVE acting as a release > > store, and the read for mod->state == LIVE acting as an acquire load. > > > > Also, to be more precise, we're now synchronizing using btf_mod->flags, not > mod->state, so I should atleast update the comment, but the idea is the same. So the concern is that cpu can execute mod->state = MODULE_STATE_LIVE; from kernel/module.c earlier than stores inside __btf_populate_kfunc_set that are called from do_one_initcall() couple lines earlier in kernel/module.c ? Let's assume cpu is not Intel, since Intel never reorders stores. (as far as I remember the only weak store ordering architecture ever produced is Alpha). But it's not mod->state, it's btf_mod->flags (as you said) which is done under btf_module_mutex. and btf_kfunc_id_set_contains() can only do that after btf_try_get_module() succeeds which is under the same btf_module_mutex. So what is the race ? I think it's important to be concerned about race conditions, but they gotta be real. So please spell out a precise scenario if you still believe it's there.
