On 04/13, Andrii Nakryiko wrote:
On Wed, Apr 13, 2022 at 11:33 AM Stanislav Fomichev <sdf@xxxxxxxxxx>
wrote:
>
> Commit 7d08c2c91171 ("bpf: Refactor BPF_PROG_RUN_ARRAY family of macros
> into functions") switched a bunch of BPF_PROG_RUN macros to inline
> routines. This changed the semantic a bit. Due to arguments expansion
> of macros, it used to be:
>
> rcu_read_lock();
> array = rcu_dereference(cgrp->bpf.effective[atype]);
> ...
>
> Now, with with inline routines, we have:
> array_rcu = rcu_dereference(cgrp->bpf.effective[atype]);
> /* array_rcu can be kfree'd here */
> rcu_read_lock();
> array = rcu_dereference(array_rcu);
>
So subtle difference, wow...
But this open-coding of rcu_read_lock() seems very unfortunate as
well. Would making BPF_PROG_RUN_ARRAY back to a macro which only does
rcu lock/unlock and grabs effective array and then calls static inline
function be a viable solution?
#define BPF_PROG_RUN_ARRAY_CG_FLAGS(array_rcu, ctx, run_prog, ret_flags) \
({
int ret;
rcu_read_lock();
ret =
__BPF_PROG_RUN_ARRAY_CG_FLAGS(rcu_dereference(array_rcu), ....);
rcu_read_unlock();
ret;
})
where __BPF_PROG_RUN_ARRAY_CG_FLAGS is what
BPF_PROG_RUN_ARRAY_CG_FLAGS is today but with __rcu annotation dropped
(and no internal rcu stuff)?
Yeah, that should work. But why do you think it's better to hide them?
I find those automatic rcu locks deep in the call stack a bit obscure
(when reasoning about sleepable vs non-sleepable contexts/bpf).
I, as the caller, know that the effective array is rcu-managed (it
has __rcu annotation) and it seems natural for me to grab rcu lock
while work with it; I might grab it for some other things like cgroup
anyway.
