netdev - Re: [PATCH bpf-next] bpf: move rcu lock management out of BPF_PROG

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Date:   Thu, 14 Apr 2022 00:31:07 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Stanislav Fomichev <sdf@...gle.com>
Cc:     Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>
Subject: Re: [PATCH bpf-next] bpf: move rcu lock management out of
 BPF_PROG_RUN routines

On 4/13/22 9:52 PM, Andrii Nakryiko wrote:
> On Wed, Apr 13, 2022 at 12:39 PM <sdf@...gle.com> wrote:
>> On 04/13, Andrii Nakryiko wrote:
>>> On Wed, Apr 13, 2022 at 11:33 AM Stanislav Fomichev <sdf@...gle.com>
>>> 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.
> 
> If you think that having this more explicitly is better, I'm fine with
> that as well. I thought a simpler invocation pattern would be good,
> given we call bpf_prog_run_array variants in quite a lot of places. So
> count me indifferent. I'm curious what others think.

+1 for explicit, might also be easier to review/audit compared to hidden
in macro.