Re: [PATCH bpf-next] bpf: Use separate RCU callbacks for freeing selem

[Martin KaFai Lau <martin.lau@linux.dev>]

On 3/3/23 9:45 AM, Kumar Kartikeya Dwivedi wrote:
> On Fri, 3 Mar 2023 at 16:40, Martin KaFai Lau <martin.lau@linux.dev> wrote:
>>
>> On 3/3/23 6:15 AM, Kumar Kartikeya Dwivedi wrote:
>>> Martin suggested that instead of using a byte in the hole (which he has
>>> a use for in his future patch) in bpf_local_storage_elem, we can
>>> dispatch a different call_rcu callback based on whether we need to free
>>> special fields in bpf_local_storage_elem data. The free path, described
>>> in commit 9db44fdd8105 ("bpf: Support kptrs in local storage maps"),
>>> only waits for call_rcu callbacks when there are special (kptrs, etc.)
>>> fields in the map value, hence it is necessary that we only access
>>> smap in this case.
>>>
>>> Therefore, dispatch different RCU callbacks based on the BPF map has a
>>> valid btf_record, which dereference and use smap's btf_record only when
>>> it is valid.
>>>
>>> Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
>>
>> Thanks for your patch. I have already made a similar change in my local branch
>> which has some differences like refactored it a little for my work. The set is
>> almost ready. Do you mind if I include your patch in my set and keep your SOB?
>>
> 
> No problem, please do.

Please ignore my previous message. I will make some adjustments on my set.

Applied with some changes on de-referencing SDATA(selem)->smap to address these 
warnings:

# this is addressed by rcu_dereference_protected(..., true)
../kernel/bpf/bpf_local_storage.c:117:41: warning: dereference of noderef expression

# this is addressed by directly using the earlier dereferenced smap pointer
../kernel/bpf/bpf_local_storage.c:200:27: warning: dereference of noderef expression