Re: [External] Re: [PATCH bpf-next] bpf: add bpf_map_lookup_percpu_elem for percpu map

[Alexei Starovoitov]

On Mon, May 9, 2022 at 7:41 PM Feng Zhou <zhoufeng.zf@xxxxxxxxxxxxx> wrote:
>
> 在 2022/5/10 上午9:04, Yosry Ahmed 写道:
> > On Mon, May 9, 2022 at 5:34 PM Andrii Nakryiko
> > <andrii.nakryiko@xxxxxxxxx> wrote:
> >> On Fri, May 6, 2022 at 7:49 PM Feng zhou <zhoufeng.zf@xxxxxxxxxxxxx> wrote:
> >>> From: Feng Zhou <zhoufeng.zf@xxxxxxxxxxxxx>
> >>>
> >>> Trace some functions, such as enqueue_task_fair, need to access the
> >>> corresponding cpu, not the current cpu, and bpf_map_lookup_elem percpu map
> >>> cannot do it. So add bpf_map_lookup_percpu_elem to accomplish it for
> >>> percpu_array_map, percpu_hash_map, lru_percpu_hash_map.
> >>>
> >>> The implementation method is relatively simple, refer to the implementation
> >>> method of map_lookup_elem of percpu map, increase the parameters of cpu, and
> >>> obtain it according to the specified cpu.
> >>>
> >> I don't think it's safe in general to access per-cpu data from another
> >> CPU. I'd suggest just having either a ARRAY_OF_MAPS or adding CPU ID
> >> as part of the key, if you need such a custom access pattern.
> > I actually just sent an RFC patch series containing a similar patch
> > for the exact same purpose. There are instances in the kernel where
> > per-cpu data is accessed from other cpus (e.g.
> > mem_cgroup_css_rstat_flush()). I believe, like any other variable,
> > percpu data can be safe or not safe to access, based on the access
> > pattern. It is up to the user to coordinate accesses to the variable.
> >
> > For example, in my use case, one of the accessors only reads percpu
> > values of different cpus, so it should be safe. If a user accesses
> > percpu data of another cpu without guaranteeing safety, they corrupt
> > their own data. I understand that the main purpose of percpu data is
> > lockless (and therefore fast) access, but in some use cases the user
> > may be able to safely (and locklessly) access the data concurrently.
> >
>
> Regarding data security, I think users need to consider before using it,
> such
> as hook enqueue_task_fair, the function itself takes the rq lock of the
> corresponding cpu, there is no problem, and the kernel only provides a
> method,
> like bpf_per_cpu_ptr and bpf_this_cpu_ptr, data security needs to be
> guaranteed
> by users in different scenarios, such as using bpf_spin_lock.

Right. The new helper looks useful and is safe.
Please add a selftest and respin.