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.
