Re: [PATCH bpf-next v3] bpf: using rcu_read_lock for bpf_sk_storage_map iterator

[Martin KaFai Lau]

On Tue, Sep 15, 2020 at 11:16:49PM -0700, Yonghong Song wrote:
[ ... ]

> diff --git a/net/core/bpf_sk_storage.c b/net/core/bpf_sk_storage.c
> index 4a86ea34f29e..d43c3d6d0693 100644
> --- a/net/core/bpf_sk_storage.c
> +++ b/net/core/bpf_sk_storage.c
> @@ -678,6 +678,7 @@ struct bpf_iter_seq_sk_storage_map_info {
>  static struct bpf_local_storage_elem *
>  bpf_sk_storage_map_seq_find_next(struct bpf_iter_seq_sk_storage_map_info 
> *info,
>                                struct bpf_local_storage_elem *prev_selem)
> +     __acquires(RCU) __releases(RCU)
>  {
>       struct bpf_local_storage *sk_storage;
>       struct bpf_local_storage_elem *selem;
In the while loop earlier in this function, if I read it correctly,
it is sort of continuing the earlier hlist_for_each_entry_rcu() for the
same bucket, so the hlist_entry_safe() needs to be changed also.
Something like this (uncompiled code):

        while (selem) {
-               selem = hlist_entry_safe(selem->map_node.next,
+               selem = 
hlist_entry_safe(rcu_dereference(hlist_next_rcu(&selem->map_node)),
                                         struct bpf_local_storage_elem, 
map_node);
                if (!selem) {
                        /* not found, unlock and go to the next bucket */

> @@ -701,11 +702,11 @@ bpf_sk_storage_map_seq_find_next(struct 
> bpf_iter_seq_sk_storage_map_info *info,
>               if (!selem) {
>                       /* not found, unlock and go to the next bucket */
>                       b = &smap->buckets[bucket_id++];
> -                     raw_spin_unlock_bh(&b->lock);
> +                     rcu_read_unlock();
>                       skip_elems = 0;
>                       break;
>               }
> -             sk_storage = rcu_dereference_raw(selem->local_storage);
> +             sk_storage = rcu_dereference(selem->local_storage);
>               if (sk_storage) {
>                       info->skip_elems = skip_elems + count;
>                       return selem;