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Date:   Wed, 2 Sep 2020 18:25:57 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Yonghong Song <yhs@...com>
Cc:     bpf <bpf@...r.kernel.org>, Lorenz Bauer <lmb@...udflare.com>,
        Martin KaFai Lau <kafai@...com>,
        Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH bpf 1/2] bpf: do not use bucket_lock for hashmap iterator

On Wed, Sep 2, 2020 at 4:56 PM Yonghong Song <yhs@...com> wrote:
>
> Currently, for hashmap, the bpf iterator will grab a bucket lock, a
> spinlock, before traversing the elements in the bucket. This can ensure
> all bpf visted elements are valid. But this mechanism may cause
> deadlock if update/deletion happens to the same bucket of the
> visited map in the program. For example, if we added bpf_map_update_elem()
> call to the same visited element in selftests bpf_iter_bpf_hash_map.c,
> we will have the following deadlock:
>

[...]

>
> Compared to old bucket_lock mechanism, if concurrent updata/delete happens,
> we may visit stale elements, miss some elements, or repeat some elements.
> I think this is a reasonable compromise. For users wanting to avoid

I agree, the only reliable way to iterate map without duplicates and
missed elements is to not update that map during iteration (unless we
start supporting point-in-time snapshots, which is a very different
matter).


> stale, missing/repeated accesses, bpf_map batch access syscall interface
> can be used.
>
> Signed-off-by: Yonghong Song <yhs@...com>
> ---
>  kernel/bpf/hashtab.c | 15 ++++-----------
>  1 file changed, 4 insertions(+), 11 deletions(-)
>
> diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c
> index 78dfff6a501b..7df28a45c66b 100644
> --- a/kernel/bpf/hashtab.c
> +++ b/kernel/bpf/hashtab.c
> @@ -1622,7 +1622,6 @@ struct bpf_iter_seq_hash_map_info {
>         struct bpf_map *map;
>         struct bpf_htab *htab;
>         void *percpu_value_buf; // non-zero means percpu hash
> -       unsigned long flags;
>         u32 bucket_id;
>         u32 skip_elems;
>  };
> @@ -1632,7 +1631,6 @@ bpf_hash_map_seq_find_next(struct bpf_iter_seq_hash_map_info *info,
>                            struct htab_elem *prev_elem)
>  {
>         const struct bpf_htab *htab = info->htab;
> -       unsigned long flags = info->flags;
>         u32 skip_elems = info->skip_elems;
>         u32 bucket_id = info->bucket_id;
>         struct hlist_nulls_head *head;
> @@ -1656,19 +1654,18 @@ bpf_hash_map_seq_find_next(struct bpf_iter_seq_hash_map_info *info,
>
>                 /* not found, unlock and go to the next bucket */
>                 b = &htab->buckets[bucket_id++];
> -               htab_unlock_bucket(htab, b, flags);
> +               rcu_read_unlock();

Just double checking as I don't yet completely understand all the
sleepable BPF implications. If the map is used from a sleepable BPF
program, we are still ok doing just rcu_read_lock/rcu_read_unlock when
accessing BPF map elements, right? No need for extra
rcu_read_lock_trace/rcu_read_unlock_trace?

>                 skip_elems = 0;
>         }
>

[...]

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