On Tue, 1 Dec 2020 11:44:38 -0800 Cong Wang wrote:
> From: Dongdong Wang <wangdongd...@bytedance.com>
>
> The per-cpu bpf_redirect_info is shared among all skb_do_redirect()
> and BPF redirect helpers. Callers on RX path are all in BH context,
> disabling preemption is not sufficient to prevent BH interruption.
>
> In production, we observed strange packet drops because of the race
> condition between LWT xmit and TC ingress, and we verified this issue
> is fixed after we disable BH.
>
> Although this bug was technically introduced from the beginning, that
> is commit 3a0af8fd61f9 ("bpf: BPF for lightweight tunnel infrastructure"),
> at that time call_rcu() had to be call_rcu_bh() to match the RCU context.
> So this patch may not work well before RCU flavor consolidation has been
> completed around v5.0.
>
> Update the comments above the code too, as call_rcu() is now BH friendly.
>
> Cc: Thomas Graf <tg...@suug.ch>
> Cc: b...@vger.kernel.org
> Reviewed-by: Cong Wang <cong.w...@bytedance.com>
> Signed-off-by: Dongdong Wang <wangdongd...@bytedance.com>
> ---
> net/core/lwt_bpf.c | 8 ++++----
> 1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/net/core/lwt_bpf.c b/net/core/lwt_bpf.c
> index 7d3438215f32..4f3cb7c15ddf 100644
> --- a/net/core/lwt_bpf.c
> +++ b/net/core/lwt_bpf.c
> @@ -39,12 +39,11 @@ static int run_lwt_bpf(struct sk_buff *skb, struct
> bpf_lwt_prog *lwt,
> {
> int ret;
>
> - /* Preempt disable is needed to protect per-cpu redirect_info between
> - * BPF prog and skb_do_redirect(). The call_rcu in bpf_prog_put() and
> - * access to maps strictly require a rcu_read_lock() for protection,
> - * mixing with BH RCU lock doesn't work.
> + /* Preempt disable and BH disable are needed to protect per-cpu
> + * redirect_info between BPF prog and skb_do_redirect().
> */
> preempt_disable();
> + local_bh_disable();
Why not remove the preempt_disable()? Disabling BH must also disable
preemption AFAIK.
> bpf_compute_data_pointers(skb);
> ret = bpf_prog_run_save_cb(lwt->prog, skb);
>
> @@ -78,6 +77,7 @@ static int run_lwt_bpf(struct sk_buff *skb, struct
> bpf_lwt_prog *lwt,
> break;
> }
>
> + local_bh_enable();
> preempt_enable();
>
> return ret;
