On Thu, 22 Feb 2018 17:36:46 +0800
Jason Wang <jasow...@redhat.com> wrote:
> Commit 762c330d670e ("tuntap: add missing xdp flush") tries to fix the
> devmap stall caused by missed xdp flush by counting the pending xdp
> redirected packets and flush when it exceeds NAPI_POLL_WEIGHT or
> MSG_MORE is clear. This may lead to BUG() since xdp_do_flush() was
> called in the process context with preemption enabled. Simply
> disabling preemption may silence the warning but be not enough since
> process may move between different CPUS during a batch which cause
> xdp_do_flush() misses some CPU where the process run
> previously. Consider the fallouts, that commit was reverted. To fix
> the issue correctly, we can simply call xdp_do_flush() immediately
> after xdp_do_redirect(), a side effect is that this removes any
> possibility of batching which could be addressed in the future.
>
> Reported-by: Christoffer Dall <christoffer.d...@linaro.org>
> Fixes: 762c330d670e ("tuntap: add missing xdp flush")
> Signed-off-by: Jason Wang <jasow...@redhat.com>
> ---
> drivers/net/tun.c | 1 +
> 1 file changed, 1 insertion(+)
>
> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index 2823a4a..a363ea2 100644
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -1662,6 +1662,7 @@ static struct sk_buff *tun_build_skb(struct tun_struct
> *tun,
> get_page(alloc_frag->page);
> alloc_frag->offset += buflen;
> err = xdp_do_redirect(tun->dev, &xdp, xdp_prog);
> + xdp_do_flush_map();
> if (err)
> goto err_redirect;
> rcu_read_unlock();
As you have noticed, the xdp_do_redirect() + xdp_do_flush_map() rely
heavily on being executed in softirq/napi_schedule context.
Particularly the map infra devmap[1]+cpumap depend on the enqueue and
flush operation MUST happen on the same CPU (e.g. stores which
devices needs flushing in a this_cpu_ptr bitmap [1]).
What context is tun_build_skb() invoked under?
Even when you call xdp_do_redirect and xdp_do_flush_map right after
each-other, are we sure we cannot be preempted here?
[1] https://github.com/torvalds/linux/blob/master/kernel/bpf/devmap.c#L209-L215
--
Best regards,
Jesper Dangaard Brouer
MSc.CS, Principal Kernel Engineer at Red Hat
LinkedIn: http://www.linkedin.com/in/brouer
