Re: [PATCH net v3 2/2] tuntap: correctly add the missing xdp flush

[Jason Wang <jasowang@redhat.com>]

On 2018年02月23日 01:46, Jesper Dangaard Brouer wrote:
> On Thu, 22 Feb 2018 17:36:46 +0800
> Jason Wang <jasowang@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.dall@linaro.org>
>> Fixes: 762c330d670e ("tuntap: add missing xdp flush")
>> Signed-off-by: Jason Wang <jasowang@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?

Ok, I miss the fact that we can be preempted here with preemptible RCU. 
Let me disable preemption here and post a V4.

Thanks

>
>
> [1] https://github.com/torvalds/linux/blob/master/kernel/bpf/devmap.c#L209-L215