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

[Jason Wang]

On 2018å02æ23æ 01:46, Jesper Dangaard Brouer wrote:
> On Thu, 22 Feb 2018 17:36:46 +0800
> Jason Wang <jasowang@xxxxxxxxxx> 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@xxxxxxxxxx>
> > Fixes: 762c330d670e ("tuntap: add missing xdp flush")
> > Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
> > ---
> >   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