Messages in this thread | | | Date | Thu, 22 Feb 2018 18:46:47 +0100 | From | Jesper Dangaard Brouer <> | Subject | Re: [PATCH net v3 2/2] tuntap: correctly add the missing xdp flush |
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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?
[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
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