On 2018年02月23日 01:46, Jesper Dangaard Brouer wrote:
On Thu, 22 Feb 2018 17:36:46 +0800 Jason Wang <[email protected]> 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 <[email protected]> Fixes: 762c330d670e ("tuntap: add missing xdp flush") Signed-off-by: Jason Wang <[email protected]> --- 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
