From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx3-rdu2.redhat.com ([66.187.233.73]:48524 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1750916AbeBWB7t (ORCPT ); Thu, 22 Feb 2018 20:59:49 -0500 Subject: Re: [PATCH net v3 2/2] tuntap: correctly add the missing xdp flush To: Jesper Dangaard Brouer Cc: netdev@vger.kernel.org, linux-kernel@vger.kernel.org, mst@redhat.com, christoffer.dall@linaro.org, sergei.shtylyov@cogentembedded.com References: <1519292206-6384-1-git-send-email-jasowang@redhat.com> <1519292206-6384-2-git-send-email-jasowang@redhat.com> <20180222184647.0e1d6631@redhat.com> From: Jason Wang Message-ID: <6b0a7fac-1b4a-c621-e391-b5e763f007f7@redhat.com> Date: Fri, 23 Feb 2018 09:59:41 +0800 MIME-Version: 1.0 In-Reply-To: <20180222184647.0e1d6631@redhat.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US Sender: netdev-owner@vger.kernel.org List-ID: On 2018年02月23日 01:46, Jesper Dangaard Brouer wrote: > On Thu, 22 Feb 2018 17:36:46 +0800 > Jason Wang 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 >> Fixes: 762c330d670e ("tuntap: add missing xdp flush") >> Signed-off-by: Jason Wang >> --- >> 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