Re: Possible race with xsk_flush

Shawn Bohrer
2022-12-16T16:48:25Z

On Fri, Dec 16, 2022 at 11:05:19AM +0100, Magnus Karlsson wrote:
> To summarize, we are expecting this ordering:
>
> CPU 0 __xsk_rcv_zc()
> CPU 0 __xsk_map_flush()
> CPU 2 __xsk_rcv_zc()
> CPU 2 __xsk_map_flush()
>
> But we are seeing this order:
>
> CPU 0 __xsk_rcv_zc()
> CPU 2 __xsk_rcv_zc()
> CPU 0 __xsk_map_flush()
> CPU 2 __xsk_map_flush()

Yes exactly, and I think I've proved that this really is the order,
and the race is occurring. See my cookie/poisoning below.

> Here is the veth NAPI poll loop:
>
> static int veth_poll(struct napi_struct *napi, int budget)
> {
> struct veth_rq *rq =
> container_of(napi, struct veth_rq, xdp_napi);
> struct veth_stats stats = {};
> struct veth_xdp_tx_bq bq;
> int done;
>
> bq.count = 0;
>
> xdp_set_return_frame_no_direct();
> done = veth_xdp_rcv(rq, budget, &bq, &stats);
>
> if (done < budget && napi_complete_done(napi, done)) {
> /* Write rx_notify_masked before reading ptr_ring */
> smp_store_mb(rq->rx_notify_masked, false);
> if (unlikely(!__ptr_ring_empty(&rq->xdp_ring))) {
> if (napi_schedule_prep(&rq->xdp_napi)) {
> WRITE_ONCE(rq->rx_notify_masked, true);
> __napi_schedule(&rq->xdp_napi);
> }
> }
> }
>
> if (stats.xdp_tx > 0)
> veth_xdp_flush(rq, &bq);
> if (stats.xdp_redirect > 0)
> xdp_do_flush();
> xdp_clear_return_frame_no_direct();
>
> return done;
> }
>
> Something I have never seen before is that there is
> napi_complete_done() and a __napi_schedule() before xdp_do_flush().
> Let us check if this has something to do with it. So two suggestions
> to be executed separately:
>
> * Put a probe at the __napi_schedule() above and check if it gets
> triggered before this problem
> * Move the "if (stats.xdp_redirect > 0) xdp_do_flush();" to just
> before "if (done < budget && napi_complete_done(napi, done)) {"
>
> This might provide us some hints on what is going on.

Excellent observation, I haven't really looked at what
napi_complete_done() does yet. I did notice it could call
__napi_schedule() and that seemed like it might be fine. I'll also
note that veth_xdp_flush() can also ultimately call __napi_schedule().
I'll see what I can do to explore these ideas.

> > I've additionally updated my application to put a bad "cookie"
> > descriptor address back in the RX ring before updating the consumer
> > pointer. My hope is that if we then ever receive that cookie it
> > proves the kernel raced and failed to update the correct address.

I guess this is more like poisoning the old descriptors rather than a
cookie. This ran last night and one of my machines read back my
0xdeadbeefdeadbeef poisoned cookie value:

iperf2-125483 [003] d.Z1. 792878.867088: __xsk_rcv_zc_L7: (__xsk_rcv_zc+0xa7/0x250) addr=0x8d4900 len=0x42 xs=0xffff8bbc542a5000 fq=0xffff8bbc1c464e40
iperf2-125483 [003] d.Z1. 792878.867093: xsk_flush: (__xsk_map_flush+0x4e/0x180) xs=0xffff8bbc542a5000
iperf2-125491 [001] d.Z1. 792878.867219: __xsk_rcv_zc_L7: (__xsk_rcv_zc+0xa7/0x250) addr=0xc79900 len=0x42 xs=0xffff8bbc542a5000 fq=0xffff8bbc1c464e40
iperf2-125491 [001] d.Z1. 792878.867229: xsk_flush: (__xsk_map_flush+0x4e/0x180) xs=0xffff8bbc542a5000
iperf2-125491 [001] d.Z1. 792878.867291: __xsk_rcv_zc_L7: (__xsk_rcv_zc+0xa7/0x250) addr=0x18e1900 len=0x42 xs=0xffff8bbc542a5000 fq=0xffff8bbc1c464e40
iperf2-125483 [003] d.Z1. 792878.867441: __xsk_rcv_zc_L7: (__xsk_rcv_zc+0xa7/0x250) addr=0xc0a900 len=0x42 xs=0xffff8bbc542a5000 fq=0xffff8bbc1c464e40
iperf2-125491 [001] d.Z1. 792878.867457: xsk_flush: (__xsk_map_flush+0x4e/0x180) xs=0xffff8bbc542a5000
flowtrackd-zjTA-201813 [001] ..... 792878.867496: tracing_mark_write: ingress q:2 0x8d4900 FILL -> RX
flowtrackd-zjTA-201813 [001] ..... 792878.867503: tracing_mark_write: ingress q:2 0xc79900 FILL -> RX
flowtrackd-zjTA-201813 [001] ..... 792878.867506: tracing_mark_write: ingress q:2 0x18e1900 FILL -> RX
flowtrackd-zjTA-201813 [001] ..... 792878.867524: tracing_mark_write: read invalid descriptor cookie: 0xdeadbeefdeadbeef

This shows what I've seen before where the xsk_flush() of CPU 1 runs
after (during?) __xsk_rcv_zc() of CPU 3. In this trace we never see
the xsk_flush() from CPU 3 but I stop tracing when the bug occurs so
it probably just hasn't happened yet.

So at least to me this does confirm there is definitely a race here
where we can flush an updated producer pointer before the descriptor
address has been filled in.

--
Shawn