Re: Possible race with xsk_flush

Magnus Karlsson
2022-12-20T15:59:48Z

On Tue, Dec 20, 2022 at 4:26 PM Shawn Bohrer <sbohrer@cloudflare.com> wrote:
>
> On Tue, Dec 20, 2022 at 10:06:48AM +0100, Magnus Karlsson wrote:
> > On Tue, Dec 20, 2022 at 2:32 AM Shawn Bohrer <sbohrer@cloudflare.com> wrote:
> > >
> > > 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()
> > > >
> > > > 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.
> > >
> > > After staring at this code for way too long I finally made a
> > > breakthrough! I could not understand how this race could occur when
> > > napi_poll() calls netpoll_poll_lock(). Here is netpoll_poll_lock():
> > >
> > > ```
> > > static inline void *netpoll_poll_lock(struct napi_struct *napi)
> > > {
> > > struct net_device *dev = napi->dev;
> > >
> > > if (dev && dev->npinfo) {
> > > int owner = smp_processor_id();
> > >
> > > while (cmpxchg(&napi->poll_owner, -1, owner) != -1)
> > > cpu_relax();
> > >
> > > return napi;
> > > }
> > > return NULL;
> > > }
> > > ```
> > > If dev or dev->npinfo are NULL then it doesn't acquire a lock at all!
> > > Adding some more trace points I see:
> > >
> > > ```
> > > iperf2-1325 [002] ..s1. 264246.626880: __napi_poll: (__napi_poll+0x0/0x150) n=0xffff91c885bff000 poll_owner=-1 dev=0xffff91c881d4e000 npinfo=0x0
> > > iperf2-1325 [002] d.Z1. 264246.626882: __xsk_rcv_zc_L7: (__xsk_rcv_zc+0x3b/0xc0) addr=0x1503100 len=0x42 xs=0xffff91c8bfe77000 fq=0xffff91c8c1a43f80 dev=0xffff91c881d4e000
> > > iperf2-1325 [002] d.Z1. 264246.626883: __xsk_rcv_zc_L7: (__xsk_rcv_zc+0x42/0xc0) addr=0x1503100 len=0x42 xs=0xffff91c8bfe77000 fq=0xffff91c8c1a43f80 dev=0xffff91c881d4e000
> > > iperf2-1325 [002] d.Z1. 264246.626884: xsk_flush: (__xsk_map_flush+0x32/0xb0) xs=0xffff91c8bfe77000
> > > ```
> > >
> > > Here you can see that poll_owner=-1 meaning the lock was never
> > > acquired because npinfo is NULL. This means that the same veth rx
> > > queue can be napi_polled from multiple CPU and nothing stops it from
> > > running concurrently. They all look like this, just most of the time
> > > there aren't concurrent napi_polls running for the same queue. They
> > > do however move around CPUs as I explained earlier.
> > >
> > > I'll note that I've ran with your suggested change of moving
> > > xdp_do_flush() before napi_complete_done() all weekend and I have not
> > > reproduced the issue. I don't know if that truly means the issue is
> > > fixed by that change or not. I suspect it does fix the issue because
> > > it prevents the napi_struct from being scheduled again before the
> > > first poll has completed, and nap_schedule_prep() ensures that only
> > > one instance is ever running.
> >
> > Thanks Shawn! Good news that the patch seems to fix the problem. To
> > me, napi_schedule_prep() makes sure that only one NAPI instance is
> > running. Netpoll is an optional feature and I do not even have it
> > compiled into my kernel. At least I do not have it defined in my
> > .config and I cannot find any netpoll symbols with a readelf command.
> > If netpoll is not used, I would suspect that npinfo == NULL. So to me,
> > it is still a mystery why this is happening.
>
> Oh I don't think it is a mystery anymore. The napi_complete_done()
> signals that this instance of of the napi_poll is complete. As you
> said nap_schedule_prep() checks to ensure that only one instance of
> napi_poll is running at a time, but we just indicated it was done with
> napi_complete_done(). This allows this CPU or more importantly any
> other CPU to reschedule napi polling for this receive queue, but we
> haven't called xdp_do_flush() yet so the flush can race. I'll note
> that the napi_schedule_prep()/__napi_schedule() in veth_poll really
> isn't the problem since it will schedule itself back on the same CPU.
> The problem is simply that another CPU is free to call
> napi_scheulde_prep()/__napi_schedule() in that window after
> napi_complete_done() and before xdp_do_flush(). The veth driver can
> schedule a napi_poll from the transmit path which is what starts the
> poll on a second CPU.

Bingo! Would you like to prepare a patch or would you like me to do
it? This has been broken since the support was put in veth in 4.19.

> I was simply blinded by that stupid netpoll_poll_lock() but it is
> completely unnecessary.
>
> > To validate or disprove that there are two instances of the same napi
> > running, could you please put a probe in veth_poll() right after
> > veth_xdp_rcv() and one at xdp_do_flush() and dump the napi id in both
> > cases? And run the original code with the bug :-). It would be good to
> > know what exactly happens in the code between these points when this
> > bug occurs. Maybe we could do this by enabling the trace buffer and
> > dumping it when the bug occurs.
>
> The two instances that race are definitely different napi_polls, the
> second one is scheduled from the veth transmit path.
>
> > > If we think this is the correct fix I'll let it run for another day or
> > > two and prepare a patch.
> >
> > There is one more advantage to this bug fix and that is performance.
> > By calling __xdp_map_flush() immediately after the receive function,
> > user space can start to work on the packets quicker so performance
> > will improve.
>
> Yes definitely.
>
> Thanks,
> Shawn Bohrer