Re: Possible race with xsk_flush

Shawn Bohrer
2022-12-20T16:02:36Z

On Tue, Dec 20, 2022 at 04:59:30PM +0100, Magnus Karlsson wrote:
> 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'll prepare the patch today. I feel like I've spent enough of my
life chasing this down and appreciate your help!

Thanks,
Shawn Bohrer