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McKenney" writes: > On Fri, Apr 16, 2021 at 11:22:52AM -0700, Martin KaFai Lau wrote: >> On Fri, Apr 16, 2021 at 03:45:23PM +0200, Jesper Dangaard Brouer wrote: >> > On Thu, 15 Apr 2021 17:39:13 -0700 >> > Martin KaFai Lau wrote: >> >=20 >> > > On Thu, Apr 15, 2021 at 10:29:40PM +0200, Toke H=C3=B8iland-J=C3=B8r= gensen wrote: >> > > > Jesper Dangaard Brouer writes: >> > > >=20=20=20 >> > > > > On Thu, 15 Apr 2021 10:35:51 -0700 >> > > > > Martin KaFai Lau wrote: >> > > > >=20=20 >> > > > >> On Thu, Apr 15, 2021 at 11:22:19AM +0200, Toke H=C3=B8iland-J= =C3=B8rgensen wrote:=20=20 >> > > > >> > Hangbin Liu writes: >> > > > >> >=20=20=20=20=20 >> > > > >> > > On Wed, Apr 14, 2021 at 05:17:11PM -0700, Martin KaFai Lau = wrote:=20=20=20=20 >> > > > >> > >> > static void bq_xmit_all(struct xdp_dev_bulk_queue *bq, = u32 flags) >> > > > >> > >> > { >> > > > >> > >> > struct net_device *dev =3D bq->dev; >> > > > >> > >> > - int sent =3D 0, err =3D 0; >> > > > >> > >> > + int sent =3D 0, drops =3D 0, err =3D 0; >> > > > >> > >> > + unsigned int cnt =3D bq->count; >> > > > >> > >> > + int to_send =3D cnt; >> > > > >> > >> > int i; >> > > > >> > >> >=20=20 >> > > > >> > >> > - if (unlikely(!bq->count)) >> > > > >> > >> > + if (unlikely(!cnt)) >> > > > >> > >> > return; >> > > > >> > >> >=20=20 >> > > > >> > >> > - for (i =3D 0; i < bq->count; i++) { >> > > > >> > >> > + for (i =3D 0; i < cnt; i++) { >> > > > >> > >> > struct xdp_frame *xdpf =3D bq->q[i]; >> > > > >> > >> >=20=20 >> > > > >> > >> > prefetch(xdpf); >> > > > >> > >> > } >> > > > >> > >> >=20=20 >> > > > >> > >> > - sent =3D dev->netdev_ops->ndo_xdp_xmit(dev, bq->count,= bq->q, flags); >> > > > >> > >> > + if (bq->xdp_prog) {=20=20=20=20 >> > > > >> > >> bq->xdp_prog is used here >> > > > >> > >>=20=20=20=20=20 >> > > > >> > >> > + to_send =3D dev_map_bpf_prog_run(bq->xdp_prog, bq->q,= cnt, dev); >> > > > >> > >> > + if (!to_send) >> > > > >> > >> > + goto out; >> > > > >> > >> > + >> > > > >> > >> > + drops =3D cnt - to_send; >> > > > >> > >> > + } >> > > > >> > >> > +=20=20=20=20 >> > > > >> > >>=20 >> > > > >> > >> [ ... ] >> > > > >> > >>=20=20=20=20=20 >> > > > >> > >> > static void bq_enqueue(struct net_device *dev, struct x= dp_frame *xdpf, >> > > > >> > >> > - struct net_device *dev_rx) >> > > > >> > >> > + struct net_device *dev_rx, struct bpf_prog *xd= p_prog) >> > > > >> > >> > { >> > > > >> > >> > struct list_head *flush_list =3D this_cpu_ptr(&dev_flu= sh_list); >> > > > >> > >> > struct xdp_dev_bulk_queue *bq =3D this_cpu_ptr(dev->xd= p_bulkq); >> > > > >> > >> > @@ -412,18 +466,22 @@ static void bq_enqueue(struct net_= device *dev, struct xdp_frame *xdpf, >> > > > >> > >> > /* Ingress dev_rx will be the same for all xdp_frame's= in >> > > > >> > >> > * bulk_queue, because bq stored per-CPU and must be f= lushed >> > > > >> > >> > * from net_device drivers NAPI func end. >> > > > >> > >> > + * >> > > > >> > >> > + * Do the same with xdp_prog and flush_list since thes= e fields >> > > > >> > >> > + * are only ever modified together. >> > > > >> > >> > */ >> > > > >> > >> > - if (!bq->dev_rx) >> > > > >> > >> > + if (!bq->dev_rx) { >> > > > >> > >> > bq->dev_rx =3D dev_rx; >> > > > >> > >> > + bq->xdp_prog =3D xdp_prog;=20=20=20=20 >> > > > >> > >> bp->xdp_prog is assigned here and could be used later in b= q_xmit_all(). >> > > > >> > >> How is bq->xdp_prog protected? Are they all under one rcu_= read_lock()? >> > > > >> > >> It is not very obvious after taking a quick look at xdp_do= _flush[_map]. >> > > > >> > >>=20 >> > > > >> > >> e.g. what if the devmap elem gets deleted.=20=20=20=20 >> > > > >> > > >> > > > >> > > Jesper knows better than me. From my veiw, based on the des= cription of >> > > > >> > > __dev_flush(): >> > > > >> > > >> > > > >> > > On devmap tear down we ensure the flush list is empty befor= e completing to >> > > > >> > > ensure all flush operations have completed. When drivers up= date the bpf >> > > > >> > > program they may need to ensure any flush ops are also comp= lete.=20=20=20=20 >> > > > >> >> > > > >> AFAICT, the bq->xdp_prog is not from the dev. It is from a devm= ap's elem. >> >=20 >> > The bq->xdp_prog comes form the devmap "dev" element, and it is stored >> > in temporarily in the "bq" structure that is only valid for this >> > softirq NAPI-cycle. I'm slightly worried that we copied this pointer >> > the the xdp_prog here, more below (and Q for Paul). >> >=20 >> > > > >> >=20 >> > > > >> > Yeah, drivers call xdp_do_flush() before exiting their NAPI p= oll loop, >> > > > >> > which also runs under one big rcu_read_lock(). So the storage= in the >> > > > >> > bulk queue is quite temporary, it's just used for bulking to = increase >> > > > >> > performance :)=20=20=20=20 >> > > > >> >> > > > >> I am missing the one big rcu_read_lock() part. For example, in= i40e_txrx.c, >> > > > >> i40e_run_xdp() has its own rcu_read_lock/unlock(). dst->xdp_pr= og used to run >> > > > >> in i40e_run_xdp() and it is fine. >> > > > >>=20 >> > > > >> In this patch, dst->xdp_prog is run outside of i40e_run_xdp() w= here the >> > > > >> rcu_read_unlock() has already done. It is now run in xdp_do_fl= ush_map(). >> > > > >> or I missed the big rcu_read_lock() in i40e_napi_poll()? >> > > > >> >> > > > >> I do see the big rcu_read_lock() in mlx5e_napi_poll().=20=20 >> > > > > >> > > > > I believed/assumed xdp_do_flush_map() was already protected unde= r an >> > > > > rcu_read_lock. As the devmap and cpumap, which get called via >> > > > > __dev_flush() and __cpu_map_flush(), have multiple RCU objects t= hat we >> > > > > are operating on.=20=20 >> > > >> > > What other rcu objects it is using during flush? >> >=20 >> > Look at code: >> > kernel/bpf/cpumap.c >> > kernel/bpf/devmap.c >> >=20 >> > The devmap is filled with RCU code and complicated take-down steps.=20= =20 >> > The devmap's elements are also RCU objects and the BPF xdp_prog is >> > embedded in this object (struct bpf_dtab_netdev). The call_rcu >> > function is __dev_map_entry_free(). >> >=20 >> >=20 >> > > > > Perhaps it is a bug in i40e?=20=20 >> > > >> > > A quick look into ixgbe falls into the same bucket. >> > > didn't look at other drivers though. >> >=20 >> > Intel driver are very much in copy-paste mode. >> >=20=20 >> > > > > >> > > > > We are running in softirq in NAPI context, when xdp_do_flush_map= () is >> > > > > call, which I think means that this CPU will not go-through a RC= U grace >> > > > > period before we exit softirq, so in-practice it should be safe.= =20=20 >> > > >=20 >> > > > Yup, this seems to be correct: rcu_softirq_qs() is only called bet= ween >> > > > full invocations of the softirq handler, which for networking is >> > > > net_rx_action(), and so translates into full NAPI poll cycles.=20= =20 >> > > >> > > I don't know enough to comment on the rcu/softirq part, may be someo= ne >> > > can chime in. There is also a recent napi_threaded_poll(). >> >=20 >> > CC added Paul. (link to patch[1][2] for context) >> Updated Paul's email address. >>=20 >> >=20 >> > > If it is the case, then some of the existing rcu_read_lock() is unne= cessary? >> >=20 >> > Well, in many cases, especially depending on how kernel is compiled, >> > that is true. But we want to keep these, as they also document the >> > intend of the programmer. And allow us to make the kernel even more >> > preempt-able in the future. >> >=20 >> > > At least, it sounds incorrect to only make an exception here while k= eeping >> > > other rcu_read_lock() as-is. >> >=20 >> > Let me be clear: I think you have spotted a problem, and we need to >> > add rcu_read_lock() at least around the invocation of >> > bpf_prog_run_xdp() or before around if-statement that call >> > dev_map_bpf_prog_run(). (Hangbin please do this in V8). >> >=20 >> > Thank you Martin for reviewing the code carefully enough to find this >> > issue, that some drivers don't have a RCU-section around the full XDP >> > code path in their NAPI-loop. >> >=20 >> > Question to Paul. (I will attempt to describe in generic terms what >> > happens, but ref real-function names). >> >=20 >> > We are running in softirq/NAPI context, the driver will call a >> > bq_enqueue() function for every packet (if calling xdp_do_redirect) , >> > some driver wrap this with a rcu_read_lock/unlock() section (other have >> > a large RCU-read section, that include the flush operation). >> >=20 >> > In the bq_enqueue() function we have a per_cpu_ptr (that store the >> > xdp_frame packets) that will get flushed/send in the call >> > xdp_do_flush() (that end-up calling bq_xmit_all()). This flush will >> > happen before we end our softirq/NAPI context. >> >=20 >> > The extension is that the per_cpu_ptr data structure (after this patch) >> > store a pointer to an xdp_prog (which is a RCU object). In the flush >> > operation (which we will wrap with RCU-read section), we will use this >> > xdp_prog pointer. I can see that it is in-principle wrong to pass >> > this-pointer between RCU-read sections, but I consider this safe as we >> > are running under softirq/NAPI and the per_cpu_ptr is only valid in >> > this short interval. >> >=20 >> > I claim a grace/quiescent RCU cannot happen between these two RCU-read >> > sections, but I might be wrong? (especially in the future or for RT). > > If I am reading this correctly (ha!), a very high-level summary of the > code in question is something like this: > > void foo(void) > { > local_bh_disable(); > > rcu_read_lock(); > p =3D rcu_dereference(gp); > do_something_with(p); > rcu_read_unlock(); > > do_something_else(); > > rcu_read_lock(); > do_some_other_thing(p); > rcu_read_unlock(); > > local_bh_enable(); > } > > void bar(struct blat *new_gp) > { > struct blat *old_gp; > > spin_lock(my_lock); > old_gp =3D rcu_dereference_protected(gp, lock_held(my_lock)); > rcu_assign_pointer(gp, new_gp); > spin_unlock(my_lock); > synchronize_rcu(); > kfree(old_gp); > } Yeah, something like that (the object is freed using call_rcu() - but I think that's equivalent, right?). And the question is whether we need to extend foo() so that is has one big rcu_read_lock() that covers the whole lifetime of p. > I need to check up on -rt. > > But first... In recent mainline kernels, the local_bh_disable() region > will look like one big RCU read-side critical section. But don't try > this prior to v4.20!!! In v4.19 and earlier, you would need to use > both synchronize_rcu() and synchronize_rcu_bh() to make this work, or, > for less latency, synchronize_rcu_mult(call_rcu, call_rcu_bh). OK. Variants of this code has been around since before then, but I honestly have no idea what it looked like back then exactly... > Except that in that case, why not just drop the inner rcu_read_unlock() > and rcu_read_lock() pair? Awkward function boundaries or some such? Well if we can just treat such a local_bh_disable()/enable() pair as the equivalent of rcu_read_lock()/unlock() then I suppose we could just get rid of the inner ones. What about tools like lockdep; do they understand this, or are we likely to get complaints if we remove it? > Especially given that if this works on -rt, it is probably because > their variant of do_softirq() holds rcu_read_lock() across each > softirq handler invocation. They do something similar for rwlocks. Right. Guess we'll wait for your confirmation of that, then. Thanks! :) -Toke