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McKenney" writes: > On Mon, Apr 19, 2021 at 08:12:27PM +0200, Toke H=C3=B8iland-J=C3=B8rgense= n wrote: >> "Paul E. McKenney" writes: >>=20 >> > On Sat, Apr 17, 2021 at 02:27:19PM +0200, Toke H=C3=B8iland-J=C3=B8rge= nsen wrote: >> >> "Paul E. McKenney" writes: >> >>=20 >> >> > 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 w= rote: >> >> >> > 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=B8rgensen 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=B8ila= nd-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 KaFa= i 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, st= ruct xdp_frame *xdpf, >> >> >> > > > >> > >> > - struct net_device *dev_rx) >> >> >> > > > >> > >> > + struct net_device *dev_rx, struct bpf_pr= og *xdp_prog) >> >> >> > > > >> > >> > { >> >> >> > > > >> > >> > struct list_head *flush_list =3D this_cpu_ptr(&d= ev_flush_list); >> >> >> > > > >> > >> > struct xdp_dev_bulk_queue *bq =3D this_cpu_ptr(d= ev->xdp_bulkq); >> >> >> > > > >> > >> > @@ -412,18 +466,22 @@ static void bq_enqueue(struc= t net_device *dev, struct xdp_frame *xdpf, >> >> >> > > > >> > >> > /* Ingress dev_rx will be the same for all xdp_f= rame's in >> >> >> > > > >> > >> > * bulk_queue, because bq stored per-CPU and mus= t be flushed >> >> >> > > > >> > >> > * from net_device drivers NAPI func end. >> >> >> > > > >> > >> > + * >> >> >> > > > >> > >> > + * Do the same with xdp_prog and flush_list sinc= e these 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 late= r in bq_xmit_all(). >> >> >> > > > >> > >> How is bq->xdp_prog protected? Are they all under on= e 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 t= he description of >> >> >> > > > >> > > __dev_flush(): >> >> >> > > > >> > > >> >> >> > > > >> > > On devmap tear down we ensure the flush list is empty= before completing to >> >> >> > > > >> > > ensure all flush operations have completed. When driv= ers update the bpf >> >> >> > > > >> > > program they may need to ensure any flush ops are als= o complete.=20=20=20=20 >> >> >> > > > >> >> >> >> > > > >> AFAICT, the bq->xdp_prog is not from the dev. It is from = a devmap'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 po= inter >> >> >> > the the xdp_prog here, more below (and Q for Paul). >> >> >> >=20 >> >> >> > > > >> >=20 >> >> >> > > > >> > Yeah, drivers call xdp_do_flush() before exiting their = NAPI poll loop, >> >> >> > > > >> > which also runs under one big rcu_read_lock(). So the s= torage in the >> >> >> > > > >> > bulk queue is quite temporary, it's just used for bulki= ng to increase >> >> >> > > > >> > performance :)=20=20=20=20 >> >> >> > > > >> >> >> >> > > > >> I am missing the one big rcu_read_lock() part. For examp= le, in i40e_txrx.c, >> >> >> > > > >> i40e_run_xdp() has its own rcu_read_lock/unlock(). dst->= xdp_prog used to run >> >> >> > > > >> in i40e_run_xdp() and it is fine. >> >> >> > > > >>=20 >> >> >> > > > >> In this patch, dst->xdp_prog is run outside of i40e_run_x= dp() where the >> >> >> > > > >> rcu_read_unlock() has already done. It is now run in xdp= _do_flush_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 protecte= d under an >> >> >> > > > > rcu_read_lock. As the devmap and cpumap, which get called= via >> >> >> > > > > __dev_flush() and __cpu_map_flush(), have multiple RCU obj= ects that 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 ste= ps.=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_flu= sh_map() is >> >> >> > > > > call, which I think means that this CPU will not go-throug= h a RCU 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 call= ed between >> >> >> > > > full invocations of the softirq handler, which for networkin= g is >> >> >> > > > net_rx_action(), and so translates into full NAPI poll cycle= s.=20=20 >> >> >> > > >> >> >> > > I don't know enough to comment on the rcu/softirq part, may be= someone >> >> >> > > 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() i= s unnecessary? >> >> >> >=20 >> >> >> > Well, in many cases, especially depending on how kernel is compi= led, >> >> >> > 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 w= hile keeping >> >> >> > > other rcu_read_lock() as-is. >> >> >> >=20 >> >> >> > Let me be clear: I think you have spotted a problem, and we nee= d 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 ful= l 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_redire= ct) , >> >> >> > some driver wrap this with a rcu_read_lock/unlock() section (oth= er 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 t= he >> >> >> > 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 us= e this >> >> >> > xdp_prog pointer. I can see that it is in-principle wrong to p= ass >> >> >> > 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 RC= U-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); >> >> > } >> >>=20 >> >> 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. >> > >> > Yes, use of call_rcu() is an asynchronous version of synchronize_rcu(). >> > In fact, synchronize_rcu() is implemented in terms of call_rcu(). ;-) >>=20 >> Right, gotcha! >>=20 >> >> > I need to check up on -rt. >> >> > >> >> > But first... In recent mainline kernels, the local_bh_disable() reg= ion >> >> > will look like one big RCU read-side critical section. But don't t= ry >> >> > 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). >> >>=20 >> >> OK. Variants of this code has been around since before then, but I >> >> honestly have no idea what it looked like back then exactly... >> > >> > I know that feeling... >> > >> >> > Except that in that case, why not just drop the inner rcu_read_unlo= ck() >> >> > and rcu_read_lock() pair? Awkward function boundaries or some such? >> >>=20 >> >> 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 g= et >> >> rid of the inner ones. What about tools like lockdep; do they underst= and >> >> this, or are we likely to get complaints if we remove it? >> > >> > If you just got rid of the first rcu_read_unlock() and the second >> > rcu_read_lock() in the code above, lockdep will understand. >>=20 >> Right, but doing so entails going through all the drivers, which is what >> we're trying to avoid :) > > I was afraid of that... ;-) > >> > However, if you instead get rid of -all- of the rcu_read_lock() and >> > rcu_read_unlock() invocations in the code above, you would need to let >> > lockdep know by adding rcu_read_lock_bh_held(). So instead of this: >> > >> > p =3D rcu_dereference(gp); >> > >> > You would do this: >> > >> > p =3D rcu_dereference_check(gp, rcu_read_lock_bh_held()); >> > >> > This would be needed for mainline, regardless of -rt. >>=20 >> OK. And as far as I can tell this is harmless for code paths that call >> the same function but from a regular rcu_read_lock()-protected section >> instead from a bh-disabled section, right? > > That is correct. That rcu_dereference_check() invocation will make > lockdep be OK with rcu_read_lock() or with softirq being disabled. > Or both, for that matter. OK, great, thank you for confirming my understanding! >> What happens, BTW, if we *don't* get rid of all the existing >> rcu_read_lock() sections? Going back to your foo() example above, what >> we're discussing is whether to add that second rcu_read_lock() around >> do_some_other_thing(p). I.e., the first one around the rcu_dereference() >> is already there (in the particular driver we're discussing), and the >> local_bh_disable/enable() pair is already there. AFAICT from our >> discussion, there really is not much point in adding that second >> rcu_read_lock/unlock(), is there? > > From an algorithmic point of view, the second rcu_read_lock() > and rcu_read_unlock() are redundant. Of course, there are also > software-engineering considerations, including copy-pasta issues. > >> And because that first rcu_read_lock() around the rcu_dereference() is >> already there, lockdep is not likely to complain either, so we're >> basically fine? Except that the code is somewhat confusing as-is, of >> course; i.e., we should probably fix it but it's not terribly urgent. Or? > > I am concerned about copy-pasta-induced bugs. Someone looks just at > the code, fails to note the fact that softirq is disabled throughout, > and decides that leaking a pointer from one RCU read-side critical > section to a later one is just fine. :-/ Yup, totally agreed that we need to fix this for the sake of the humans reading the code; just wanted to make sure my understanding was correct that we don't strictly need to do anything as far as the machines executing it are concerned :) >> Hmm, looking at it now, it seems not all the lookup code is actually >> doing rcu_dereference() at all, but rather just a plain READ_ONCE() with >> a comment above it saying that RCU ensures objects won't disappear[0]; >> so I suppose we're at least safe from lockdep in that sense :P - but we >> should definitely clean this up. >>=20 >> [0] Exhibit A: https://elixir.bootlin.com/linux/latest/source/kernel/bpf= /devmap.c#L391 > > That use of READ_ONCE() will definitely avoid lockdep complaints, > including those complaints that point out bugs. It also might get you > sparse complaints if the RCU-protected pointer is marked with __rcu. It's not; it's the netdev_map member of this struct: struct bpf_dtab { struct bpf_map map; struct bpf_dtab_netdev **netdev_map; /* DEVMAP type only */ struct list_head list; /* these are only used for DEVMAP_HASH type maps */ struct hlist_head *dev_index_head; spinlock_t index_lock; unsigned int items; u32 n_buckets; }; Will adding __rcu to such a dynamic array member do the right thing when paired with rcu_dereference() on array members (i.e., in place of the READ_ONCE in the code linked above)? Also, while you're being so nice about confirming my understanding of things: I always understood the point of rcu_dereference() (and __rcu on struct members) to be annotations that document the lifetime expectations of the object being pointed to, rather than a functional change vs READ_ONCE()? Documentation that the static checkers can turn into warnings, of course, but totally transparent in terms of the generated code. Right? :) >> >> > 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. >> >>=20 >> >> Right. Guess we'll wait for your confirmation of that, then. Thanks! = :) >> > >> > Looking at v5.11.4-rt11... >> > >> > And __local_bh_disable_ip() has added the required rcu_read_lock(), >> > so dropping all the rcu_read_lock() and rcu_read_unlock() calls would >> > do the right thing in -rt. And lockdep would understand without the >> > rcu_read_lock_bh_held(), but that is still required for mainline. >>=20 >> Great, thanks for checking! >>=20 >> So this brings to mind another question: Are there any performance >> implications to nesting rcu_read_locks() inside each other? One >> thing that would be fairly easy to do (in terms of how much code we have >> to touch) is to just add a top-level rcu_read_lock() around the >> napi_poll() call in the core dev code, thus making -rt and mainline >> equivalent in that respect. Also, this would make it obvious that all >> the RCU usage inside of NAPI is safe, without having to know about >> bh_disable() and all that. But we obviously don't want to do that if it >> is going to slow things down; WDYT? > > Both rcu_read_lock() and rcu_read_unlock() are quite lightweight (zero for > CONFIG_PREEMPT=3Dn and about two nanoseconds per pair for CONFIG_PREEMPT= =3Dy > on 2GHz x86) and can be nested quite deeply. So that approach should > be fine from that viewpoint. OK, that may be fine, then. Guess I'll try it and benchmark (and compare with the rcu_dereference_check() approach). > However, remaining in a single RCU read-side critical section forever > will eventually OOM the system, so the code should periodically exit > its top-level RCU read-side critical section, say, every few tens of > milliseconds. Yup, NAPI already does this (there's a poll budget), so that should be fine. -Toke