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McKenney" writes: > On Mon, Apr 19, 2021 at 11:21:41PM +0200, Toke H=C3=B8iland-J=C3=B8rgense= n wrote: >> "Paul E. McKenney" writes: >>=20 >> > On Mon, Apr 19, 2021 at 08:12:27PM +0200, Toke H=C3=B8iland-J=C3=B8rge= nsen wrote: >> >> "Paul E. McKenney" writes: >> >>=20 >> >> > On Sat, Apr 17, 2021 at 02:27:19PM +0200, Toke H=C3=B8iland-J=C3=B8= rgensen 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 Broue= r 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=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=B8= iland-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 K= aFai Lau wrote:=20=20=20=20 >> >> >> >> > > > >> > >> > static void bq_xmit_all(struct xdp_dev_bulk_qu= eue *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, b= q->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_pro= g, 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 xdp_frame *xdpf, >> >> >> >> > > > >> > >> > - struct net_device *dev_rx) >> >> >> >> > > > >> > >> > + struct net_device *dev_rx, struct bpf= _prog *xdp_prog) >> >> >> >> > > > >> > >> > { >> >> >> >> > > > >> > >> > struct list_head *flush_list =3D this_cpu_ptr= (&dev_flush_list); >> >> >> >> > > > >> > >> > struct xdp_dev_bulk_queue *bq =3D this_cpu_pt= r(dev->xdp_bulkq); >> >> >> >> > > > >> > >> > @@ -412,18 +466,22 @@ static void bq_enqueue(st= ruct net_device *dev, struct xdp_frame *xdpf, >> >> >> >> > > > >> > >> > /* Ingress dev_rx will be the same for all xd= p_frame's in >> >> >> >> > > > >> > >> > * bulk_queue, because bq stored per-CPU and = must be flushed >> >> >> >> > > > >> > >> > * from net_device drivers NAPI func end. >> >> >> >> > > > >> > >> > + * >> >> >> >> > > > >> > >> > + * Do the same with xdp_prog and flush_list s= ince 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 l= ater in bq_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 o= n the description of >> >> >> >> > > > >> > > __dev_flush(): >> >> >> >> > > > >> > > >> >> >> >> > > > >> > > On devmap tear down we ensure the flush list is em= pty before completing to >> >> >> >> > > > >> > > ensure all flush operations have completed. When d= rivers update the bpf >> >> >> >> > > > >> > > program they may need to ensure any flush ops are = also complete.=20=20=20=20 >> >> >> >> > > > >> >> >> >> >> > > > >> AFAICT, the bq->xdp_prog is not from the dev. It is fr= om 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 t= his >> >> >> >> > 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 the= ir NAPI poll loop, >> >> >> >> > > > >> > which also runs under one big rcu_read_lock(). So th= e storage in the >> >> >> >> > > > >> > bulk queue is quite temporary, it's just used for bu= lking to increase >> >> >> >> > > > >> > performance :)=20=20=20=20 >> >> >> >> > > > >> >> >> >> >> > > > >> I am missing the one big rcu_read_lock() part. For ex= ample, in i40e_txrx.c, >> >> >> >> > > > >> i40e_run_xdp() has its own rcu_read_lock/unlock(). ds= t->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_ru= n_xdp() 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 prote= cted under an >> >> >> >> > > > > rcu_read_lock. As the devmap and cpumap, which get cal= led via >> >> >> >> > > > > __dev_flush() and __cpu_map_flush(), have multiple RCU = objects 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 = steps.=20=20 >> >> >> >> > The devmap's elements are also RCU objects and the BPF xdp_pr= og is >> >> >> >> > embedded in this object (struct bpf_dtab_netdev). The call_r= cu >> >> >> >> > 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-thr= ough 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 c= alled between >> >> >> >> > > > full invocations of the softirq handler, which for networ= king is >> >> >> >> > > > net_rx_action(), and so translates into full NAPI poll cy= cles.=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(= ) is unnecessary? >> >> >> >> >=20 >> >> >> >> > Well, in many cases, especially depending on how kernel is co= mpiled, >> >> >> >> > that is true. But we want to keep these, as they also docume= nt the >> >> >> >> > intend of the programmer. And allow us to make the kernel ev= en more >> >> >> >> > preempt-able in the future. >> >> >> >> >=20 >> >> >> >> > > At least, it sounds incorrect to only make an exception her= e while keeping >> >> >> >> > > 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 f= ind 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 ter= ms 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_red= irect) , >> >> >> >> > 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 stor= e the >> >> >> >> > xdp_frame packets) that will get flushed/send in the call >> >> >> >> > xdp_do_flush() (that end-up calling bq_xmit_all()). This flu= sh will >> >> >> >> > happen before we end our softirq/NAPI context. >> >> >> >> >=20 >> >> >> >> > The extension is that the per_cpu_ptr data structure (after t= his patch) >> >> >> >> > store a pointer to an xdp_prog (which is a RCU object). In t= he 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 t= o pass >> >> >> >> > this-pointer between RCU-read sections, but I consider this s= afe as we >> >> >> >> > are running under softirq/NAPI and the per_cpu_ptr is only va= lid 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); >> >> >> > } >> >> >>=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 n= eed 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_rc= u(). >> >> > 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() = 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 wor= k, 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_u= nlock() >> >> >> > and rcu_read_lock() pair? Awkward function boundaries or some s= uch? >> >> >>=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 jus= t get >> >> >> rid of the inner ones. What about tools like lockdep; do they unde= rstand >> >> >> 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 w= hat >> >> 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. >>=20 >> OK, great, thank you for confirming my understanding! >>=20 >> >> 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_dereferenc= e() >> >> 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. :-/ >>=20 >> 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 :) >>=20 >> >> 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() w= ith >> >> 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. >>=20 >> It's not; it's the netdev_map member of this struct: >>=20 >> struct bpf_dtab { >> struct bpf_map map; >> struct bpf_dtab_netdev **netdev_map; /* DEVMAP type only */ >> struct list_head list; >>=20 >> /* 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; >> }; >>=20 >> 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)? > > The only thing __rcu will do is provide information to the sparse static > analysis tool. Which will then gripe at you for applying READ_ONCE() > to a __rcu pointer. But it is already griping at you for applying > rcu_dereference() to something not marked __rcu, so... ;-) Right, hence the need for a cleanup ;) My question was more if it understood arrays, though. I.e., that 'netdev_map' is an array of RCU pointers, not an RCU pointer to an array... Or am I maybe thinking that tool is way smarter than it is, and it just complains for any access to that field that doesn't use rcu_dereference()? >> 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? :) > > Yes for __rcu. > > Maybe for rcu_dereference(). Yes in that it is functionally the same > as READ_ONCE(), no in that it is not the same as a simple C-language load. Right, was going for "functionally the same" - cool! >> >> >> > Especially given that if this works on -rt, it is probably becau= se >> >> >> > their variant of do_softirq() holds rcu_read_lock() across each >> >> >> > softirq handler invocation. They do something similar for rwlock= s. >> >> >>=20 >> >> >> Right. Guess we'll wait for your confirmation of that, then. Thank= s! :) >> >> > >> >> > 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 wou= ld >> >> > 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 h= ave >> >> 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_PREEM= PT=3Dy >> > on 2GHz x86) and can be nested quite deeply. So that approach should >> > be fine from that viewpoint. >>=20 >> OK, that may be fine, then. Guess I'll try it and benchmark (and compare >> with the rcu_dereference_check() approach). > > Sounds good! Awesome! Thanks a lot for explaining, and for bearing me and all my stupid questions - I feel like I get closer to understanding RCU each time I speak with you about it :) >> > 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. >>=20 >> Yup, NAPI already does this (there's a poll budget), so that should be >> fine. > > Whew!!! ;-) I know, right? ;) Although I do seem to recall you quite recently helping me fix a case where it didn't quite interrupt itself enough, and was causing hangs... -Toke