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 <kafai@xxxxxx> wrote:
> >
> > > On Thu, Apr 15, 2021 at 10:29:40PM +0200, Toke Høiland-Jørgensen wrote:
> > > > Jesper Dangaard Brouer <brouer@xxxxxxxxxx> writes:
> > > >
> > > > > On Thu, 15 Apr 2021 10:35:51 -0700
> > > > > Martin KaFai Lau <kafai@xxxxxx> wrote:
> > > > >
> > > > >> On Thu, Apr 15, 2021 at 11:22:19AM +0200, Toke Høiland-Jørgensen wrote:
> > > > >> > Hangbin Liu <liuhangbin@xxxxxxxxx> writes:
> > > > >> >
> > > > >> > > On Wed, Apr 14, 2021 at 05:17:11PM -0700, Martin KaFai Lau wrote:
> > > > >> > >> > static void bq_xmit_all(struct xdp_dev_bulk_queue *bq, u32 flags)
> > > > >> > >> > {
> > > > >> > >> > struct net_device *dev = bq->dev;
> > > > >> > >> > - int sent = 0, err = 0;
> > > > >> > >> > + int sent = 0, drops = 0, err = 0;
> > > > >> > >> > + unsigned int cnt = bq->count;
> > > > >> > >> > + int to_send = cnt;
> > > > >> > >> > int i;
> > > > >> > >> >
> > > > >> > >> > - if (unlikely(!bq->count))
> > > > >> > >> > + if (unlikely(!cnt))
> > > > >> > >> > return;
> > > > >> > >> >
> > > > >> > >> > - for (i = 0; i < bq->count; i++) {
> > > > >> > >> > + for (i = 0; i < cnt; i++) {
> > > > >> > >> > struct xdp_frame *xdpf = bq->q[i];
> > > > >> > >> >
> > > > >> > >> > prefetch(xdpf);
> > > > >> > >> > }
> > > > >> > >> >
> > > > >> > >> > - sent = dev->netdev_ops->ndo_xdp_xmit(dev, bq->count, bq->q, flags);
> > > > >> > >> > + if (bq->xdp_prog) {
> > > > >> > >> bq->xdp_prog is used here
> > > > >> > >>
> > > > >> > >> > + to_send = dev_map_bpf_prog_run(bq->xdp_prog, bq->q, cnt, dev);
> > > > >> > >> > + if (!to_send)
> > > > >> > >> > + goto out;
> > > > >> > >> > +
> > > > >> > >> > + drops = cnt - to_send;
> > > > >> > >> > + }
> > > > >> > >> > +
> > > > >> > >>
> > > > >> > >> [ ... ]
> > > > >> > >>
> > > > >> > >> > 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 = this_cpu_ptr(&dev_flush_list);
> > > > >> > >> > struct xdp_dev_bulk_queue *bq = this_cpu_ptr(dev->xdp_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 flushed
> > > > >> > >> > * from net_device drivers NAPI func end.
> > > > >> > >> > + *
> > > > >> > >> > + * Do the same with xdp_prog and flush_list since these fields
> > > > >> > >> > + * are only ever modified together.
> > > > >> > >> > */
> > > > >> > >> > - if (!bq->dev_rx)
> > > > >> > >> > + if (!bq->dev_rx) {
> > > > >> > >> > bq->dev_rx = dev_rx;
> > > > >> > >> > + bq->xdp_prog = xdp_prog;
> > > > >> > >> bp->xdp_prog is assigned here and could be used later 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].
> > > > >> > >>
> > > > >> > >> e.g. what if the devmap elem gets deleted.
> > > > >> > >
> > > > >> > > Jesper knows better than me. From my veiw, based on the 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 drivers update the bpf
> > > > >> > > program they may need to ensure any flush ops are also complete.
> > > > >>
> > > > >> AFAICT, the bq->xdp_prog is not from the dev. It is from a devmap's elem.
> >
> > 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).
> >
> > > > >> >
> > > > >> > Yeah, drivers call xdp_do_flush() before exiting their NAPI poll 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 :)
> > > > >>
> > > > >> 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_prog used to run
> > > > >> in i40e_run_xdp() and it is fine.
> > > > >>
> > > > >> In this patch, dst->xdp_prog is run outside of i40e_run_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().
> > > > >
> > > > > I believed/assumed xdp_do_flush_map() was already protected under an
> > > > > rcu_read_lock. As the devmap and cpumap, which get called via
> > > > > __dev_flush() and __cpu_map_flush(), have multiple RCU objects that we
> > > > > are operating on.
> > >
> > > What other rcu objects it is using during flush?
> >
> > Look at code:
> > kernel/bpf/cpumap.c
> > kernel/bpf/devmap.c
> >
> > The devmap is filled with RCU code and complicated take-down steps.
> > 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().
> >
> >
> > > > > Perhaps it is a bug in i40e?
> > >
> > > A quick look into ixgbe falls into the same bucket.
> > > didn't look at other drivers though.
> >
> > Intel driver are very much in copy-paste mode.
> >
> > > > >
> > > > > 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 RCU grace
> > > > > period before we exit softirq, so in-practice it should be safe.
> > > >
> > > > Yup, this seems to be correct: rcu_softirq_qs() is only called between
> > > > full invocations of the softirq handler, which for networking is
> > > > net_rx_action(), and so translates into full NAPI poll cycles.
> > >
> > > 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().
> >
> > CC added Paul. (link to patch[1][2] for context)
> Updated Paul's email address.
>
> >
> > > If it is the case, then some of the existing rcu_read_lock() is unnecessary?
> >
> > 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.
> >
> > > At least, it sounds incorrect to only make an exception here while keeping
> > > other rcu_read_lock() as-is.
> >
> > 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).
> >
> > 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.
> >
> > Question to Paul. (I will attempt to describe in generic terms what
> > happens, but ref real-function names).
> >
> > 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).
> >
> > 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.
> >
> > 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.
> >
> > 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 = 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 = rcu_dereference_protected(gp, lock_held(my_lock));
rcu_assign_pointer(gp, new_gp);
spin_unlock(my_lock);
synchronize_rcu();
kfree(old_gp);
}
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).
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?
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.
Thanx, Paul
> > --
> > Best regards,
> > Jesper Dangaard Brouer
> > MSc.CS, Principal Kernel Engineer at Red Hat
> > LinkedIn: http://www.linkedin.com/in/brouer
> >
> > [1] https://lore.kernel.org/netdev/20210414122610.4037085-2-liuhangbin@xxxxxxxxx/
> > [2] https://patchwork.kernel.org/project/netdevbpf/patch/20210414122610.4037085-2-liuhangbin@xxxxxxxxx/
> >
