Re: [PATCH RFC net 1/1] net/sched: Fix mirred to self recursion

[Jamal Hadi Salim <jhs@mojatatu.com>]

On Wed, Mar 27, 2024 at 6:57 PM Jamal Hadi Salim <jhs@mojatatu.com> wrote:
>
> On Wed, Mar 27, 2024 at 9:23 AM Eric Dumazet <edumazet@google.com> wrote:
> >
> > On Wed, Mar 27, 2024 at 12:03 AM Jamal Hadi Salim <jhs@mojatatu.com> wrote:
> > >
> > > When the mirred action is used on a classful egress qdisc and a packet is
> > > mirrored or redirected to self we hit a qdisc lock deadlock.
> > > See trace below.
> > >
> > > [..... other info removed for brevity....]
> > > [   82.890906]
> > > [   82.890906] ============================================
> > > [   82.890906] WARNING: possible recursive locking detected
> > > [   82.890906] 6.8.0-05205-g77fadd89fe2d-dirty #213 Tainted: G        W
> > > [   82.890906] --------------------------------------------
> > > [   82.890906] ping/418 is trying to acquire lock:
> > > [   82.890906] ffff888006994110 (&sch->q.lock){+.-.}-{3:3}, at:
> > > __dev_queue_xmit+0x1778/0x3550
> > > [   82.890906]
> > > [   82.890906] but task is already holding lock:
> > > [   82.890906] ffff888006994110 (&sch->q.lock){+.-.}-{3:3}, at:
> > > __dev_queue_xmit+0x1778/0x3550
> > > [   82.890906]
> > > [   82.890906] other info that might help us debug this:
> > > [   82.890906]  Possible unsafe locking scenario:
> > > [   82.890906]
> > > [   82.890906]        CPU0
> > > [   82.890906]        ----
> > > [   82.890906]   lock(&sch->q.lock);
> > > [   82.890906]   lock(&sch->q.lock);
> > > [   82.890906]
> > > [   82.890906]  *** DEADLOCK ***
> > > [   82.890906]
> > > [..... other info removed for brevity....]
> > >
> > > Example setup (eth0->eth0) to recreate
> > > tc qdisc add dev eth0 root handle 1: htb default 30
> > > tc filter add dev eth0 handle 1: protocol ip prio 2 matchall \
> > >      action mirred egress redirect dev eth0
> > >
> > > Another example(eth0->eth1->eth0) to recreate
> > > tc qdisc add dev eth0 root handle 1: htb default 30
> > > tc filter add dev eth0 handle 1: protocol ip prio 2 matchall \
> > >      action mirred egress redirect dev eth1
> > >
> > > tc qdisc add dev eth1 root handle 1: htb default 30
> > > tc filter add dev eth1 handle 1: protocol ip prio 2 matchall \
> > >      action mirred egress redirect dev eth0
> > >
> > > We fix this by adding a per-cpu, per-qdisc recursion counter which is
> > > incremented the first time a root qdisc is entered and on a second attempt
> > > enter the same root qdisc from the top, the packet is dropped to break the
> > > loop.
> > >
> > > Reported-by: renmingshuai@huawei.com
> > > Closes: https://lore.kernel.org/netdev/20240314111713.5979-1-renmingshuai@huawei.com/
> > > Fixes: 3bcb846ca4cf ("net: get rid of spin_trylock() in net_tx_action()")
> > > Fixes: e578d9c02587 ("net: sched: use counter to break reclassify loops")
> > > Co-developed-by: Victor Nogueira <victor@mojatatu.com>
> > > Signed-off-by: Victor Nogueira <victor@mojatatu.com>
> > > Signed-off-by: Jamal Hadi Salim <jhs@mojatatu.com>
> > > ---
> > >  include/net/sch_generic.h |  2 ++
> > >  net/core/dev.c            |  9 +++++++++
> > >  net/sched/sch_api.c       | 12 ++++++++++++
> > >  net/sched/sch_generic.c   |  2 ++
> > >  4 files changed, 25 insertions(+)
> > >
> > > diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h
> > > index cefe0c4bdae3..f9f99df037ed 100644
> > > --- a/include/net/sch_generic.h
> > > +++ b/include/net/sch_generic.h
> > > @@ -125,6 +125,8 @@ struct Qdisc {
> > >         spinlock_t              busylock ____cacheline_aligned_in_smp;
> > >         spinlock_t              seqlock;
> > >
> > > +       u16 __percpu            *xmit_recursion;
> > > +
> > >         struct rcu_head         rcu;
> > >         netdevice_tracker       dev_tracker;
> > >         /* private data */
> > > diff --git a/net/core/dev.c b/net/core/dev.c
> > > index 9a67003e49db..2b712388c06f 100644
> > > --- a/net/core/dev.c
> > > +++ b/net/core/dev.c
> > > @@ -3789,6 +3789,13 @@ static inline int __dev_xmit_skb(struct sk_buff *skb, struct Qdisc *q,
> > >         if (unlikely(contended))
> > >                 spin_lock(&q->busylock);
> >
> > This could hang here (busylock)
>
> Notice the goto free_skb_list has an spin_unlock(&q->busylock);  in
> its code vicinity. Am I missing something?
>
> >
> > >
> > > +       if (__this_cpu_read(*q->xmit_recursion) > 0) {
> > > +               __qdisc_drop(skb, &to_free);
> > > +               rc = NET_XMIT_DROP;
> > > +               goto free_skb_list;
> > > +       }
> >
> >
> > I do not think we want to add yet another cache line miss and
> > complexity in tx fast path.
> >
>
> I empathize. The cache miss is due to a per-cpu variable? Otherwise
> that seems to be in the vicinity of the other fields being accessed in
> __dev_xmit_skb()
>
> > I think that mirred should  use a separate queue to kick a transmit
> > from the top level.
> >
> > (Like netif_rx() does)
> >
>
> Eric, here's my concern: this would entail restructuring mirred
> totally just to cater for one use case which is in itself _a bad
> config_ for egress qdisc case only. Mirred is very heavily used in
> many use cases and changing its behavior could likely introduce other
> corner cases for some use cases which we would be chasing for a while.
> Not to forget now we have to go via an extra transient queue.
> If i understood what you are suggesting is to add an equivalent of
> backlog queu for the tx side? I am assuming in a very similar nature
> to backlog, meaning per cpu fired by softirq? or is it something
> closer to qdisc->gso_skb
> For either of those cases, the amount of infrastructure code there is
> not a few lines of code. And then there's the desire to break the loop
> etc.
>
> Some questions regarding your proposal - something I am not following
> And i may have misunderstood what you are suggesting, but i am missing
> what scenario mirred can directly call tcf_dev_queue_xmit() (see my
> comment below)..
>
> > Using a softnet.xmit_qdisc_recursion (not a qdisc-per-cpu thing),
> > would allow mirred to bypass this additional queue
> > in most cases.
> >
> > diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> > index cb37817d6382c29117afd8ce54db6dba94f8c930..62ba5ef554860496ee928f7ed6b7c3ea46b8ee1d
> > 100644
> > --- a/include/linux/netdevice.h
> > +++ b/include/linux/netdevice.h
> > @@ -3217,7 +3217,8 @@ struct softnet_data {
> >  #endif
> >         /* written and read only by owning cpu: */
> >         struct {
> > -               u16 recursion;
> > +               u8 recursion;
> > +               u8 qdisc_recursion;
> >                 u8  more;
> >  #ifdef CONFIG_NET_EGRESS
> >                 u8  skip_txqueue;
> > diff --git a/net/core/dev.c b/net/core/dev.c
> > index 9a67003e49db87f3f92b6c6296b3e7a5ca9d9171..7ac59835edef657e9558d4d4fc0a76b171aace93
> > 100644
> > --- a/net/core/dev.c
> > +++ b/net/core/dev.c
> > @@ -4298,7 +4298,9 @@ int __dev_queue_xmit(struct sk_buff *skb, struct
> > net_device *sb_dev)
> >
> >         trace_net_dev_queue(skb);
> >         if (q->enqueue) {
> > +               __this_cpu_inc(softnet_data.xmit.qdisc_recursion);
>
> This increments the count by 1..
>
> >                 rc = __dev_xmit_skb(skb, q, dev, txq);
> > +               __this_cpu_dec(softnet_data.xmit.qdisc_recursion);
> >                 goto out;
> >         }
> >
> > diff --git a/net/sched/act_mirred.c b/net/sched/act_mirred.c
> > index 5b38143659249e66718348e0ec4ed3c7bc21c13d..0f5f02e6744397d33ae2a72670ba7131aaa6942e
> > 100644
> > --- a/net/sched/act_mirred.c
> > +++ b/net/sched/act_mirred.c
> > @@ -237,8 +237,13 @@ tcf_mirred_forward(bool at_ingress, bool
> > want_ingress, struct sk_buff *skb)
> >  {
> >         int err;
> >
> > -       if (!want_ingress)
> > -               err = tcf_dev_queue_xmit(skb, dev_queue_xmit);
> > +       if (!want_ingress) {
> > +               if (__this_cpu_read(softnet_data.xmit.qdisc_recursion)) {
>
> Where does the defered
> So this will always be 1 assuming the defer queue will have to be
> something like a workqueue

Sorry, sent too fast - meant we would always enter here..

> > +                       // Queue to top level, or drop
> > +               } else {
>
> and we'll never enter this..
>
> > +                       err = tcf_dev_queue_xmit(skb, dev_queue_xmit);
> > +               }
> > +       }
> >         else if (!at_ingress)
> >                 err = netif_rx(skb);
> >         else
>
> cheers,
> jamal