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Date: Mon, 15 Apr 2024 11:29:50 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: Jamal Hadi Salim <jhs@...atatu.com>
Cc: davem@...emloft.net, kuba@...nel.org, pabeni@...hat.com, jiri@...nulli.us,
xiyou.wangcong@...il.com, netdev@...r.kernel.org, renmingshuai@...wei.com,
Victor Nogueira <victor@...atatu.com>
Subject: Re: [PATCH RFC net 1/1] net/sched: Fix mirred to self recursion
On Mon, Apr 15, 2024 at 11:20 AM Eric Dumazet <edumazet@...gle.com> wrote:
>
> On Wed, Apr 10, 2024 at 10:30 PM Jamal Hadi Salim <jhs@...atatu.com> wrote:
> >
> > On Tue, Apr 2, 2024 at 1:35 PM Jamal Hadi Salim <jhs@...atatu.com> wrote:
> > >
> > > On Tue, Apr 2, 2024 at 12:47 PM Eric Dumazet <edumazet@...gle.com> wrote:
> > > >
> > > > On Wed, Mar 27, 2024 at 11:58 PM Jamal Hadi Salim <jhs@...atatu.com> wrote:
> > > > >
> > > > > On Wed, Mar 27, 2024 at 9:23 AM Eric Dumazet <edumazet@...gle.com> wrote:
> > > > > >
> > > > > > On Wed, Mar 27, 2024 at 12:03 AM Jamal Hadi Salim <jhs@...atatu.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@...wei.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@...atatu.com>
> > > > > > > Signed-off-by: Victor Nogueira <victor@...atatu.com>
> > > > > > > Signed-off-by: Jamal Hadi Salim <jhs@...atatu.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?
> > > >
> > > > The hang would happen in above spin_lock(&q->busylock), before you can
> > > > get a chance...
> > > >
> > > > If you want to test your patch, add this debugging feature, pretending
> > > > the spinlock is contended.
> > > >
> > > > diff --git a/net/core/dev.c b/net/core/dev.c
> > > > index 818699dea9d7040ee74532ccdebf01c4fd6887cc..b2fe3aa2716f0fe128ef10f9d06c2431b3246933
> > > > 100644
> > > > --- a/net/core/dev.c
> > > > +++ b/net/core/dev.c
> > > > @@ -3816,7 +3816,7 @@ static inline int __dev_xmit_skb(struct sk_buff
> > > > *skb, struct Qdisc *q,
> > > > * sent after the qdisc owner is scheduled again. To prevent this
> > > > * scenario the task always serialize on the lock.
> > > > */
> > > > - contended = qdisc_is_running(q) || IS_ENABLED(CONFIG_PREEMPT_RT);
> > > > + contended = true; // DEBUG for Jamal
> > > > if (unlikely(contended))
> > > > spin_lock(&q->busylock);
> > >
> > > Will do.
> >
> > Finally got time to look again. Probably being too clever, but moving
> > the check before the contended check resolves it as well. The only
> > strange thing is now with the latest net-next seems to be spitting
> > some false positive lockdep splat for the test of A->B->A (i am sure
> > it's fixable).
> >
> > See attached. Didnt try the other idea, see if you like this one.
>
> A spinlock can only be held by one cpu at a time, so recording the cpu
> number of the lock owner should be
> enough to avoid a deadlock.
>
> So I really do not understand your push for a per-cpu variable with
> extra cache line misses.
>
> I think the following would work just fine ? What do you think ?
>
> diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h
> index 76db6be1608315102495dd6372fc30e6c9d41a99..dcd92ed7f69fae00deaca2c88fed248a559108ea
> 100644
> --- a/include/net/sch_generic.h
> +++ b/include/net/sch_generic.h
> @@ -117,6 +117,7 @@ struct Qdisc {
> struct qdisc_skb_head q;
> struct gnet_stats_basic_sync bstats;
> struct gnet_stats_queue qstats;
> + int owner;
> unsigned long state;
> unsigned long state2; /* must be written under qdisc
> spinlock */
> struct Qdisc *next_sched;
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 854a3a28a8d85b335a9158378ae0cca6dfbf8b36..d77cac53df4b4af478548dd17e7a3a7cfe4bd792
> 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -3808,6 +3808,11 @@ static inline int __dev_xmit_skb(struct sk_buff
> *skb, struct Qdisc *q,
> return rc;
> }
>
> + if (unlikely(READ_ONCE(q->owner) == smp_processor_id())) {
> + /* add a specific drop_reason later in net-next */
> + kfree_skb_reason(skb, SKB_DROP_REASON_TC_RECLASSIFY_LOOP);
> + return NET_XMIT_DROP;
> + }
Or even better this expensive check could be moved into tcf_mirred_forward() ?
This would be done later in net-next, because this looks a bit more
complex than this simple solution.
> /*
> * Heuristic to force contended enqueues to serialize on a
> * separate lock before trying to get qdisc main lock.
> @@ -3847,7 +3852,9 @@ static inline int __dev_xmit_skb(struct sk_buff
> *skb, struct Qdisc *q,
> qdisc_run_end(q);
> rc = NET_XMIT_SUCCESS;
> } else {
> + WRITE_ONCE(q->owner, smp_processor_id());
> rc = dev_qdisc_enqueue(skb, q, &to_free, txq);
> + WRITE_ONCE(q->owner, -1);
> if (qdisc_run_begin(q)) {
> if (unlikely(contended)) {
> spin_unlock(&q->busylock);
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