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From:   "Jason A. Donenfeld" <Jason@zx2c4.com>
Date:   Sun, 26 Apr 2020 13:42:58 -0600
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Subject: Re: INFO: rcu detected stall in wg_packet_tx_worker
To:     Eric Dumazet <eric.dumazet@gmail.com>
Cc:     syzbot <syzbot+0251e883fe39e7a0cb0a@syzkaller.appspotmail.com>,
        David Miller <davem@davemloft.net>,
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On Sun, Apr 26, 2020 at 1:40 PM Eric Dumazet <eric.dumazet@gmail.com> wrote:
>
>
>
> On 4/26/20 10:57 AM, syzbot wrote:
> > syzbot has bisected this bug to:
> >
> > commit e7096c131e5161fa3b8e52a650d7719d2857adfd
> > Author: Jason A. Donenfeld <Jason@zx2c4.com>
> > Date:   Sun Dec 8 23:27:34 2019 +0000
> >
> >     net: WireGuard secure network tunnel
> >
> > bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=15258fcfe00000
> > start commit:   b2768df2 Merge branch 'for-linus' of git://git.kernel.org/..
> > git tree:       upstream
> > final crash:    https://syzkaller.appspot.com/x/report.txt?x=17258fcfe00000
> > console output: https://syzkaller.appspot.com/x/log.txt?x=13258fcfe00000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=b7a70e992f2f9b68
> > dashboard link: https://syzkaller.appspot.com/bug?extid=0251e883fe39e7a0cb0a
> > userspace arch: i386
> > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=15f5f47fe00000
> > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=11e8efb4100000
> >
> > Reported-by: syzbot+0251e883fe39e7a0cb0a@syzkaller.appspotmail.com
> > Fixes: e7096c131e51 ("net: WireGuard secure network tunnel")
> >
> > For information about bisection process see: https://goo.gl/tpsmEJ#bisection
> >
>
> I have not looked at the repro closely, but WireGuard has some workers
> that might loop forever, cond_resched() might help a bit.

I'm working on this right now. Having a bit difficult of a time
getting it to reproduce locally...

The reports show the stall happening always at:

static struct sk_buff *
sfq_dequeue(struct Qdisc *sch)
{
       struct sfq_sched_data *q = qdisc_priv(sch);
       struct sk_buff *skb;
       sfq_index a, next_a;
       struct sfq_slot *slot;

       /* No active slots */
       if (q->tail == NULL)
               return NULL;

next_slot:
       a = q->tail->next;
       slot = &q->slots[a];

Which is kind of interesting, because it's not like that should block
or anything, unless there's some kasan faulting happening.