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SubjectRe: INFO: rcu detected stall in br_handle_frame (2)
syzbot <syzbot+dc9071cc5a85950bdfce@syzkaller.appspotmail.com> wrote:

[ CC Eric, fq related ]

> syzbot found the following crash on:
>
> HEAD commit: 7e0165b2 Merge branch 'akpm' (patches from Andrew)
> git tree: upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=116ec09ee00000
> kernel config: https://syzkaller.appspot.com/x/.config?x=1b59a3066828ac4c
> dashboard link: https://syzkaller.appspot.com/bug?extid=dc9071cc5a85950bdfce
> compiler: gcc (GCC) 9.0.0 20181231 (experimental)
> syz repro: https://syzkaller.appspot.com/x/repro.syz?x=159182c1e00000
> C reproducer: https://syzkaller.appspot.com/x/repro.c?x=1221218ee00000
>
> Bisection is inconclusive: the bug happens on the oldest tested release.
>
> bisection log: https://syzkaller.appspot.com/x/bisect.txt?x=158224c1e00000
> final crash: https://syzkaller.appspot.com/x/report.txt?x=178224c1e00000
> console output: https://syzkaller.appspot.com/x/log.txt?x=138224c1e00000
>
> IMPORTANT: if you fix the bug, please add the following tag to the commit:
> Reported-by: syzbot+dc9071cc5a85950bdfce@syzkaller.appspotmail.com
>
> rcu: INFO: rcu_preempt detected stalls on CPUs/tasks:
> (detected by 0, t=10502 jiffies, g=10149, q=201)
> rcu: All QSes seen, last rcu_preempt kthread activity 10502
> (4294978441-4294967939), jiffies_till_next_fqs=1, root ->qsmask 0x0
> sshd R running task 26584 10034 9965 0x00000008
> Call Trace:
> <IRQ>
> sched_show_task kernel/sched/core.c:5954 [inline]
[..]

The reproducer sets up 'fq' sched with TCA_FQ_QUANTUM == 0x80000000

This causes infinite loop in fq_dequeue:

if (f->credit <= 0) {
f->credit += q->quantum;
goto begin;
}

... because f->credit is either 0 or -2147483648.

Eric, what is a 'sane' ->quantum value?

One could simply add a 'quantum > 0 && quantum < INT_MAX'
constraint afaics.

If you don't have a better idea/suggestion for an upperlimit INT_MAX
would be enough to prevent perpetual <= 0 condition.

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