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SubjectRe: INFO: rcu detected stall in do_idle
On Sat, 13 Oct 2018, syzbot wrote:
> syzbot found the following crash on:
>
> HEAD commit: 6b3944e42e2e afs: Fix cell proc list
> git tree: upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=1545a479400000
> kernel config: https://syzkaller.appspot.com/x/.config?x=88e9a8a39dc0be2d
> dashboard link: https://syzkaller.appspot.com/bug?extid=385468161961cee80c31
> compiler: gcc (GCC) 8.0.1 20180413 (experimental)
> syz repro: https://syzkaller.appspot.com/x/repro.syz?x=14825d5e400000
> C reproducer: https://syzkaller.appspot.com/x/repro.c?x=122cc531400000
>
> IMPORTANT: if you fix the bug, please add the following tag to the commit:
> Reported-by: syzbot+385468161961cee80c31@syzkaller.appspotmail.com
>
> sshd (5193) used greatest stack depth: 15496 bytes left
> sched: DL replenish lagged too much
> hrtimer: interrupt took 417330 ns
> rcu: INFO: rcu_sched detected stalls on CPUs/tasks:

It does reproduce here but with a kworker stall. Looking at the reproducer:

*(uint32_t*)0x20000000 = 0;
*(uint32_t*)0x20000004 = 6;
*(uint64_t*)0x20000008 = 0;
*(uint32_t*)0x20000010 = 0;
*(uint32_t*)0x20000014 = 0;
*(uint64_t*)0x20000018 = 0x9917;
*(uint64_t*)0x20000020 = 0xffff;
*(uint64_t*)0x20000028 = 0;
syscall(__NR_sched_setattr, 0, 0x20000000, 0);

which means:

struct sched_attr {
.size = 0,
.policy = 6,
.flags = 0,
.nice = 0,
.priority = 0,
.deadline = 0x9917,
.runtime = 0xffff,
.period = 0,
}

policy 6 is SCHED_DEADLINE

That makes the thread hog the CPU and prevents all kind of stuff to run.

Peter, is that expected behaviour?

Thanks,

tglx

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