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Date:   Tue, 16 Oct 2018 15:24:06 +0200 (CEST)
From:   Thomas Gleixner <tglx@linutronix.de>
To:     syzbot <syzbot+385468161961cee80c31@syzkaller.appspotmail.com>
cc:     Borislav Petkov <bp@alien8.de>, "H. Peter Anvin" <hpa@zytor.com>,
        LKML <linux-kernel@vger.kernel.org>, mingo@redhat.com,
        nstange@suse.de, syzkaller-bugs@googlegroups.com, x86@kernel.org,
        Peter Zijlstra <peterz@infradead.org>
Subject: Re: INFO: rcu detected stall in do_idle
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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