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Date:   Fri, 13 Sep 2019 16:35:57 -0400 (EDT)
From:   Alan Stern <stern@rowland.harvard.edu>
To:     syzbot <syzbot+b24d736f18a1541ad550@syzkaller.appspotmail.com>
cc:     andreyknvl@google.com, <balbi@kernel.org>,
        <chunfeng.yun@mediatek.com>, <gregkh@linuxfoundation.org>,
        <linux-kernel@vger.kernel.org>, <linux-usb@vger.kernel.org>,
        <syzkaller-bugs@googlegroups.com>
Subject: Re: INFO: rcu detected stall in dummy_timer
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On Fri, 13 Sep 2019, syzbot wrote:

> syzbot has found a reproducer for the following crash on:
> 
> HEAD commit:    f0df5c1b usb-fuzzer: main usb gadget fuzzer driver
> git tree:       https://github.com/google/kasan.git usb-fuzzer
> console output: https://syzkaller.appspot.com/x/log.txt?x=1146550d600000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=5c6633fa4ed00be5
> dashboard link: https://syzkaller.appspot.com/bug?extid=b24d736f18a1541ad550
> compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=11203fa5600000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=162cd335600000
> 
> IMPORTANT: if you fix the bug, please add the following tag to the commit:
> Reported-by: syzbot+b24d736f18a1541ad550@syzkaller.appspotmail.com
> 
> yurex 3-1:0.101: yurex_interrupt - unknown status received: -71
> yurex 5-1:0.101: yurex_interrupt - unknown status received: -71
> yurex 6-1:0.101: yurex_interrupt - unknown status received: -71
> rcu: INFO: rcu_sched self-detected stall on CPU

Andrey:

This problem may be a result of overloading dummy_timer.  The kernel 
config you are using has CONFIG_HZ=100, but dummy-hcd needs 
CONFIG_HZ=1000 (see the comment on line 1789).  That is, lower values 
of HZ will occasionally lead to trouble, and this may be an example.

Can you change the config value for HZ and see if the bug still 
reproduces?

Alan Stern