linux-kernel - Re: upstream boot error: BUG: soft lockup in __do

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Message-ID: <7e93543a-ff0c-2433-7bed-4053da59d5c2@infradead.org>
Date:   Fri, 31 Jul 2020 09:08:11 -0700
From:   Randy Dunlap <rdunlap@...radead.org>
To:     Dmitry Vyukov <dvyukov@...gle.com>,
        syzbot <syzbot+8472ea265fe32cc3bf78@...kaller.appspotmail.com>
Cc:     Borislav Petkov <bp@...en8.de>, "H. Peter Anvin" <hpa@...or.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        the arch/x86 maintainers <x86@...nel.org>,
        "Paul E. McKenney" <paulmck@...nel.org>
Subject: Re: upstream boot error: BUG: soft lockup in __do_softirq

On 7/30/20 11:50 PM, Dmitry Vyukov wrote:
> On Fri, Jul 31, 2020 at 8:44 AM syzbot
> <syzbot+8472ea265fe32cc3bf78@...kaller.appspotmail.com> wrote:
>>
>> Hello,
>>
>> syzbot found the following issue on:
>>
>> HEAD commit:    92ed3019 Linux 5.8-rc7
>> git tree:       upstream
>> console output: https://syzkaller.appspot.com/x/log.txt?x=10e84cdf100000
>> kernel config:  https://syzkaller.appspot.com/x/.config?x=b45e47f6d958ae82
>> dashboard link: https://syzkaller.appspot.com/bug?extid=8472ea265fe32cc3bf78
>> compiler:       gcc (GCC) 10.1.0-syz 20200507
>>
>> IMPORTANT: if you fix the issue, please add the following tag to the commit:
>> Reported-by: syzbot+8472ea265fe32cc3bf78@...kaller.appspotmail.com
> 
> This is a qemu-kvm instance killing the host kernel somehow, the host
> kernel itself running qemu's is full of rcu stalls. I think this is
> not a bug in the tested kernel.
> We change rcu stall timeout to 120 seconds from the default 21s, but
> this happens only after boot using sysctls. I did not find any way to
> change the rcu timeout via cmdline/config (would be useful).

(adding Paul)


Documentation/RCU/stallwarn.rst says there is a Kconfig:

CONFIG_RCU_CPU_STALL_TIMEOUT

	This kernel configuration parameter defines the period of time
	that RCU will wait from the beginning of a grace period until it
	issues an RCU CPU stall warning.  This time period is normally
	21 seconds.

and Documentation/admin-guide/kernel-parameters.txt has 2 RCU stall timeouts,
one for CPU and one for tasks:

	rcupdate.rcu_cpu_stall_timeout= [KNL]
			Set timeout for RCU CPU stall warning messages.

	rcupdate.rcu_task_stall_timeout= [KNL]
			Set timeout in jiffies for RCU task stall warning
			messages.  Disable with a value less than or equal
			to zero.


-- 
~Randy