linux-kernel - Re: console: lockup on boot

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Date:	Wed, 11 Jun 2014 23:07:04 -0400
From:	Sasha Levin <sasha.levin@...cle.com>
To:	Jan Kara <jack@...e.cz>
CC:	Peter Hurley <peter@...leysoftware.com>, pmladek@...e.cz,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jet Chen <jet.chen@...el.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: console: lockup on boot

On 06/11/2014 05:31 PM, Jan Kara wrote:
> On Wed 11-06-14 22:34:36, Jan Kara wrote:
>> > On Wed 11-06-14 10:55:55, Sasha Levin wrote:
>>> > > On 06/10/2014 11:59 AM, Peter Hurley wrote:
>>>> > > > On 06/06/2014 03:05 PM, Sasha Levin wrote:
>>>>> > > >> On 05/30/2014 10:07 AM, Jan Kara wrote:
>>>>>> > > >>> On Fri 30-05-14 09:58:14, Peter Hurley wrote:
>>>>>>>> > > >>>>> On 05/30/2014 09:11 AM, Sasha Levin wrote:
>>>>>>>>>> > > >>>>>>> Hi all,
>>>>>>>>>> > > >>>>>>>
>>>>>>>>>> > > >>>>>>> I sometime see lockups when booting my KVM guest with the latest -next kernel,
>>>>>>>>>> > > >>>>>>> it basically hangs right when it should start 'init', and after a while I get
>>>>>>>>>> > > >>>>>>> the following spew:
>>>>>>>>>> > > >>>>>>>
>>>>>>>>>> > > >>>>>>> [   30.790833] BUG: spinlock lockup suspected on CPU#1, swapper/1/0
>>>>>>>> > > >>>>>
>>>>>>>> > > >>>>> Maybe related to this report: https://lkml.org/lkml/2014/5/30/26
>>>>>>>> > > >>>>> from Jet Chen which was bisected to
>>>>>>>> > > >>>>>
>>>>>>>> > > >>>>> commit bafe980f5afc7ccc693fd8c81c8aa5a02fbb5ae0
>>>>>>>> > > >>>>> Author:     Jan Kara <jack@...e.cz>
>>>>>>>> > > >>>>> AuthorDate: Thu May 22 10:43:35 2014 +1000
>>>>>>>> > > >>>>> Commit:     Stephen Rothwell <sfr@...b.auug.org.au>
>>>>>>>> > > >>>>> CommitDate: Thu May 22 10:43:35 2014 +1000
>>>>>>>> > > >>>>>
>>>>>>>> > > >>>>>      printk: enable interrupts before calling console_trylock_for_printk()
>>>>>>>> > > >>>>>          We need interrupts disabled when calling console_trylock_for_printk() only
>>>>>>>> > > >>>>>      so that cpu id we pass to can_use_console() remains valid (for other
>>>>>>>> > > >>>>>      things console_sem provides all the exclusion we need and deadlocks on
>>>>>>>> > > >>>>>      console_sem due to interrupts are impossible because we use
>>>>>>>> > > >>>>>      down_trylock()).  However if we are rescheduled, we are guaranteed to run
>>>>>>>> > > >>>>>      on an online cpu so we can easily just get the cpu id in
>>>>>>>> > > >>>>>      can_use_console().
>>>>>>>> > > >>>>>          We can lose a bit of performance when we enable interrupts in
>>>>>>>> > > >>>>>      vprintk_emit() and then disable them again in console_unlock() but OTOH it
>>>>>>>> > > >>>>>      can somewhat reduce interrupt latency caused by console_unlock()
>>>>>>>> > > >>>>>      especially since later in the patch series we will want to spin on
>>>>>>>> > > >>>>>      console_sem in console_trylock_for_printk().
>>>>>>>> > > >>>>>          Signed-off-by: Jan Kara <jack@...e.cz>
>>>>>>>> > > >>>>>      Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
>>>>>>>> > > >>>>>
>>>>>>>> > > >>>>> ?
>>>>>> > > >>>    Yeah, very likely. I think I see the problem, I'll send the fix shortly.
>>>>> > > >>
>>>>> > > >> Hi Jan,
>>>>> > > >>
>>>>> > > >> It seems that the issue I'm seeing is different from the "[prink]  BUG: spinlock
>>>>> > > >> lockup suspected on CPU#0, swapper/1".
>>>>> > > >>
>>>>> > > >> Is there anything else I could try here? The issue is very common during testing.
>>>> > > > 
>>>> > > > Sasha,
>>>> > > > 
>>>> > > > Is this bisectable? Maybe that's the best way forward here.
>>> > > 
>>> > > I've ran a bisection again and ended up at the same commit as Jet Chen
>>> > > (the commit unfortunately already made it to Linus's tree).
>>> > > 
>>> > > Note that I did try Jan's proposed fix and that didn't solve the issue
>>> > > for me, I believe we're seeing different issues caused by the same
>>> > > commit.
>> >   Sorry it has been busy time lately and I didn't have as much time to look
>> > into this as would be needed.
>   Oops, pressed send too early... So I have two debug patches for you. Can
> you try whether the problem reproduces with the first one or with both of
> them applied?

The first patch fixed it (I assumed that there's no need to try the second).


Thanks,
Sasha
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