linux-kernel - Re: console: lockup on boot

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Date:	Wed, 11 Jun 2014 22:34:36 +0200
From:	Jan Kara <jack@...e.cz>
To:	Sasha Levin <sasha.levin@...cle.com>
Cc:	Peter Hurley <peter@...leysoftware.com>, Jan Kara <jack@...e.cz>,
	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 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.
> 
> 
> 939f04bec1a4ef6ba4370b0f34b01decc844b1b1 is the first bad commit
> commit 939f04bec1a4ef6ba4370b0f34b01decc844b1b1
> Author: Jan Kara <jack@...e.cz>
> Date:   Wed Jun 4 16:11:37 2014 -0700
> 
>     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>
>     Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
> 
> 
> Thanks,
> Sasha
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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