linux-kernel - Re: [BUG][tip/master] kernel panic while locking selftest at qspinlock

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Date:	Sat, 11 Jul 2015 09:32:57 +0900
From:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
To:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>
CC:	Ingo Molnar <mingo@...e.hu>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Waiman Long <Waiman.Long@...com>
Subject: Re: [BUG][tip/master] kernel panic while locking selftest at qspinlock_paravirt.h:137!

On 2015/07/10 23:28, Peter Zijlstra wrote:
> On Fri, Jul 10, 2015 at 03:57:46PM +0200, Ingo Molnar wrote:
>> * Peter Zijlstra <peterz@...radead.org> wrote:
> 
>>> Do we want to make double unlock non-fatal unconditionally?
>>
>> No, just don't BUG() out, don't crash the system - generate a warning?
> 
> So that would be a yes..
> 
> Something like so then? Won't this generate a splat on that locking self
> test then? And upset people?

Hmm, yes, this still noisy...
Can't we avoid double-unlock completely? it seems that this warning can
happen randomly, which means pv-spinlock randomly broken, doesn't it?

Thank you,

> ---
>  kernel/locking/qspinlock_paravirt.h | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/locking/qspinlock_paravirt.h b/kernel/locking/qspinlock_paravirt.h
> index 04ab18151cc8..286e8978a562 100644
> --- a/kernel/locking/qspinlock_paravirt.h
> +++ b/kernel/locking/qspinlock_paravirt.h
> @@ -133,8 +133,14 @@ static struct pv_node *pv_unhash(struct qspinlock *lock)
>  	 * This guarantees a limited lookup time and is itself guaranteed by
>  	 * having the lock owner do the unhash -- IFF the unlock sees the
>  	 * SLOW flag, there MUST be a hash entry.
> +	 *
> +	 * This can trigger due to double-unlock. In which case, return a
> +	 * random pointer so that __pv_queued_spin_unlock() can dereference it
> +	 * without crashing.
>  	 */
> -	BUG();
> +	WARN_ON_ONCE(true);
> +
> +	return (struct pv_node *)this_cpu_ptr(&mcs_nodes[0]);
>  }
>  
>  /*
> 


-- 
Masami HIRAMATSU
Linux Technology Research Center, System Productivity Research Dept.
Center for Technology Innovation - Systems Engineering
Hitachi, Ltd., Research & Development Group
E-mail: masami.hiramatsu.pt@...achi.com
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