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

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Message-ID: <20150711102718.GA28523@gmail.com>
Date:	Sat, 11 Jul 2015 12:27:18 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	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!


* Peter Zijlstra <peterz@...radead.org> 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?
> 
> ---
>  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]);

Yeah, just please also use debug_locks_silent to make the self-test execute 
properly or so.

Thanks,

	Ingo
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