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

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Message-ID: <55A0A413.7020507@hitachi.com>
Date:	Sat, 11 Jul 2015 14:05:23 +0900
From:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
To:	Waiman Long <waiman.long@...com>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>, Ingo Molnar <mingo@...e.hu>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [BUG][tip/master] kernel panic while locking selftest at qspinlock_paravirt.h:137!

On 2015/07/11 10:27, Waiman Long wrote:
> On 07/10/2015 08:32 PM, Masami Hiramatsu wrote:
>> 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?
> 
> It shouldn't randomly happen. The message should be printed at the first 
> instance of double-unlock. If that is not case, there may be some 
> problem in the code.

Ah, OK. That comes from locking selftest. In that case, do we really
need the warning while selftest, since we know it always fails ?

> Anyway, I have an alternative fix that should better capture the problem:

Do we need both Peter's BUG() removing patch and this?

Thank you,

> -------------------------------
> diff --git a/kernel/locking/qspinlock_paravirt.h 
> b/kernel/locking/qspinlock_paravirt.h
> index 04ab181..92fc54f 100644
> --- a/kernel/locking/qspinlock_paravirt.h
> +++ b/kernel/locking/qspinlock_paravirt.h
> @@ -286,15 +286,24 @@ __visible void __pv_queued_spin_unlock(struct 
> qspinlock *lock)
>   {
>       struct __qspinlock *l = (void *)lock;
>       struct pv_node *node;
> +    u8 lockval = cmpxchg(&l->locked, _Q_LOCKED_VAL, 0);
> 
>       /*
>        * We must not unlock if SLOW, because in that case we must first
>        * unhash. Otherwise it would be possible to have multiple @lock
>        * entries, which would be BAD.
>        */
> -    if (likely(cmpxchg(&l->locked, _Q_LOCKED_VAL, 0) == _Q_LOCKED_VAL))
> +    if (likely(lockval == _Q_LOCKED_VAL))
>           return;
> 
> +    if (unlikely(lockval != _Q_SLOW_VAL)) {
> +        printk(KERN_WARNING
> +               "pvqspinlock: lock 0x%lx has corrupted value 0x%x!\n",
> +               (unsigned long)lock, atomic_read(&lock->val));
> +        WARN_ON_ONCE(1);
> +        return;
> +    }
> +
>       /*
>        * Since the above failed to release, this must be the SLOW path.
>        * Therefore start by looking up the blocked node and unhashing it.
> 
> 


-- 
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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