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

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Date:	Sat, 11 Jul 2015 12:22:19 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Waiman Long <waiman.long@...com>
Cc:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	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 Fri, Jul 10, 2015 at 09:27:45PM -0400, Waiman Long wrote:
> Anyway, I have an alternative fix that should better capture the problem:
> 
> -------------------------------
> 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);

	WARN_ONCE(1, "foo");

> +        return;
> +    }

Right, so since this should not ever happen in 'sane' code, its a shame
to have to put in this condition. But yes, this works too.
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