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SubjectRe: [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@infradead.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@hitachi.com


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