linux-kernel - Re: [PATCH v2 2/5] locking/pvqspinlock: Fix missed PV wakeup problem

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Date:	Sun, 17 Jul 2016 19:10:55 -0400
From:	Waiman Long <waiman.long@....com>
To:	Boqun Feng <boqun.feng@...il.com>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Pan Xinhui <xinhui@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...hat.com>, <linux-kernel@...r.kernel.org>,
	Scott J Norton <scott.norton@....com>,
	Douglas Hatch <doug.hatch@....com>
Subject: Re: [PATCH v2 2/5] locking/pvqspinlock: Fix missed PV wakeup problem

On 07/17/2016 07:07 PM, Waiman Long wrote:
> On 07/15/2016 09:16 PM, Boqun Feng wrote:
>> On Fri, Jul 15, 2016 at 06:35:56PM +0200, Peter Zijlstra wrote:
>>> On Fri, Jul 15, 2016 at 12:07:03PM +0200, Peter Zijlstra wrote:
>>>>> So if we are kicked by the unlock_slowpath, and the lock is 
>>>>> stealed by
>>>>> someone else,  we need hash its node again and set l->locked to
>>>>> _Q_SLOW_VAL, then enter pv_wait.
>>>> Right, let me go think about this a bit.
>>> Urgh, brain hurt.
>>>
>>> So I _think_ the below does for it but I could easily have missed yet
>>> another case.
>>>
>>> Waiman's patch has the problem that it can have two pv_hash() calls for
>>> the same lock in progress and I'm thinking that means we can hit the
>>> BUG() in pv_hash() due to that.
>>>
>> I think Waiman's patch does have the problem of two pv_hash() calls for
>> the same lock in progress. As I mentioned in the first version:
>>
>> http://lkml.kernel.org/g/20160527074331.GB8096@insomnia
>>
>> And he tried to address this in the patch #3 of this series. However,
>> I think what is proper here is either to reorder patch 2 and 3 or to
>> merge patch 2 and 3, otherwise, we are introducing a bug in the middle
>> of this series.
>>
>> Thoughts, Waiman?
>
> Patches 2 and 3 can be reversed.
>
>>
>> That said, I found Peter's way is much simpler and easier to understand
>> ;-)
>
> I agree. Peter's patch is better than mine.
>
>>> If we can't, it still has a problem because its not telling us either.
>>>
>>>
>>>
>>> --- a/kernel/locking/qspinlock_paravirt.h
>>> +++ b/kernel/locking/qspinlock_paravirt.h
>>> @@ -20,7 +20,8 @@
>>>    * native_queued_spin_unlock().
>>>    */
>>>
>>> -#define _Q_SLOW_VAL    (3U<<  _Q_LOCKED_OFFSET)
>>> +#define _Q_HASH_VAL    (3U<<  _Q_LOCKED_OFFSET)
>>> +#define _Q_SLOW_VAL    (7U<<  _Q_LOCKED_OFFSET)
>>>
>>>   /*
>>>    * Queue Node Adaptive Spinning
>>> @@ -36,14 +37,11 @@
>>>    */
>>>   #define PV_PREV_CHECK_MASK    0xff
>>>
>>> -/*
>>> - * Queue node uses: vcpu_running&  vcpu_halted.
>>> - * Queue head uses: vcpu_running&  vcpu_hashed.
>>> - */
>>>   enum vcpu_state {
>>> -    vcpu_running = 0,
>>> -    vcpu_halted,        /* Used only in pv_wait_node */
>>> -    vcpu_hashed,        /* = pv_hash'ed + vcpu_halted */
>>> +    vcpu_node_running = 0,
>>> +    vcpu_node_halted,
>>> +    vcpu_head_running,
>> We actually don't need this extra running state, right? Because nobody
>> cares about the difference between two running states right now.
>
> That addresses the problem in Xinhui patch that changed the state from 
> halted to hashed. With that separation, that change is no longer 
> necessary.

Oh, I meant Wanpeng's double hash race patch, not Xinhui's patch.

Cheers,
Longman