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

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Message-ID: <CANRm+CytA0-bz8ArUEd=SHUFRu5OEXiqB0RmaxB51MhtABM+8Q@mail.gmail.com>
Date:	Mon, 18 Jul 2016 07:22:39 +0800
From:	Wanpeng Li <kernellwp@...il.com>
To:	Waiman Long <waiman.long@....com>
Cc:	Boqun Feng <boqun.feng@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Pan Xinhui <xinhui@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...hat.com>,
	"linux-kernel@...r.kernel.org" <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

2016-07-18 7:10 GMT+08:00 Waiman Long <waiman.long@....com>:
> 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.

This patch can base on the top of mine, I think it has already done this way. :)

Regards,
Wanpeng Li