linux-kernel - Re: [PATCH v3] locking/pvqspinlock: restore/set vcpu_hashed state after failing adaptive locking spinning

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Date:	Fri, 15 Jul 2016 15:45:58 +0800
From:	Wanpeng Li <kernellwp@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Waiman Long <waiman.long@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Wanpeng Li <wanpeng.li@...mail.com>,
	Ingo Molnar <mingo@...nel.org>,
	Davidlohr Bueso <dave@...olabs.net>
Subject: Re: [PATCH v3] locking/pvqspinlock: restore/set vcpu_hashed state
 after failing adaptive locking spinning

2016-07-15 15:09 GMT+08:00 Peter Zijlstra <peterz@...radead.org>:
> On Fri, Jul 15, 2016 at 05:26:40AM +0800, Wanpeng Li wrote:
>> 2016-07-14 22:52 GMT+08:00 Waiman Long <waiman.long@....com>:
>> [...]
>> > As pv_kick_node() is called immediately after designating the next node as
>> > the queue head, the chance of this racing is possible, but is not likely
>> > unless the lock holder vCPU gets preempted for a long time at that right
>> > moment. This change does not do any harm though, so I am OK with that.
>> > However, I do want you to add a comment about the possible race in the code
>> > as it isn't that obvious or likely.
>>
>> How about something like:
>>
>> /*
>>  * If the lock holder vCPU gets preempted for a long time, pv_kick_node will
>>  * advance its state and hash the lock, restore/set the vcpu_hashed state to
>>  * avoid the race.
>>  */
>
> So I'm not sure. Yes it was a bug, but its fairly 'obvious' it should be

I believe Waiman can give a better comments. :)

> vcpu_hashed, we did after all hash the thing.
>
>> Btw, do you think patch title should be improved, what do you like?
>
> I changed it to: "locking/pvqspinlock: Fix double hash race"

Thanks. :)

Regards,
Wanpeng Li