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

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Message-ID: <20160715070925.GO30154@twins.programming.kicks-ass.net>
Date:	Fri, 15 Jul 2016 09:09:25 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Wanpeng Li <kernellwp@...il.com>
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

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
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"