linux-kernel - Re: sem_lock() vs qspinlocks

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Date:	Fri, 20 May 2016 16:47:43 -0400
From:	Waiman Long <waiman.long@....com>
To:	Davidlohr Bueso <dave@...olabs.net>
CC:	Peter Zijlstra <peterz@...radead.org>, <manfred@...orfullife.com>,
	<mingo@...nel.org>, <torvalds@...ux-foundation.org>,
	<ggherdovich@...e.com>, <mgorman@...hsingularity.net>,
	<linux-kernel@...r.kernel.org>
Subject: Re: sem_lock() vs qspinlocks

On 05/20/2016 11:00 AM, Davidlohr Bueso wrote:
> On Fri, 20 May 2016, Peter Zijlstra wrote:
>
>> On Thu, May 19, 2016 at 10:39:26PM -0700, Davidlohr Bueso wrote:
>>>  In addition, this makes me wonder if queued_spin_is_locked() should 
>>> then be:
>>>
>>> -    return atomic_read(&lock->val);
>>> +    return atomic_read(&lock->val) & _Q_LOCKED_MASK;
>>>
>>> And avoid considering pending waiters as locked.
>>
>> Probably
>
> Similarly, and I know you hate it, but afaict, then semantically
> queued_spin_is_contended() ought to be:
>
> -       return atomic_read(&lock->val) & ~_Q_LOCKED_MASK;
> +       return atomic_read(&lock->val);
>
> Thanks,
> Davidlohr

Looking for contended lock, you need to consider the lock waiters also. 
So looking at the whole word is right.

Cheers,
Longman