linux-kernel - Re: sem_lock() vs qspinlocks

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Date:	Fri, 20 May 2016 08:25:24 -0700
From:	Davidlohr Bueso <dave@...olabs.net>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	manfred@...orfullife.com, Waiman.Long@....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 Fri, 20 May 2016, Peter Zijlstra wrote:

>On Fri, May 20, 2016 at 08:00:49AM -0700, 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);
>
>Nah, that would make it return true for (0,0,1), ie. uncontended locked.

Right, and we want:

(*, 1, 1)
(*, 1, 0)
(n, 0, 0)

I may be missing some combinations, its still early.