linux-kernel - Re: sem_lock() vs qspinlocks

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Date:	Sat, 21 May 2016 00:01:00 -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 08:59 PM, Davidlohr Bueso wrote:
> On Fri, 20 May 2016, Peter Zijlstra wrote:
>
>> On Fri, May 20, 2016 at 04:47:43PM -0400, Waiman Long wrote:
>>
>>> >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);
>>> >
>>
>>> Looking for contended lock, you need to consider the lock waiters 
>>> also. So
>>> looking at the whole word is right.
>>
>> No, you _only_ need to look at the lock waiters.
>
> Is there anyway to do this in a single atomic_read? My thought is that 
> otherwise
> we could further expand the race window of when the lock is and isn't
> contended (as returned to by the user). Ie avoiding crap like:
>
> atomic_read(&lock->val) && atomic_read(&lock->val) != _Q_LOCKED_VAL
>
> In any case, falsely returning for the 'locked, uncontended' case, vs 
> completely
> ignoring waiters is probably the lesser evil :).
>
> Thanks,
> Davidlohr

The existing code is doing that, but I would argue that including the 
locked, but uncontended case isn't a bad idea.

Cheers,
Longman