linux-kernel - Re: sem_lock() vs qspinlocks

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Date:	Fri, 20 May 2016 17:59:21 -0700
From:	Davidlohr Bueso <dave@...olabs.net>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Waiman Long <waiman.long@....com>, 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 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