linux-kernel - Re: sem_lock() vs qspinlocks

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Message-ID: <20160520150505.GG3193@twins.programming.kicks-ass.net>
Date:	Fri, 20 May 2016 17:05:05 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Davidlohr Bueso <dave@...olabs.net>
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, 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.