linux-kernel - Re: [PATCH 03/11] qspinlock: Add pending bit

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Date:	Wed, 18 Jun 2014 09:36:56 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>, Waiman.Long@...com,
	tglx@...utronix.de, mingo@...nel.org, linux-arch@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
	xen-devel@...ts.xenproject.org, kvm@...r.kernel.org,
	paolo.bonzini@...il.com, boris.ostrovsky@...cle.com,
	paulmck@...ux.vnet.ibm.com, riel@...hat.com,
	torvalds@...ux-foundation.org, raghavendra.kt@...ux.vnet.ibm.com,
	david.vrabel@...rix.com, oleg@...hat.com, gleb@...hat.com,
	scott.norton@...com, chegu_vinod@...com,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 03/11] qspinlock: Add pending bit

On Wed, Jun 18, 2014 at 01:29:48PM +0200, Paolo Bonzini wrote:
> Il 17/06/2014 22:36, Konrad Rzeszutek Wilk ha scritto:
> >+	/* One more attempt - but if we fail mark it as pending. */
> >+	if (val == _Q_LOCKED_VAL) {
> >+		new = Q_LOCKED_VAL |_Q_PENDING_VAL;
> >+
> >+		old = atomic_cmpxchg(&lock->val, val, new);
> >+		if (old == _Q_LOCKED_VAL) /* YEEY! */
> >+			return;
> >+		val = old;
> >+	}
> 
> Note that Peter's code is in a for(;;) loop:
> 
> 
> +	for (;;) {
> +		/*
> +		 * If we observe any contention; queue.
> +		 */
> +		if (val & ~_Q_LOCKED_MASK)
> +			goto queue;
> +
> +		new = _Q_LOCKED_VAL;
> +		if (val == new)
> +			new |= _Q_PENDING_VAL;
> +
> +		old = atomic_cmpxchg(&lock->val, val, new);
> +		if (old == val)
> +			break;
> +
> +		val = old;
> +	}
> +
> +	/*
> +	 * we won the trylock
> +	 */
> +	if (new == _Q_LOCKED_VAL)
> +		return;
> 
> So what you'd have is basically:
> 
> 	/*
> 	 * One more attempt if no one is already in queue.  Perhaps
> 	 * they have unlocked the spinlock already.
> 	 */
> 	if (val == _Q_LOCKED_VAL && atomic_read(&lock->val) == 0) {
> 		old = atomic_cmpxchg(&lock->val, 0, _Q_LOCKED_VAL);
> 		if (old == 0) /* YEEY! */
> 			return;
> 		val = old;
> 	}
> 
> But I agree with Waiman that this is unlikely to trigger often enough. It
> does have to be handled in the slowpath for correctness, but the most likely
> path is (0,0,1)->(0,1,1).

<nods>
> 
> Paolo
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