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

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Date:	Wed, 18 Jun 2014 13:29:48 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
CC:	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

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).

Paolo
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