linux-kernel - Re: [PATCH 05/11] qspinlock: Optimize for smaller NR

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Date:	Mon, 07 Jul 2014 17:08:17 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
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, konrad.wilk@...cle.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
Subject: Re: [PATCH 05/11] qspinlock: Optimize for smaller NR_CPUS

Il 07/07/2014 16:35, Peter Zijlstra ha scritto:
> On Wed, Jun 18, 2014 at 01:39:52PM +0200, Paolo Bonzini wrote:
>> Il 15/06/2014 14:47, Peter Zijlstra ha scritto:
>>>
>>> -	for (;;) {
>>> -		new = (val & ~_Q_PENDING_MASK) | _Q_LOCKED_VAL;
>>> -
>>> -		old = atomic_cmpxchg(&lock->val, val, new);
>>> -		if (old == val)
>>> -			break;
>>> -
>>> -		val = old;
>>> -	}
>>> +	clear_pending_set_locked(lock, val);
>>> 	return;
>>
>>
>> Might as well add clear_pending_set_locked already in patch 3.
>
> Patch 4, if anywhere.
>

This code is added in patch 3:

+	/*
+	 * take ownership and clear the pending bit.
+	 *
+	 * *,1,0 -> *,0,1
+	 */
+	for (;;) {
+		new = (val & ~_Q_PENDING_MASK) | _Q_LOCKED_VAL;
+
+		old = atomic_cmpxchg(&lock->val, val, new);
+		if (old == val)
+			break;
+
+		val = old;
+	}
+	return;

Unlike the change in patch 4, clear_pending_set_locked doesn't change 
how qspinlock moves from a state to the next.

Paolo
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