Linux Virtualization - Jul 2014 - [PATCH 05/11] qspinlock: Optimize for smaller NR_CPUS

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.

Paolo

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.
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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