linux-kernel - Re: [PATCH 2/4, v2] x86: enlightenment for ticket spin locks

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Date:	Wed, 30 Jun 2010 15:36:37 +0100
From:	"Jan Beulich" <JBeulich@...ell.com>
To:	"Jeremy Fitzhardinge" <jeremy@...p.org>
Cc:	"mingo@...e.hu" <mingo@...e.hu>,
	"Stefano Stabellini" <stefano.stabellini@...citrix.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"Ky Srinivasan" <KSrinivasan@...ell.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"hpa@...or.com" <hpa@...or.com>
Subject: Re: [PATCH 2/4, v2] x86: enlightenment for ticket spin locks -
	 Xen implementation

>>> On 30.06.10 at 16:25, Jeremy Fitzhardinge <jeremy@...p.org> wrote:
> On 06/30/2010 04:03 PM, Jan Beulich wrote:
>> Afaics the unlock still involves a function call *in all cases* with
>> pvops spinlocks, whereas it's a single inline instruction without.
>>   
> 
> No.  The unlock path can see if there are any further waiters by looking
> at the ticket in the, and only do the kick call if there are some.

Are we perhaps talking about different things? I'm referring to

static __always_inline void arch_spin_unlock(struct arch_spinlock *lock)
{
	PVOP_VCALL1(pv_lock_ops.spin_unlock, lock);
}

which is an indirect call which, as I understand it, gets replaced
with a direct one at runtime. But it remains to be a call (as opposed
to being a single inc instructions without CONFIG_PARAVIRT_SPINLOCKS).

Jan

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