Re: [PATCH 09/14] xen/pvticketlock: Xen implementation for PV ticket locks

From: "Jan Beulich" <JBeulich@novell.com>
To: "Jeremy Fitzhardinge" <jeremy@goop.org>
Cc: "Jeremy Fitzhardinge" <jeremy.fitzhardinge@citrix.com>,
	"Eric Dumazet" <dada1@cosmosbay.com>, <xiyou.wangcong@gmail.com>,
	"Peter Zijlstra" <peterz@infradead.org>,
	"Nick Piggin" <npiggin@kernel.dk>,
	"Srivatsa Vaddagiri" <vatsa@linux.vnet.ibm.com>,
	"Linux Virtualization"
	<virtualization@lists.linux-foundation.org>,
	"Xen-devel" <xen-devel@lists.xensource.com>,
	"Mathieu Desnoyers" <mathieu.desnoyers@polymtl.ca>,
	"Avi Kivity" <avi@redhat.com>,
	"Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>,
	"H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [PATCH 09/14] xen/pvticketlock: Xen implementation for PV ticket locks
Date: Wed, 17 Nov 2010 08:11:24 +0000	[thread overview]
Message-ID: <4CE39C3C0200007800022AE2@vpn.id2.novell.com> (raw)
In-Reply-To: <aa32da076143b8e13c23c1f589d7e6cbedb22907.1289940821.git.jeremy.fitzhardinge@citrix.com>

>>> On 16.11.10 at 22:08, Jeremy Fitzhardinge <jeremy@goop.org> wrote:
> +static void xen_lock_spinning(struct arch_spinlock *lock, unsigned want)
>  {
> -	struct xen_spinlock *xl = (struct xen_spinlock *)lock;
> -	struct xen_spinlock *prev;
>  	int irq = __get_cpu_var(lock_kicker_irq);
> -	int ret;
> +	struct xen_lock_waiting *w = &__get_cpu_var(lock_waiting);
> +	int cpu = smp_processor_id();
>  	u64 start;
>  
>  	/* If kicker interrupts not initialized yet, just spin */
>  	if (irq == -1)
> -		return 0;
> +		return;
>  
>  	start = spin_time_start();
>  
> -	/* announce we're spinning */
> -	prev = spinning_lock(xl);
> +	w->want = want;
> +	w->lock = lock;
> +
> +	/* This uses set_bit, which atomic and therefore a barrier */
> +	cpumask_set_cpu(cpu, &waiting_cpus);

Since you don't allow nesting, don't you need to disable
interrupts before you touch per-CPU state?

Jan