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

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Date:	Wed, 17 Nov 2010 08:11:24 +0000
From:	"Jan Beulich" <JBeulich@...ell.com>
To:	"Jeremy Fitzhardinge" <jeremy@...p.org>
Cc:	"Jeremy Fitzhardinge" <jeremy.fitzhardinge@...rix.com>,
	"Eric Dumazet" <dada1@...mosbay.com>, <xiyou.wangcong@...il.com>,
	"Peter Zijlstra" <peterz@...radead.org>,
	"Nick Piggin" <npiggin@...nel.dk>,
	"Srivatsa Vaddagiri" <vatsa@...ux.vnet.ibm.com>,
	"Linux Virtualization" <virtualization@...ts.linux-foundation.org>,
	"Xen-devel" <xen-devel@...ts.xensource.com>,
	"Mathieu Desnoyers" <mathieu.desnoyers@...ymtl.ca>,
	"Avi Kivity" <avi@...hat.com>,
	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH 09/14] xen/pvticketlock: Xen implementation for PV
	 ticket locks

>>> On 16.11.10 at 22:08, Jeremy Fitzhardinge <jeremy@...p.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

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