linux-kernel - Re: [patch v3 21/36] Hexagon: Add SMP support

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Date:	Sun, 11 Sep 2011 11:51:52 -0300
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Richard Kuo <rkuo@...eaurora.org>
Cc:	linux-arch@...r.kernel.org, linux-hexagon@...r.kernel.org,
	linux-kernel@...r.kernel.org, Linas Vepstas <linas@...eaurora.org>
Subject: Re: [patch v3 21/36] Hexagon: Add SMP support

On Thu, 2011-09-08 at 20:09 -0500, Richard Kuo wrote:

> +irqreturn_t handle_ipi(int irq, void *desc)
> +{
> +	int cpu = smp_processor_id();
> +	struct ipi_data *ipi = &per_cpu(ipi_data, cpu);
> +	unsigned long ops;
> +
> +	while ((ops = xchg(&ipi->bits, 0)) != 0)
> +		__handle_ipi(&ops, ipi, cpu);
> +	return IRQ_HANDLED;
> +}

So on the consumer side you are using xchg() ...

> +void send_ipi(const struct cpumask *cpumask, enum ipi_message_type msg)
> +{
> +	unsigned long flags;
> +	unsigned long cpu;
> +	unsigned long retval;
> +
> +	local_irq_save(flags);
> +
> +	for_each_cpu(cpu, cpumask) {
> +		struct ipi_data *ipi = &per_cpu(ipi_data, cpu);
> +
> +		spin_lock(&ipi->lock);
> +		ipi->bits |= 1 << msg;  /*  one bit per message  */
> +		spin_unlock(&ipi->lock);
> +
> +		retval = __vmintop_post(BASE_IPI_IRQ+cpu);
> +		if (retval != 0) {
> +			printk(KERN_ERR "interrupt %ld not configured?\n",
> +				BASE_IPI_IRQ+cpu);
> +		}
> +	}
> +
> +	local_irq_restore(flags);
> +}

And on the producer side, a spinlock for one or... that looks like wheel
re-inventing to me :-)

In fact your bit iteration loop also re-invents find_*_bit interfaces.

Any reason why you don't simply use bitops or atomics here ?

Also what is your memory model ? ordered or weakly ordered ? Do you need
some kind of memory barrier between setting ipi->bits and
__vmintop_post() ?

Cheers,
Ben.


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