linux-kernel - Re: [RFC PATCH v2 2/6] PM / Runtime: introduce pm_runtime_set_memalloc

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Date:	Mon, 22 Oct 2012 10:33:13 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Ming Lei <ming.lei@...onical.com>
cc:	linux-kernel@...r.kernel.org, Oliver Neukum <oneukum@...e.de>,
	Minchan Kim <minchan@...nel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>, Jens Axboe <axboe@...nel.dk>,
	"David S. Miller" <davem@...emloft.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	<netdev@...r.kernel.org>, <linux-usb@...r.kernel.org>,
	<linux-pm@...r.kernel.org>, <linux-mm@...ck.org>
Subject: Re: [RFC PATCH v2 2/6] PM / Runtime: introduce pm_runtime_set_memalloc_noio()

On Mon, 22 Oct 2012, Ming Lei wrote:

> +void pm_runtime_set_memalloc_noio(struct device *dev, bool enable)
> +{
> +	dev->power.memalloc_noio_resume = enable;
> +
> +	if (!dev->parent)
> +		return;
> +
> +	if (enable) {
> +		pm_runtime_set_memalloc_noio(dev->parent, 1);
> +	} else {
> +		/* only clear the flag for one device if all
> +		 * children of the device don't set the flag.
> +		 */
> +		if (device_for_each_child(dev->parent, NULL,
> +					  dev_memalloc_noio))
> +			return;
> +
> +		pm_runtime_set_memalloc_noio(dev->parent, 0);
> +	}
> +}
> +EXPORT_SYMBOL_GPL(pm_runtime_set_memalloc_noio);

Tail recursion should be implemented as a loop, not as an explicit
recursion.  That is, the function should be:

void pm_runtime_set_memalloc_noio(struct device *dev, bool enable)
{
	do {
		dev->power.memalloc_noio_resume = enable;

		if (!enable) {
			/*
			 * Don't clear the parent's flag if any of the
			 * parent's children have their flag set.
			 */
			if (device_for_each_child(dev->parent, NULL,
					  dev_memalloc_noio))
				return;
		}
		dev = dev->parent;
	} while (dev);
}

except that you need to add locking, for two reasons:

	There's a race.  What happens if another child sets the flag
	between the time device_for_each_child() runs and the next loop
	iteration?

	Even without a race, access to bitfields is not SMP-safe 
	without locking.

Alan Stern

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