linux-kernel - Re: [PATCHv6 07/16] pps: move idr stuff to pps.c

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Date:	Fri, 17 Dec 2010 16:13:28 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Alexander Gordeev <lasaine@....cs.msu.su>
Cc:	linux-kernel@...r.kernel.org,
	"Nikita V\. Youshchenko" <yoush@...msu.su>,
	linuxpps@...enneenne.com, Rodolfo Giometti <giometti@...eenne.com>
Subject: Re: [PATCHv6 07/16] pps: move idr stuff to pps.c

On Fri, 17 Dec 2010 22:54:31 +0300
Alexander Gordeev <lasaine@....cs.msu.su> wrote:

> Since now idr is only used to manage char device id's and not used in
> kernel API anymore it should be moved to pps.c. This also makes it
> possible to release id only at actual device freeing so nobody can
> register a pps device with the same id while our device is not freed
> yet.
> 
> ...
>
>  int pps_register_cdev(struct pps_device *pps)
>  {
>  	int err;
>  	dev_t devt;
>  
> +	/* Get new ID for the new PPS source */
> +	if (idr_pre_get(&pps_idr, GFP_KERNEL) == 0)
> +		return -ENOMEM;
> +
> +	/* Now really allocate the PPS source.
> +	 * After idr_get_new() calling the new source will be freely available
> +	 * into the kernel.
> +	 */
> +	spin_lock_irq(&pps_idr_lock);
> +	err = idr_get_new(&pps_idr, pps, &pps->id);
> +	spin_unlock_irq(&pps_idr_lock);
> +
> +	if (err < 0)
> +		return err;

The IDR interface really sucks :(

What this code should be doing is

retry:
	if (idr_pre_get(&pps_idr, GFP_KERNEL) == 0)
		return -ENOMEM;
	spin_lock_irq(&pps_idr_lock);
	err = idr_get_new(&pps_idr, pps, &pps->id);
	spin_unlock_irq(&pps_idr_lock);
	if (err < 0) {
		if (err == -EAGAIN)
			goto retry;
		return err;
	}

this way it correctly handles the case where the idr_pre_get()
succeeded in precharging the pool, but some other task cam in and stole
your reservation.


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