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

Andrew Morton
2010-12-18T00:14:00Z

On Fri, 17 Dec 2010 22:54:31 +0300
Alexander Gordeev <lasaine@lvk.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.