On Mon, Jan 11, 2021 at 06:05:13PM +0100, Christoph Hellwig wrote:
> None of the complicated overlapping regions bits of the kobj_map are
> required for the character device lookup, so just a trivial xarray
> instead.
Thanks for doing this. We could make it more efficient for chardevs
that occupy 64 or more consecutive/aligned devices -- is it worth doing?
> +static struct cdev *cdev_lookup(dev_t dev)
> +{
> + struct cdev *cdev;
> +
> + mutex_lock(&chrdevs_lock);
> + cdev = xa_load(&cdev_map, dev);
> + if (!cdev) {
> + mutex_unlock(&chrdevs_lock);
> + if (request_module("char-major-%d-%d",
> + MAJOR(dev), MINOR(dev)) > 0)
> + /* Make old-style 2.4 aliases work */
> + request_module("char-major-%d", MAJOR(dev));
> + mutex_lock(&chrdevs_lock);
> +
> + cdev = xa_load(&cdev_map, dev);
> + }
> + if (cdev && !cdev_get(cdev))
> + cdev = NULL;
> + mutex_unlock(&chrdevs_lock);
> + return cdev;
What does the mutex protect here? Is it cdev being freed?
> @@ -593,11 +601,16 @@ static void cdev_unmap(dev_t dev, unsigned count)
> */
> void cdev_del(struct cdev *p)
> {
> - cdev_unmap(p->dev, p->count);
> + int i;
> +
> + mutex_lock(&chrdevs_lock);
> + for (i = 0; i < p->count; i++)
> + xa_erase(&cdev_map, p->dev + i);
> + mutex_unlock(&chrdevs_lock);
I don't understand what it's protecting here. It's clearly not cdev_get
as that could happen before we acquire the mutex. This also suggests
I should add an xa_erase_range() to the API.
But there's nothing wrong here, just some places that maybe could be
better, so:
Reviewed-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
