On Fri, Aug 12, 2016 at 5:17 PM, James Bottomley
<James.Bottomley@xxxxxxxxxxxxxxxxxxxxx> wrote:
> On Fri, 2016-08-12 at 14:29 -0700, Dan Williams wrote:
>> Before spending effort trying to flush the destruction of old bdi
>> instances before new ones are registered, is it rather time to
>> complete the conversion of sd to only use dynamically allocated devt?
>
> Do we have to go that far? Surely your fix is extensible: the only
> reason it doesn't work for us is that the gendisk holds the parent
> without a reference, so we can free the SCSI device before its child
> gendisk (good job no-one actually uses gendisk->parent after we've
> released it ...). If we fix that it would mean SCSI can't release the
> sdev until after the queue is dead and the bdi namespace released, so
> isn't something like this the easy fix?
>
> James
>
> ---
>
> diff --git a/block/genhd.c b/block/genhd.c
> index fcd6d4f..54ae4ae 100644
> --- a/block/genhd.c
> +++ b/block/genhd.c
> @@ -514,7 +514,7 @@ static void register_disk(struct device *parent, struct gendisk *disk)
> struct hd_struct *part;
> int err;
>
> - ddev->parent = parent;
> + ddev->parent = get_device(parent);
>
> dev_set_name(ddev, "%s", disk->disk_name);
>
> @@ -1144,6 +1144,7 @@ static void disk_release(struct device *dev)
> hd_free_part(&disk->part0);
> if (disk->queue)
> blk_put_queue(disk->queue);
> + put_device(dev->parent);
> kfree(disk);
> }
> struct class block_class = {
Looks ok at first glance to me.
We do hold a reference on the parent device, but it gets dropped at
device_unregister() time and this moves it out to the final put.
However, this does leave static devt block-device-drivers that
register a disk without a parent device susceptible to the race... I
think those exist given all the drivers still using add_disk() after
commit 52c44d93c26f "block: remove ->driverfs_dev".
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