Re: [PATCH 3/4] make nbd device wait for its users.

Markus Pargmann
2016-05-30T10:44:55Z

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Hi,

sorry I couldn't fit the review into last week.

On Tuesday 24 May 2016 14:26:27 Pranay Kr. Srivastava wrote:
> When a timeout occurs or a recv fails, then
> instead of abruplty killing nbd block device
> wait for it's users to finish.
>
> This is more required when filesystem(s) like
> ext2 or ext3 don't expect their buffer heads to
> disappear while the filesystem is mounted.
>
> Use a kref for users using this. The device will
> be released for kref count of 2, not less or more.
>
> Signed-off-by: Pranay Kr. Srivastava <pranjas@gmail.com>
> ---
> drivers/block/nbd.c | 51 +++++++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 51 insertions(+)
>
> diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
> index af86c9b..59db890 100644
> --- a/drivers/block/nbd.c
> +++ b/drivers/block/nbd.c
> @@ -71,6 +71,8 @@ struct nbd_device {
> struct dentry *dbg_dir;
> #endif
> struct work_struct ws_nbd;
> + struct kref users;
> + struct completion user_completion;
> };
>
> #if IS_ENABLED(CONFIG_DEBUG_FS)
> @@ -674,6 +676,7 @@ static void nbd_reset(struct nbd_device *nbd)
> nbd->flags = 0;
> nbd->xmit_timeout = 0;
> INIT_WORK(&nbd->ws_nbd, nbd_work_func);
> + init_completion(&nbd->user_completion);
> queue_flag_clear_unlocked(QUEUE_FLAG_DISCARD, nbd->disk->queue);
> del_timer_sync(&nbd->timeout_timer);
> }
> @@ -807,6 +810,7 @@ static int __nbd_ioctl(struct block_device *bdev, struct nbd_device *nbd,
> kthread_stop(thread);
>
> sock_shutdown(nbd);
> + wait_for_completion(&nbd->user_completion);
> mutex_lock(&nbd->tx_lock);
> nbd_clear_que(nbd);
> kill_bdev(bdev);
> @@ -858,12 +862,58 @@ static int nbd_ioctl(struct block_device *bdev, fmode_t mode,
> return error;
> }
>
> +static void nbd_kref_release(struct kref *kref_users)
> +{
> + struct nbd_device *nbd = container_of(kref_users, struct nbd_device,
> + users);
> + pr_debug("Releasing kref [%s]\n", __FUNCTION__);
> + complete(&nbd->user_completion);
> +
> +}
> +
> +static int nbd_open(struct block_device *bdev, fmode_t mode)
> +{
> + struct nbd_device *nbd_dev = bdev->bd_disk->private_data;
> +
> + kref_get(&nbd_dev->users);
> + pr_debug("Opening nbd_dev %s. Active users = %u\n",
> + bdev->bd_disk->disk_name,
> + atomic_read(&nbd_dev->users.refcount) - 1);
> + return 0;
> +}
> +
> +static void nbd_release(struct gendisk *disk, fmode_t mode)
> +{
> + struct nbd_device *nbd_dev = disk->private_data;
> + /*
> + *kref_init initializes ref count to 1, so we
> + *we check for refcount to be 2 for a final put.
> + *
> + *kref needs to be re-initialized just here as the
> + *other process holding it must see the ref count as 2.
> + */
> + kref_put(&nbd_dev->users, nbd_kref_release);
> +
> + if (atomic_read(&nbd_dev->users.refcount) == 2) {
> + kref_sub(&nbd_dev->users, 2, nbd_kref_release);
> + kref_init(&nbd_dev->users);
> + kref_get(&nbd_dev->users);

Reading the refcount directly seems not to be as it supposed to be.

Why don't you put a kref_init() and kref_put() call into NBD_DO_IT? This
way you don't have to work around the property that kref_init() starts
with a refcount of 1 but you can use it.

For example:
NBD_DO_IT:
kref_init()
...
kref_put()

nbd_thread_recv() and nbd_thread_send():
kref_get()
...
kref_put()

In nbd_open() you could use kref_get_unless_zero() to avoid
opening a not connected device.

nbd_release() would then be a very simple kref_put() without
checking for 2 and so on.

Also there are some checkpatch issues with this patch.

Best Regards,

Markus

> + }
> +
> + pr_debug("Closing nbd_dev %s. Active users = %u\n",
> + disk->disk_name,
> + atomic_read(&nbd_dev->users.refcount) - 1);
> +}
> +
> static const struct block_device_operations nbd_fops = {
> .owner = THIS_MODULE,
> .ioctl = nbd_ioctl,
> .compat_ioctl = nbd_ioctl,
> + .open = nbd_open,
> + .release = nbd_release
> };
>
> +
> static void nbd_work_func(struct work_struct *ws_nbd)
> {
> struct nbd_device *nbd_dev = container_of(ws_nbd, struct nbd_device,
> @@ -1098,6 +1148,7 @@ static int __init nbd_init(void)
> disk->first_minor = i << part_shift;
> disk->fops = &nbd_fops;
> disk->private_data = &nbd_dev[i];
> + kref_init(&nbd_dev[i].users);
> sprintf(disk->disk_name, "nbd%d", i);
> nbd_reset(&nbd_dev[i]);
> add_disk(disk);
>

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