Re: [PATCH v2 4/5]nbd: make nbd device wait for its users.

Markus Pargmann
2016-06-14T08:59:21Z

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On Thursday 02 June 2016 13:25:00 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.
>
> Each open of a nbd device is refcounted, while
> the userland program [nbd-client] doing the
> NBD_DO_IT ioctl would now wait for any other users
> of this device before invalidating the nbd device.
>
> Signed-off-by: Pranay Kr. Srivastava <pranjas@gmail.com>
> ---
> drivers/block/nbd.c | 58 +++++++++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 58 insertions(+)
>
> diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
> index d1d898d..4da40dc 100644
> --- a/drivers/block/nbd.c
> +++ b/drivers/block/nbd.c
> @@ -70,10 +70,13 @@ struct nbd_device {
> #if IS_ENABLED(CONFIG_DEBUG_FS)
> struct dentry *dbg_dir;
> #endif
> + atomic_t inuse;
> /*
> *This is specifically for calling sock_shutdown, for now.
> */
> struct work_struct ws_shutdown;
> + struct kref users;
> + struct completion user_completion;
> };
>
> #if IS_ENABLED(CONFIG_DEBUG_FS)
> @@ -104,6 +107,7 @@ static DEFINE_SPINLOCK(nbd_lock);
> * Shutdown function for nbd_dev work struct.
> */
> static void nbd_ws_func_shutdown(struct work_struct *);
> +static void nbd_kref_release(struct kref *);
>
> static inline struct device *nbd_to_dev(struct nbd_device *nbd)
> {
> @@ -682,6 +686,8 @@ static void nbd_reset(struct nbd_device *nbd)
> nbd->flags = 0;
> nbd->xmit_timeout = 0;
> INIT_WORK(&nbd->ws_shutdown, nbd_ws_func_shutdown);
> + init_completion(&nbd->user_completion);
> + kref_init(&nbd->users);
> queue_flag_clear_unlocked(QUEUE_FLAG_DISCARD, nbd->disk->queue);
> del_timer_sync(&nbd->timeout_timer);
> }
> @@ -815,6 +821,14 @@ static int __nbd_ioctl(struct block_device *bdev, struct nbd_device *nbd,
> kthread_stop(thread);
>
> sock_shutdown(nbd);
> + /*
> + * kref_init initializes with ref count as 1,
> + * nbd_client, or the user-land program executing
> + * this ioctl will make the refcount to 2[at least]
> + * so subtracting 2 from refcount.
> + */
> + kref_sub(&nbd->users, 2, nbd_kref_release);

Why don't you use a kref_put?

> + wait_for_completion(&nbd->user_completion);
> mutex_lock(&nbd->tx_lock);
> nbd_clear_que(nbd);
> kill_bdev(bdev);
> @@ -865,13 +879,56 @@ 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);

Not indented to opening bracket.

> + pr_debug("Releasing kref [%s]\n", __func__);
> + atomic_set(&nbd->inuse, 0);
> + 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;
> +
> + if (kref_get_unless_zero(&nbd_dev->users))
> + atomic_set(&nbd_dev->inuse, 1);
> +
> + pr_debug("Opening nbd_dev %s. Active users = %u\n",
> + bdev->bd_disk->disk_name,
> + atomic_read(&nbd_dev->users.refcount) - 1);

Indent to opening bracket.

> + 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.
> + */
> + if (atomic_read(&nbd_dev->inuse))
> + kref_put(&nbd_dev->users, nbd_kref_release);

What is this inuse atomic for? Everyone that releases the nbd device
will need to execute a kref_put().

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_ws_func_shutdown(struct work_struct *ws_nbd)
> {
> struct nbd_device *nbd_dev = container_of(ws_nbd, struct nbd_device,
> @@ -1107,6 +1164,7 @@ static int __init nbd_init(void)
> disk->fops = &nbd_fops;
> disk->private_data = &nbd_dev[i];
> sprintf(disk->disk_name, "nbd%d", i);
> + atomic_set(&nbd_dev[i].inuse, 0);
> nbd_reset(&nbd_dev[i]);
> add_disk(disk);
> }
>

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