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

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Date:	Tue, 14 Jun 2016 10:59:14 +0200
From:	Markus Pargmann <mpa@...gutronix.de>
To:	"Pranay Kr. Srivastava" <pranjas@...il.com>
Cc:	nbd-general@...ts.sourceforge.net, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 4/5]nbd: make nbd device wait for its users.

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@...il.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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