Re: [PATCH 6/6] nbd: reduce the nbd_index_mutex scope

[Josef Bacik <josef@toxicpanda.com>]

On 8/11/21 8:44 AM, Christoph Hellwig wrote:
> nbd_index_mutex is currently held over add_disk and inside ->open, which
> leads to lock order reversals.  Refactor the device creation code path
> so that nbd_dev_add is called without nbd_index_mutex lock held and
> only takes it for the IDR insertation.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
>   drivers/block/nbd.c | 55 +++++++++++++++++++++++----------------------
>   1 file changed, 28 insertions(+), 27 deletions(-)
> 
> diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
> index 0b46a608f879..4054cc33fc1e 100644
> --- a/drivers/block/nbd.c
> +++ b/drivers/block/nbd.c
> @@ -1681,7 +1681,7 @@ static const struct blk_mq_ops nbd_mq_ops = {
>   	.timeout	= nbd_xmit_timeout,
>   };
>   
> -static struct nbd_device *nbd_dev_add(int index)
> +static struct nbd_device *nbd_dev_add(int index, unsigned int refs)
>   {
>   	struct nbd_device *nbd;
>   	struct gendisk *disk;
> @@ -1707,6 +1707,7 @@ static struct nbd_device *nbd_dev_add(int index)
>   	if (err)
>   		goto out_free_nbd;
>   
> +	mutex_lock(&nbd_index_mutex);
>   	if (index >= 0) {
>   		err = idr_alloc(&nbd_index_idr, nbd, index, index + 1,
>   				GFP_KERNEL);
> @@ -1717,6 +1718,7 @@ static struct nbd_device *nbd_dev_add(int index)
>   		if (err >= 0)
>   			index = err;
>   	}
> +	mutex_unlock(&nbd_index_mutex);
>   	if (err < 0)
>   		goto out_free_tags;
>   	nbd->index = index;
> @@ -1743,7 +1745,7 @@ static struct nbd_device *nbd_dev_add(int index)
>   
>   	mutex_init(&nbd->config_lock);
>   	refcount_set(&nbd->config_refs, 0);
> -	refcount_set(&nbd->refs, 1);
> +	refcount_set(&nbd->refs, refs);
>   	INIT_LIST_HEAD(&nbd->list);
>   	disk->major = NBD_MAJOR;
>   	disk->first_minor = index << part_shift;
> @@ -1847,34 +1849,35 @@ static int nbd_genl_connect(struct sk_buff *skb, struct genl_info *info)
>   		nbd = idr_find(&nbd_index_idr, index);
>   	}
>   
> -	if (!nbd) {
> -		nbd = nbd_dev_add(index);
> -		if (IS_ERR(nbd)) {
> +	if (nbd) {
> +		if (test_bit(NBD_DESTROY_ON_DISCONNECT, &nbd->flags) &&
> +		    test_bit(NBD_DISCONNECT_REQUESTED, &nbd->flags)) {
> +			nbd->destroy_complete = &destroy_complete;
>   			mutex_unlock(&nbd_index_mutex);
> -			pr_err("nbd: failed to add new device\n");
> -			return PTR_ERR(nbd);
> +
> +			/* wait until the nbd device is completely destroyed */
> +			wait_for_completion(&destroy_complete);
> +			goto again;
>   		}
> -	}
>   
> -	if (test_bit(NBD_DESTROY_ON_DISCONNECT, &nbd->flags) &&
> -	    test_bit(NBD_DISCONNECT_REQUESTED, &nbd->flags)) {
> -		nbd->destroy_complete = &destroy_complete;
> +		if (!refcount_inc_not_zero(&nbd->refs)) {
> +			mutex_unlock(&nbd_index_mutex);
> +			if (index == -1)
> +				goto again;
> +			pr_err("nbd: device at index %d is going down\n",
> +		       		index);

Errant whitespace here.  Thanks,

Josef