[PATCH v2] NVMe: eliminate potential deadlock by nvme_get_ns_from_disk invoking nvme_free_ns

[hch@lst.de (Christoph Hellwig)]

On Wed, May 04, 2016@09:08:34AM +0800, Wang Sheng-Hui wrote:
> Release dev_list_lock before enter nvme_free_ns from
> nvme_get_ns_from_disk to avoid potential deadlock.
> 
> Signed-off-by: Wang Sheng-Hui <shhuiw at foxmail.com>
> ---
>  drivers/nvme/host/core.c | 15 ++++++---------
>  1 file changed, 6 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
> index 643f457..ab12892 100644
> --- a/drivers/nvme/host/core.c
> +++ b/drivers/nvme/host/core.c
> @@ -87,20 +87,17 @@ static struct nvme_ns *nvme_get_ns_from_disk(struct gendisk *disk)
>  	spin_lock(&dev_list_lock);
>  	ns = disk->private_data;
>  	if (ns) {
> +		if (!try_module_get(ns->ctrl->ops->module)) {
> +			spin_unlock(&dev_list_lock);
> +			kref_put(&ns->kref, nvme_free_ns);
> +			return NULL;
> +		}
>  		if (!kref_get_unless_zero(&ns->kref))
> +			ns = NULL;
>  	}
>  	spin_unlock(&dev_list_lock);
>  
>  	return ns;
>  }

This is incorrect - if kref_get_unless_zero fails we now fail to to to
drop the reference again.  What about this variant instead?

	ns = disk->private_data;
	if (ns) {
		if (!kref_get_unless_zero(&ns->kref))
			ns = NULL;
		else if (!try_module_get(ns->ctrl->ops->module))
			goto out_put_ns;
	}
	spin_unlock(&dev_list_lock);
	return ns;

out_put_ns:
	spin_unlock(&dev_list_lock);
	kref_put(&ns->kref, nvme_free_ns);
	return NULL;