Re: [PATCH v2] virtiofs: Export filesystem tags through sysfs

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Wed, Nov 08, 2023 at 04:33:33PM -0500, Vivek Goyal wrote:
> virtiofs filesystem is mounted using a "tag" which is exported by the
> virtiofs device. virtiofs driver knows about all the available tags but
> these are not exported to user space.
> 
> People have asked these tags to be exported to user space. Most recently
> Lennart Poettering has asked for it as he wants to scan the tags and mount
> virtiofs automatically in certain cases.
> 
> https://gitlab.com/virtio-fs/virtiofsd/-/issues/128
> 
> This patch exports tags through sysfs. One tag is associated with each
> virtiofs device. A new "tag" file appears under virtiofs device dir.
> Actual filesystem tag can be obtained by reading this "tag" file.
> 
> For example, if a virtiofs device exports tag "myfs", a new file "tag"
> will show up here. Tag has a newline char at the end.
> 
> /sys/bus/virtio/devices/virtio<N>/tag
> 
> # cat /sys/bus/virtio/devices/virtio<N>/tag
> myfs
> 
> Note, tag is available at KOBJ_BIND time and not at KOBJ_ADD event time.
> 
> v2:
> - Add a newline char at the end in tag file. (Alyssa Ross)
> - Add a line in commit logs about tag file being available at KOBJ_BIND
>   time and not KOBJ_ADD time.
> 
> Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
> Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
>  fs/fuse/virtio_fs.c | 34 ++++++++++++++++++++++++++++++++++
>  1 file changed, 34 insertions(+)
> 
> diff --git a/fs/fuse/virtio_fs.c b/fs/fuse/virtio_fs.c
> index 5f1be1da92ce..9f76c9697e6f 100644
> --- a/fs/fuse/virtio_fs.c
> +++ b/fs/fuse/virtio_fs.c
> @@ -107,6 +107,21 @@ static const struct fs_parameter_spec virtio_fs_parameters[] = {
>  	{}
>  };
>  
> +/* Forward Declarations */
> +static void virtio_fs_stop_all_queues(struct virtio_fs *fs);
> +
> +/* sysfs related */
> +static ssize_t tag_show(struct device *dev, struct device_attribute *attr,
> +			char *buf)
> +{
> +	struct virtio_device *vdev = container_of(dev, struct virtio_device,
> +						  dev);
> +	struct virtio_fs *fs = vdev->priv;
> +
> +	return sysfs_emit(buf, "%s\n", fs->tag);
> +}
> +static DEVICE_ATTR_RO(tag);

Is there a race between tag_show() and virtio_fs_remove()?
virtio_fs_mutex is not held. I'm thinking of the case where userspace
opens the sysfs file and invokes read(2) on one CPU while
virtio_fs_remove() runs on another CPU.

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

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