On Mon, Nov 13, 2023 at 06:19:39PM -0500, Vivek Goyal wrote: > On Thu, Nov 09, 2023 at 09:28:25AM +0800, Stefan Hajnoczi wrote: > > 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@xxxxxxxxxx> > > > Reviewed-by: Stefan Hajnoczi <stefanha@xxxxxxxxxx> > > > --- > > > 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. > > Hi Stefan, > > Good point. I started testing it and realized that something else > is providing mutual exclusion and race does not occur. I added > an artifial msleep(10 seconds) in tag_show() and removed the device > and let tag_show() continue, hoping kernel will crash. But that > did not happen. > > Further investation revealed that device_remove_file() call in > virtio_fs_remove() blocks till tag_show() has finished. > > I have not looked too deep but my guess is that is is probably > kernfs_node->kernfs_rwsem which is providing mutual exclusion > and eliminating this race. > > So I don't think we need to take virtio_fs_mutex in tag_show(). Nice. That explains why other ->show() functions in the kernel also don't have much in the way of explicit synchronization. Thank you! Stefan
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