On Tue, 2021-07-13 at 01:05 +0200, Matteo Croce wrote:
> From: Matteo Croce <mcroce@microsoft.com>
> 
> Associating uevents with block devices in userspace is difficult and racy:
> the uevent netlink socket is lossy, and on slow and overloaded systems has
> a very high latency. Block devices do not have exclusive owners in
> userspace, any process can set one up (e.g. loop devices). Moreover, device
> names can be reused (e.g. loop0 can be reused again and again). A userspace
> process setting up a block device and watching for its events cannot thus
> reliably tell whether an event relates to the device it just set up or
> another earlier instance with the same name.
> 
> Being able to set a UUID on a loop device would solve the race conditions.
> But it does not allow to derive orderings from uevents: if you see a uevent
> with a UUID that does not match the device you are waiting for, you cannot
> tell whether it's because the right uevent has not arrived yet, or it was
> already sent and you missed it. So you cannot tell whether you should wait
> for it or not.
> 
> Being able to set devices up in a namespace would solve the race conditions
> too, but it can work only if being namespaced is feasible in the first
> place. Many userspace processes need to set devices up for the root
> namespace, so this solution cannot always work.
> 
> Changing the loop devices naming implementation to always use
> monotonically increasing device numbers, instead of reusing the lowest
> free number, would also solve the problem, but it would be very disruptive
> to userspace and likely break many existing use cases. It would also be
> quite awkward to use on long-running machines, as the loop device name
> would quickly grow to many-digits length.
> 
> Furthermore, this problem does not affect only loop devices - partition
> probing is asynchronous and very slow on busy systems. It is very easy to
> enter races when using LO_FLAGS_PARTSCAN and watching for the partitions to
> show up, as it can take a long time for the uevents to be delivered after
> setting them up.
> 
> Associating a unique, monotonically increasing sequential number to the
> lifetime of each block device, which can be retrieved with an ioctl
> immediately upon setting it up, allows to solve the race conditions with
> uevents, and also allows userspace processes to know whether they should
> wait for the uevent they need or if it was dropped and thus they should
> move on.
> 
> This does not benefit only loop devices and block devices with multiple
> partitions, but for example also removable media such as USB sticks or
> cdroms/dvdroms/etc.
> 
> The first patch is the core one, the 2..4 expose the information in
> different ways, and the last one makes the loop device generate a media
> changed event upon attach, detach or reconfigure, so the sequence number
> is increased.
> 
> If merged, this feature will immediately used by the userspace:
> https://github.com/systemd/systemd/issues/17469#issuecomment-762919781
> 
> v4 -> v5:
> - introduce a helper to raise media changed events
> - use the new helper in loop instead of the full event code
> - unexport inc_diskseq() which is only used by the block code now
> - rebase on top of 5.14-rc1
> 
> v3 -> v4:
> - rebased on top of 5.13
> - hook the seqnum increase into the media change event
> - make the loop device raise media change events
> - merge 1/6 and 5/6
> - move the uevent part of 1/6 into a separate one
> - drop the now unneeded sysfs refactor
> - change 'diskseq' to a global static variable
> - add more comments
> - refactor commit messages
> 
> v2 -> v3:
> - rebased on top of 5.13-rc7
> - resend because it appeared archived on patchwork
> 
> v1 -> v2:
> - increase seqnum on media change
> - increase on loop detach
> 
> Matteo Croce (6):
>   block: add disk sequence number
>   block: export the diskseq in uevents
>   block: add ioctl to read the disk sequence number
>   block: export diskseq in sysfs
>   block: add a helper to raise a media changed event
>   loop: raise media_change event
> 
>  Documentation/ABI/testing/sysfs-block | 12 ++++++
>  block/disk-events.c                   | 62 +++++++++++++++++++++------
>  block/genhd.c                         | 43 +++++++++++++++++++
>  block/ioctl.c                         |  2 +
>  drivers/block/loop.c                  |  5 +++
>  include/linux/genhd.h                 |  3 ++
>  include/uapi/linux/fs.h               |  1 +
>  7 files changed, 114 insertions(+), 14 deletions(-)

For the series:

Tested-by: Luca Boccassi <bluca@debian.org>

I have implemented the basic systemd support for this (ioctl + uevent,
sysfs will be done later), and tested with this series on x86_64 and
Debian 11 userspace, everything seems to work great. Thanks Matteo!

Here's the implementation, in draft state until the kernel side is
merged:

https://github.com/systemd/systemd/pull/20257

-- 
Kind regards,
Luca Boccassi