block: add a sequence number to disks

From: Matteo Croce <[email protected]>

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(-)

Quoted 88 lines · Matteo Croce

From: Matteo Croce <[email protected]>

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

Quoted 8 lines

 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 <[email protected]>

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

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

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

From: Matteo Croce <[email protected]>

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