2021-06-24 09:38:42 +02:00
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2022-01-24 10:39:13 +01:00
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2021-06-24 09:38:42 +02:00
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2021-07-13 01:05:29 +02:00
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2021-06-24 09:38:42 +02:00
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block: add disk sequence number
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.
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.
Additionally, increment the disk sequence number when the media change,
i.e. on DISK_EVENT_MEDIA_CHANGE event.
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Matteo Croce <mcroce@microsoft.com>
Tested-by: Luca Boccassi <bluca@debian.org>
Link: https://lore.kernel.org/r/20210712230530.29323-2-mcroce@linux.microsoft.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
2021-07-13 01:05:25 +02:00
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2021-07-13 01:05:29 +02:00
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2021-06-24 09:38:42 +02:00
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2023-06-08 13:02:35 +02:00
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2021-06-24 09:38:42 +02:00
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2023-10-17 20:48:21 +02:00
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2021-06-24 09:38:42 +02:00
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2023-06-08 13:02:35 +02:00
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2021-06-24 09:38:42 +02:00
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2023-06-08 13:02:35 +02:00
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2021-06-24 09:38:42 +02:00
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2023-10-17 20:48:21 +02:00
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2021-06-24 09:38:42 +02:00
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2023-06-08 13:02:35 +02:00
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2021-06-24 09:38:42 +02:00
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2021-07-13 01:05:29 +02:00
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2023-08-11 12:08:19 +02:00
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2021-07-13 01:05:29 +02:00
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loop: fix partition scan race between udev and loop_reread_partitions()
When LOOP_CONFIGURE is called with LO_FLAGS_PARTSCAN, the following
sequence occurs:
1. disk_force_media_change() sets GD_NEED_PART_SCAN
2. Uevent suppression is lifted and a KOBJ_CHANGE uevent is sent
3. loop_global_unlock() releases the lock
4. loop_reread_partitions() calls bdev_disk_changed() to scan
There is a race between steps 2 and 4: when udev receives the uevent
and opens the device before loop_reread_partitions() runs,
blkdev_get_whole() in bdev.c sees GD_NEED_PART_SCAN set and calls
bdev_disk_changed() for a first scan. Then loop_reread_partitions()
does a second scan. The open_mutex serializes these two scans, but
does not prevent both from running.
The second scan in bdev_disk_changed() drops all partition devices
from the first scan (via blk_drop_partitions()) before re-adding
them, causing partition block devices to briefly disappear. This
breaks any systemd unit with BindsTo= on the partition device: systemd
observes the device going dead, fails the dependent units, and does
not retry them when the device reappears.
Fix this by removing the GD_NEED_PART_SCAN set from
disk_force_media_change() entirely. None of the current callers need
the lazy on-open partition scan triggered by this flag:
- floppy: sets GENHD_FL_NO_PART, so disk_has_partscan() is always
false and GD_NEED_PART_SCAN has no effect.
- loop (loop_configure, loop_change_fd): when LO_FLAGS_PARTSCAN is
set, loop_reread_partitions() performs an explicit scan. When not
set, GD_SUPPRESS_PART_SCAN prevents the lazy scan path.
- loop (__loop_clr_fd): calls bdev_disk_changed() explicitly if
LO_FLAGS_PARTSCAN is set.
- nbd (nbd_clear_sock_ioctl): capacity is set to zero immediately
after; nbd manages GD_NEED_PART_SCAN explicitly elsewhere.
With GD_NEED_PART_SCAN no longer set by disk_force_media_change(),
udev opening the loop device after the uevent no longer triggers a
redundant scan in blkdev_get_whole(), and only the single explicit
scan from loop_reread_partitions() runs.
A regression test for this bug has been submitted to blktests:
https://github.com/linux-blktests/blktests/pull/240.
Fixes: 9f65c489b68d ("loop: raise media_change event")
Signed-off-by: Daan De Meyer <daan@amutable.com>
Acked-by: Christian Brauner <brauner@kernel.org>
Link: https://patch.msgid.link/20260331105130.1077599-1-daan@amutable.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
2026-03-31 10:51:28 +00:00
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2021-07-13 01:05:29 +02:00
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2023-08-11 12:08:19 +02:00
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2021-07-13 01:05:29 +02:00
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2023-08-11 12:08:19 +02:00
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2023-06-07 13:08:37 -04:00
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2023-08-11 12:08:24 +02:00
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2021-07-13 01:05:29 +02:00
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2021-06-24 09:38:42 +02:00
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2021-06-24 09:38:43 +02:00
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2021-06-24 09:38:42 +02:00
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2021-08-18 16:45:39 +02:00
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2021-06-24 09:38:42 +02:00
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2021-08-18 16:45:39 +02:00
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2021-06-24 09:38:42 +02:00
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2026-02-21 16:37:42 -08:00
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2021-06-24 09:38:42 +02:00
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2021-08-18 16:45:39 +02:00
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2021-06-24 09:38:42 +02:00
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2021-08-18 16:45:39 +02:00
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2021-06-24 09:38:42 +02:00
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