loop: add regression test for partscan double-scan race#240
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loop: add regression test for partscan double-scan race#240daandemeyer wants to merge 1 commit intolinux-blktests:masterfrom
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Mar 31, 2026
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:
linux-blktests/blktests#240.
Fixes: 9f65c48 ("loop: raise media_change event")
Signed-off-by: Daan De Meyer <[email protected]>
blktests-ci Bot
pushed a commit
to linux-blktests/linux-block
that referenced
this pull request
Mar 31, 2026
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:
linux-blktests/blktests#240.
Fixes: 9f65c48 ("loop: raise media_change event")
Signed-off-by: Daan De Meyer <[email protected]>
Acked-by: Christian Brauner <[email protected]>
blktests-ci Bot
pushed a commit
to linux-blktests/linux-block
that referenced
this pull request
Mar 31, 2026
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:
linux-blktests/blktests#240.
Fixes: 9f65c48 ("loop: raise media_change event")
Signed-off-by: Daan De Meyer <[email protected]>
Acked-by: Christian Brauner <[email protected]>
blktests-ci Bot
pushed a commit
to linux-blktests/linux-block
that referenced
this pull request
Mar 31, 2026
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:
linux-blktests/blktests#240.
Fixes: 9f65c48 ("loop: raise media_change event")
Signed-off-by: Daan De Meyer <[email protected]>
Acked-by: Christian Brauner <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jens Axboe <[email protected]>
blktests-ci Bot
pushed a commit
to linux-blktests/linux-block
that referenced
this pull request
Apr 1, 2026
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:
linux-blktests/blktests#240.
Fixes: 9f65c48 ("loop: raise media_change event")
Signed-off-by: Daan De Meyer <[email protected]>
Acked-by: Christian Brauner <[email protected]>
blktests-ci Bot
pushed a commit
to linux-blktests/linux-block
that referenced
this pull request
Apr 2, 2026
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:
linux-blktests/blktests#240.
Fixes: 9f65c48 ("loop: raise media_change event")
Signed-off-by: Daan De Meyer <[email protected]>
Acked-by: Christian Brauner <[email protected]>
blktests-ci Bot
pushed a commit
to linux-blktests/linux-block
that referenced
this pull request
Apr 3, 2026
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:
linux-blktests/blktests#240.
Fixes: 9f65c48 ("loop: raise media_change event")
Signed-off-by: Daan De Meyer <[email protected]>
Acked-by: Christian Brauner <[email protected]>
blktests-ci Bot
pushed a commit
to linux-blktests/linux-block
that referenced
this pull request
Apr 3, 2026
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:
linux-blktests/blktests#240.
Fixes: 9f65c48 ("loop: raise media_change event")
Signed-off-by: Daan De Meyer <[email protected]>
Acked-by: Christian Brauner <[email protected]>
blktests-ci Bot
pushed a commit
to linux-blktests/linux-block
that referenced
this pull request
Apr 4, 2026
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:
linux-blktests/blktests#240.
Fixes: 9f65c48 ("loop: raise media_change event")
Signed-off-by: Daan De Meyer <[email protected]>
Acked-by: Christian Brauner <[email protected]>
blktests-ci Bot
pushed a commit
to linux-blktests/linux-block
that referenced
this pull request
Apr 4, 2026
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:
linux-blktests/blktests#240.
Fixes: 9f65c48 ("loop: raise media_change event")
Signed-off-by: Daan De Meyer <[email protected]>
Acked-by: Christian Brauner <[email protected]>
Collaborator
|
Hello @daandemeyer Today, I ran the added test case on my test environment, and found the test case fail even when I apply the fix patch to the kernel. I observed the failure with both v2 and v3 kernel patches. I used v7.0-rc7 as the baseline kernel. Is this failure expected? |
Collaborator
|
One more nit comment. Some commands in the new test case use short options: "truncate -s", "losetup -f" or so. For readability and stability, longer options are recommended, like "truncate --size=", "losetup --find". I'm fine with the current short options, but if you have chance to respin, I suggest to use longer options. I attach a diff file to show how the new test case will look like with the longer options. |
blktests-ci Bot
pushed a commit
to linux-blktests/linux-block
that referenced
this pull request
Apr 8, 2026
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:
linux-blktests/blktests#240.
Fixes: 9f65c48 ("loop: raise media_change event")
Signed-off-by: Daan De Meyer <[email protected]>
Acked-by: Christian Brauner <[email protected]>
blktests-ci Bot
pushed a commit
to linux-blktests/linux-block
that referenced
this pull request
Apr 10, 2026
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:
linux-blktests/blktests#240.
Fixes: 9f65c48 ("loop: raise media_change event")
Signed-off-by: Daan De Meyer <[email protected]>
Acked-by: Christian Brauner <[email protected]>
blktests-ci Bot
pushed a commit
to linux-blktests/linux-block
that referenced
this pull request
Apr 13, 2026
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:
linux-blktests/blktests#240.
Fixes: 9f65c48 ("loop: raise media_change event")
Signed-off-by: Daan De Meyer <[email protected]>
Acked-by: Christian Brauner <[email protected]>
Author
|
@kawasaki I ran it a bunch of times and it doesn't fail on my test setup :/ |
Collaborator
|
@daandemeyer Thank you for trying out. FYI, here I attach the kernel config of the kernel that I used. And I used Fedora 43 on QEMU VM as the userland of my test environment. Which distro do you use? Do you run your test on any VM or bare metal machine? |
Author
|
My test environment is https://github.com/DaanDeMeyer/mkosi-kernel, an Arch Linux image running in qemu (booted with systemd). (I verified the test failed without my changes as well) |
Collaborator
|
Thanks. I started trying mkosi and mkosi-kernel, but still failing to building the Arch image. Will try to allocate more time for it. |
Collaborator
|
I managed to run mkosi with mkosi-kernel to generate Fedora and Arch images. (I fell in the pit fall that Fedora 43 kernel failed to boot with the Fedora rawhide image generated by mkosi. When I specify kernel source to build, the failure disappeared.) With the Fedora and Arch images, I confirmed that the new test case works as expected. The test case fails without the kernel side fix, and the test case passes with the kernel side fix. Then, the next step is to investigate why the test case fails on my Fedora QEMU environment even with the kernel side fix. Now I'm chasing this. |
Collaborator
|
I set up "Distribution=fedora" in mkosi.local.conf. I tried both "Release=rawhide" and "Release=43", and found that the test case passes with "Release=rawhide", but fail with "Release=43" (same kernel v7.0 + kernel fix by @daandemeyer ). I guess systemd version difference between the Fedora version might be the cause of the behavior difference.
If this hypothesis is correct, it means that the new test case has systemd version dependency. I would like to confirm it, but I do not know how to replace systemd. @daandemeyer If you have any good idea to confirm it, please share. Can mkosi replace systemd version? |
Collaborator
|
If was stupid to ask "Can mkosi replace systemd version?". mkosi is the systemd origin side project to build up the OS image to help systemd development. Of course it can. And @daandemeyer already documented how to do it in README of mkosi-kernel. It was a bit tough for newbees, but I have managed to control both systemd and kernel versions to build Fedora rawhide image. With that environment, I confirmed that the new test case depends on both kernel and systemd:
I bisected further, and found that the systemd commit aa47d8ade18c is required to make the test case pass. The test case fails with systemd v258 tag or before the commit. Based on this observation, I suggest to check systemd version in the required() function. If it is older than v259, skip the test case. I attach a patch file patch.txt for reference. @daandemeyer Please let me know your thoughts. |
Author
|
@kawasaki Thank you for the investigation! The attached patch looks good to me. I will apply it and update the PR |
Add a stress test that detects spurious partition removal events when setting up a loop device with partscan enabled. The kernel bug was that disk_force_media_change() set GD_NEED_PART_SCAN, causing udev's device open to trigger a partition scan racing with the explicit scan from loop_reread_partitions(). The second scan would drop and re-add all partitions, making partition devices briefly disappear. The test monitors kernel uevents while repeatedly setting up and tearing down a loop device with partscan. Each cycle should produce exactly one add and one remove uevent for the partition device. Extra events indicate the double-scan race was triggered. Link: https://lore.kernel.org/linux-block/[email protected]/T/#u Signed-off-by: Daan De Meyer <[email protected]> Co-Authored-By: Shinichiro Kawasaki <[email protected]>
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Add a stress test that detects spurious partition removal events when setting up a loop device with partscan enabled.
The kernel bug was that disk_force_media_change() set GD_NEED_PART_SCAN, causing udev's device open to trigger a partition scan racing with the explicit scan from loop_reread_partitions(). The second scan would drop and re-add all partitions, making partition devices briefly disappear.
The test monitors kernel uevents while repeatedly setting up and tearing down a loop device with partscan. Each cycle should produce exactly one add and one remove uevent for the partition device. Extra events indicate the double-scan race was triggered.
Link: https://lore.kernel.org/linux-block/[email protected]/T/#u